Quantitative analysis. Chemical methods of analysis. Analytical chemistry. Quantitative chemical analysis for each plant, allowing all environmental aspects to be taken into account
State system for ensuring the uniformity of measurements
QUANTITATIVE CHEMICAL ANALYSIS METHODS
General requirements for development, certification and use
State system for ensuring the uniformity of measurements. Quantitative chemical analysis procedures. General requirements for development, certification and application
OKS 17.020
Date of introduction 2015-01-01
Preface
1 DEVELOPED by the Federal State Unitary Enterprise "Ural Research Institute of Metrology" (FSUE "UNIIM")
2 INTRODUCED by the Technical Committee for Standardization TC 53 "Basic norms and rules for ensuring the uniformity of measurements"
3 APPROVED AND ENTERED INTO EFFECT by Order of the Federal Agency for Technical Regulation and Metrology dated November 22, 2013 N 1940-st
4 INTRODUCED FOR THE FIRST TIME
The rules for applying these recommendations are established in GOST R 1.0-2012 (Section 8). Information about changes to these recommendations is published in the annual (as of January 1 of the current year) information index "National Standards", and the official text of changes and amendments is published in the monthly information index "National Standards". In case of revision (replacement) or cancellation of these recommendations, the corresponding notice will be published in the next issue of the monthly information index "National Standards". Relevant information, notices and texts are also posted in the public information system - on the official website of the Federal Agency for Technical Regulation and Metrology on the Internet (gost.ru)
Introduction
Introduction
Methods of quantitative chemical analysis (hereinafter - MCHA), which are one of the varieties of measurement methods and are used in the analytical control of the composition or properties of substances, materials, environmental objects, objects of technical regulation, biological and other objects, as well as in transferring the size of units from standards and for certification of reference materials, constitute a significant part of the measurement techniques used both in the field of state regulation of ensuring the uniformity of measurements and outside it. At the same time, regardless of the scope of application, MCCA have common specifics associated with the presence and implementation of special procedures inherent in quantitative chemical analysis, such as the presence of different procedures for sampling and stabilization of samples for different objects, the presence of special conditions for storing and transporting samples of chemically aggressive objects , the presence of specific procedures for opening samples (chemical, thermal decomposition, etc.), the presence of special procedures for preparing samples for analysis related to the transfer of the analyte (component) into a state convenient for analysis (measurement) (various methods of extraction, concentration) and etc. Each of the above procedures can determine its own, sometimes quite significant contribution to the overall error (uncertainty) of the analysis results, causing them to be unreliable if any of the listed factors was not sufficiently well analyzed, assessed and taken into account in the process of developing the ICA and in the assessment its compliance with the intended purpose - MKHA validation (suitability assessment according to GOST ISO/IEC 17025-2009). Depending on the scope of application of the MKHA, the final stage of its development may be the methodology validation procedure in accordance with GOST ISO/IEC 17025-2009 (for MKHA intended for use outside the scope of state regulation to ensure the uniformity of measurements) or the certification procedure (in accordance with Federal Law N 102-FZ " On ensuring the uniformity of measurements" and GOST R 8.563-2009) for MKHA used in the field of state regulation of ensuring the uniformity of measurements), which can be carried out based on the results of validation of the MKHA. In this case, the validation of the IKHA is carried out by the developer or user of the methodology, and the certification of the IKHA is carried out by legal entities (individual entrepreneurs) accredited for this type of activity in the field of ensuring the uniformity of measurements.
The reliability and traceability of the analysis results obtained when using a specific MCCA depends on its metrological level, which, in turn, is determined by the quality of the implementation of the MCCA development procedure itself and its final stages - validation, certification.
The purpose of these recommendations is to describe a system of provisions and recommendations that should be taken into account when carrying out procedures for the development of MCCA, taking into account the above-mentioned specifics of quantitative chemical analysis and the need to apply various procedures for assessing its suitability for its intended purpose, as the final stage of development of MCCA (depending on the scope of its application), as well as the features and procedure for using MCCA, including MCCA, developed on the basis of international standards regulating standardized measurement (analysis) methods.
These recommendations have been developed to develop the provisions of GOST R 8.563-2009.
1 area of use
1.1 These recommendations define a system of provisions and recommendations that should be taken into account when developing, validating, certifying and applying quantitative chemical analysis techniques, which are one of the types of measurement techniques.
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According to GOST ISO/IEC 17025 - suitability assessment.
1.2 These recommendations apply to methods of quantitative chemical analysis (hereinafter referred to as MCCA), as well as to test methods, methods of testing, measurements, analysis, if they represent or contain MCCA.
2 Normative references
These recommendations use regulatory references to the following regulatory documents:
GOST 8.315-97 State system for ensuring the uniformity of measurements. Standard samples of the composition and properties of substances and materials. Basic provisions
GOST 8.417-2002 State system for ensuring the uniformity of measurements. Units of quantities
GOST 17.4.3.03-85 Nature conservation. Soils. General requirements for methods for determining pollutants
GOST 17.2.4.02-81 Nature conservation. Atmosphere. General requirements for methods for determining pollutants
GOST 27384-2002 Water. Standards of error for measurements of composition and properties indicators
GOST 28473-90. Cast iron, steel, ferroalloys, chrome, manganese metal. General requirements for analysis methods
GOST ISO 9000-2011 Quality management systems. Fundamentals and Vocabulary
GOST ISO/IEC 17025-2009 General requirements for the competence of testing and calibration laboratories
GOST R 8.563-2009 State system for ensuring the uniformity of measurements. Measurement techniques (methods)
GOST R 8.568-97 State system for ensuring the uniformity of measurements. Certification of testing equipment. Basic provisions
GOST R 8.596-2002 State system for ensuring the uniformity of measurements. Metrological support of measuring systems. Basic provisions
GOST R 8.654-2009 State system for ensuring the uniformity of measurements. Requirements for software of measuring instruments. Basic provisions
GOST R 8.736-2011 State system for ensuring the uniformity of measurements. Multiple direct measurements. Methods for processing measurement results. Basic provisions
GOST R 52361-2005 Analytical control of an object. Terms and Definitions
GOST R 52599-2006 Precious metals and their alloys. General requirements for analysis methods
GOST R 54569-2011 Cast iron, steel, ferroalloys, chromium and manganese metals. Accuracy standards for quantitative chemical analysis
GOST R ISO 5725-1-2002 Accuracy (correctness and precision) of measurement methods and results. Part 1. Basic provisions and definitions
GOST R ISO 5725-2-2002 Accuracy (correctness and precision) of measurement methods and results. Part 2: Basic method for determining repeatability and reproducibility of a standard measurement method
GOST R ISO 5725-3-2002 Accuracy (correctness and precision) of measurement methods and results. Part 3. Intermediate indicators of precision of a standard measurement method
GOST R ISO 5725-4-2002 Accuracy (correctness and precision) of measurement methods and results. Part 4. Basic methods for determining the correctness of a standard measurement method
GOST R ISO 5725-6-2002 Accuracy (correctness and precision) of measurement methods and results. Part 6: Using Accuracy Values in Practice
RMG 54-2002 State system for ensuring the uniformity of measurements. Characteristics of calibration instruments for measuring the composition and properties of substances and materials. Methodology for performing measurements using standard samples
RMG 60-2003 State system for ensuring the uniformity of measurements. Certified mixtures. General development requirements
RMG 61-2010 State system for ensuring the uniformity of measurements. Indicators of accuracy, correctness, precision of methods of quantitative chemical analysis. Assessment methods
RMG 62-2003 State system for ensuring the uniformity of measurements. Ensuring the effectiveness of measurements in process control. Estimation of measurement error with limited initial information
RMG 63-2003 State system for ensuring the uniformity of measurements. Ensuring the effectiveness of measurements in process control. Metrological examination of technical documentation
RMG 64-2003 State system for ensuring the uniformity of measurements. Ensuring the effectiveness of measurements in process control. Methods and methods for increasing measurement accuracy
RMG 76-2004 State system for ensuring the uniformity of measurements. Internal quality control of quantitative chemical analysis results.
PMG 44-2001 Rules for interstate standardization. The procedure for recognizing measurement methods
PMG 96-2009 State system for ensuring the uniformity of measurements. Results and quality characteristics of measurements. Forms of presentation
R 50.2.008-2001 State system for ensuring the uniformity of measurements. Methods of quantitative chemical analysis. Contents and procedure for metrological examination
R 50.2.028-2003 State system for ensuring the uniformity of measurements. Algorithms for constructing calibration characteristics of instruments for measuring the composition of substances and materials and assessing their errors (uncertainties). Estimation of the error (uncertainty) of linear calibration characteristics using the least squares method
R 50.2.060-2008 State system for ensuring the uniformity of measurements. Introduction of standardized methods for quantitative chemical analysis in laboratories. Confirmation of compliance with established requirements
Note - When using these recommendations, it is advisable to check the effect of reference documents and classifiers in the public information system - on the official website of the Federal Agency for Technical Regulation and Metrology on the Internet or using the annual information index "National Standards", which was published as of January 1 of the current year, and according to the releases of the monthly information index "National Standards" for the current year. If a referenced document to which an undated reference is given is replaced, it is recommended that the current version of that document be used, taking into account any changes made to that version. If a reference document to which a dated reference is given is replaced, it is recommended to use the version of this document with the year of approval (acceptance) indicated above. If, after the approval of these recommendations, a change is made to the referenced document to which a dated reference is made that affects the provision referred to, it is recommended that that provision be applied without regard to that change. If the reference document is canceled without replacement, then the provision in which a reference to it is given is recommended to be applied in the part that does not affect this reference.
3 Terms and definitions
These recommendations use terms according to GOST R 8.563, GOST R 52361, GOST ISO 9000, GOST R ISO 5725-1, PMG 96, RMG 61, as well as the following terms with corresponding definitions:
3.1 quantitative chemical analysis; QCA: Experimental quantitative determination in the object of analysis (substance, material) of the content (mass concentration, mass fraction, volume fraction, etc.) of one or more components by chemical, physicochemical, physical methods.
Note - The result of QCA is the established content of a substance component in a sample, expressed in units of physical quantities approved for use in the country, indicating the characteristics of its error (uncertainty) or their statistical estimates. The QCA result is a type of measurement result.
3.2 methods of quantitative chemical analysis; MKHA: A set of specifically described operations, the implementation of which provides the results of a quantitative chemical analysis with established accuracy indicators.
Notes
1 Quantitative chemical analysis technique is a type of measurement technique.
2. The content of one or more components of the object of analysis is taken as the measured characteristic.
Note - As an indicator of the accuracy of the measurement technique, measurement error characteristics can be used in accordance with , uncertainty indicators in accordance with *, accuracy indicators in accordance with GOST R ISO 5725-1.
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* See the Bibliography section, hereinafter. - Database manufacturer's note.
3.4 MCHA accuracy indicator: An indicator of measurement accuracy established for any QCA result obtained in compliance with the requirements and rules of a given ICA.
Note - Accuracy indicator values can be assigned to any QCA result obtained in compliance with the requirements and rules regulated in the MCA document.
3.5 measurement accuracy standard: Accuracy index values allowed for certain measurement purposes.
3.6 MKHA validation: A documented procedure for confirming the suitability of the MCHA to achieve its intended purposes, including examination and provision of objective evidence that the specific requirements for the specific intended use of the technique are met.
3.7 metrological requirements for MKHA: Requirements for the characteristics (parameters) of the measurement procedure provided for by the IKHA that affect the results and accuracy indicators, and the conditions under which these characteristics (parameters) must be ensured.
3.8 influencing factors of the sample: Interfering components and other properties (factors) of the sample that influence the result and value of the measurement accuracy indicator.
3.9 influencing factors of the technique: Factors whose values determine the conditions for carrying out measurements according to MKHA, influencing the result and value of the measurement accuracy indicator.
4 General provisions
4.1 MKHA are developed and used to ensure measurements of indicators of the composition and properties of substances, materials, objects of technical regulation, biological and other objects subject to analytical control, in accordance with established metrological requirements for measurements, including requirements for measurement accuracy.
4.2 Metrological requirements for measurements performed during analytical control are established taking into account the specifics of the objects being controlled and the purposes for using the measurement results.
4.3 Metrological requirements for measurements performed during analytical control include the requirements for:
- type and characteristics of the measured value (indicator);
- unit of the measured value (indicator);
- range of measurements of a value (indicator);
- measurement accuracy;
- ensuring traceability of measurement results;
- to the conditions of measurements;
- to the number of digits as a result of measurements (rounding of measurement results) - if necessary.
4.4 For MKHA related to the sphere of state regulation of ensuring the uniformity of measurements, in accordance with the federal executive authorities determine mandatory metrological requirements for measurements, including indicators of measurement accuracy.
MKHA, intended, according to , to confirm the compliance of objects of technical regulation with the requirements of technical regulations, must also provide mandatory requirements in terms of compliance:
- measured quantities (indicators) of the controlled object of technical regulation and the list of safety indicators established in it;
- units of measurements according to the IKHA units of quantities determined by the technical regulations;
- measurement range according to the IKHA established (acceptable) levels of safety indicators of technical regulation objects;
- values of the MKHA accuracy indicators to the measurement accuracy standards defined by the technical regulations (if any).
When developing an MKHA, additional metrological requirements may be determined by the customer (developer).
4.5 For microchemical equipment not related to the scope of state regulation of ensuring the uniformity of measurements, metrological requirements for measurements are determined by the customer (developer) of the methodology.
4.6 The development of MKHA is carried out on the basis of plans, national (industry) standardization programs, plans for modernizing the organization’s production, etc., depending on its purpose and scope of application.
4.7 The final stage in the development of the MKHA used in the field of state regulation of ensuring the uniformity of measurements is its certification. The final stage in the development of MKHA, which is not intended for use in the field of state regulation of ensuring the uniformity of measurements, is its validation or certification, performed on a voluntary basis.
4.8 The document for the MKHA is developed in accordance with the requirements of GOST R 8.563, these recommendations and the procedure established for the corresponding rank of document in the field of standardization, which involves the approval of a specific MKHA.
4.9 MKHA certification is carried out in accordance with the procedure defined by GOST R 8.563 and these recommendations. Certification of MKHA related to the sphere of state regulation of ensuring the uniformity of measurements is carried out by legal entities and individual entrepreneurs duly accredited to carry out work on certification of measurement methods in accordance with the approved scope of their accreditation.
4.10 Validation of the MKHA is carried out by its developer or, on his behalf, by a third-party organization competent in the field of metrological support of KKhA in accordance with these recommendations.
4.11 The use of MKHA in a specific laboratory that is not the developer of MKHA must be preceded by a procedure for its verification (implementation), confirming its feasibility in the conditions of this laboratory with established accuracy indicators.
4.12 MKHA are used in strict accordance with their purpose and scope, which are regulated in the approved document on MKHA.
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QUANTITATIVE CHEMICAL ANALYSIS, determination of the quantitative content of the components of the analyzed substance; one of the main types of chemical analysis. Based on the nature of the particles being determined, isotope analysis, elemental analysis, molecular analysis, phase analysis, structural group (functional) analysis and other types of analysis are distinguished. The content of the determined component (analyte) is characterized by the following quantities: amount of substance, mass, mass fraction, mole fraction, concentration, molar or mass ratios of components. The main characteristic is the amount of substance (v, mol). More often, the mass fraction of the analyte (ω, %) is determined proportional to the amount of substance.
Quantitative chemical analysis is a type of indirect measurements (see the article Metrology of chemical analysis). Quantitative chemical analysis is fundamentally different from conventional measurements in the absence of a standard unit of quantity of a substance (mole). In addition, in quantitative chemical analysis, non-measuring stages (sampling, sample preparation, identification) play an important role, therefore the error of the analysis result is higher than the total error of the initial measurements (mass, volume, etc.). Achieving uniformity of measurements in quantitative chemical analysis is difficult and is achieved in specific ways - using standard composition samples, as well as by comparing results obtained in different laboratories.
To carry out quantitative chemical analysis, chemical, physicochemical, physical, as well as biochemical and biological methods are used. Their relative importance varied: in the 18th and 19th centuries, gravimetry and titrimetry were the main ones, in the mid-20th century - spectral analysis, photometric analysis and electrochemical methods of analysis. At the turn of the 20th and 21st centuries, chromatography, various types of spectroscopy and mass spectrometry play a leading role. The general theoretical and metrological foundations of quantitative chemical analysis are rapidly developing, chemometric methods have begun to be used, computerization and automation of analysis continues, and attention to economic aspects is growing.
The analysis technique specifies the chosen method and regulates the sequence, methods and conditions for performing all operations when analyzing objects of a known type into specified components. The analyte must be previously detected and identified by qualitative chemical analysis methods. It is advisable to know in advance the approximate content of the analyte, as well as substances that may interfere with the analysis. The technique is characterized by the lower limit of detectable content (LLC), that is, the minimum content of the analyte at which the relative error of analysis with a probability of 0.95 remains below the specified limit. Typically, NGOS is an order of magnitude higher than the detection limit - the minimum analyte content required for its detection using a given method with a given reliability. There are also upper limits for the determined contents.
Most methods of quantitative chemical analysis include the following stages: sample collection, sample preparation (grinding, decomposition, dissolution, separation or masking of interfering substances, transformation of the analyte into a new form), measurement of the analytical signal, calculation of the analyte content. Some techniques (for example, those using chemical sensors or chemical analysis test methods) do not require sampling or sample preparation. To calculate the analyte content, the analytical signal (I) is measured - a physical quantity functionally related to the analyte content in the sample (in semi-quantitative methods the signal is assessed visually). The nature of the analytical signal is different: in gravimetry it is the mass of the reaction product, in titrimetry it is the volume of the titrant, in potentiometry it is the electrode potential, in atomic emission spectral analysis it is the radiation intensity at a certain wavelength. The measurement of the analytical signal is often combined with a chemical reaction (physicochemical methods of analysis) or with the separation of components (hybrid methods of analysis).
Calculation of the analyte content (c) usually requires knowledge of the calibration characteristic - a dependence of the form I = f(c). In relative methods of quantitative chemical analysis (most methods), this dependence is set using reference samples for which the analyte content is precisely known, and analytical signals are measured by the same means and under the same conditions as in subsequent analyzes. In absolute methods (for example, gravimetry, titrimetry, coulometry), reference samples are usually not used, and calibration characteristics are obtained based on general chemical information (reaction stoichiometry, law of equivalents, Faraday’s law, etc.).
The results of quantitative chemical analysis are subjected to mathematical processing, which includes the rejection of gross errors, assessment of the compatibility of the results of repeated analyzes, their averaging to reduce the influence of random errors, exclusion of systematic errors, calculation of a confidence interval in which with probability P (usually P = 0.95) should get the actual analyte content. When processing the results of quantitative chemical analysis, comparing them with each other or with technical standards, the statistical distribution of the results of repeated analyzes is taken into account.
When selecting and evaluating methods for quantitative chemical analysis, high accuracy (random and systematic errors should be as small as possible), high sensitivity (characterized by the slope of the calibration characteristic dl/de), absence or constancy of the background (signal arising in the absence of the analyte), high selectivity (the signal should not depend on the content of other components of the sample), rapidity (the duration of the analysis should be as short as possible). Other characteristics of the technique are also important (sample weight, cost and complexity of equipment, labor intensity of analysis, possibility of automation of analysis, continuous signal recording, simultaneous determination of a number of analytes). Continuous automated quantitative chemical analysis is important for effective process control, environmental monitoring, etc.
Lit.: Fundamentals of Analytical Chemistry: In 2 books. / Edited by Yu. A. Zolotov. M., 2004; Zolotov Yu. A., Vershinin V. I. History and methodology of analytical chemistry. 2nd ed. M., 2008.
Our laboratory offers a wide range of analyzes necessary when performing the following work:
Environmental monitoring
· Waste certification (development of a hazardous waste passport)
Determination of the component composition of industrial waste
· Calculation of waste hazard class
· Analysis of water, air, products and many others.
When developing a passport for hazardous waste, it is necessary to determine the composition of the waste. A mandatory document when approving a waste passport is a QCA (quantitative chemical analysis) protocol, which is done by our laboratory, which is accredited for this type of activity. The chemical analysis protocol is drawn up after the sample has been analyzed and contains information about the component composition of the waste.
The composition is indicated in mg/kg of dry matter and in % of the dry matter. The QCA protocol also contains information about regulatory documents on the measurement methodology. In addition, the protocol for quantitative chemical analysis of hazardous waste contains information about a legal entity or individual entrepreneur (name of organization and legal address), as well as information about the laboratory that analyzed the hazardous waste sample.
When preparing documents to obtain a license to carry out activities for the collection, use, neutralization, transportation, disposal of waste of I-IV hazard classes, KHA protocols for hazardous waste are also required. In this case, CCA protocols are used to indicate information about the component composition of waste of hazard classes I-IV declared in the license.
When conducting QCA, it is very important to take into account the assessment of quality indicators of quantitative chemical analysis (QCA) methods.
Protecting the environment from the increasing impact of chemicals is receiving increasing attention throughout the world. In our country, on the basis of the Law of the Russian Federation “On Ensuring the Uniformity of Measurements,” environmental protection falls within the scope of state metrological control and supervision.
The basis of all measures to prevent or reduce environmental pollution is the control of the content of harmful substances. Monitoring is necessary to obtain information about the level of pollution. The assessment of pollution of environmental objects is the maximum permissible concentration (MPC). Normalized maximum permissible concentrations should formulate requirements for the accuracy of pollution control and regulate the required level of metrological support for the state of the environment.
Quantitative chemical analysis (QCA) is an experimental determination of the mass or volume fraction of one or more components in a sample using physical, chemical and physicochemical methods.
CCA is the main tool for ensuring the reliability of the results obtained from the analysis of environmental objects.
The peculiarity of QCA is that the composition of multicomponent systems is measured. Measuring the composition is complicated by the effects of mutual influence of the components, which determines the complexity of the chemical analysis procedure. Characteristic of analysis as a measuring process is that the component being determined, distributed in the sample matrix, is chemically bonded to the components of the matrix.
The measurement result and the indicator of their accuracy can also be influenced by other physicochemical factors of the sample. This leads to the need:
firstly, normalization of influencing quantities for each technique,
secondly, the use of certified substances that are adequate to the analyzed samples (at the stage of monitoring the accuracy of measurement results).
The main goal of metrological support for measurements in environmental monitoring and control is to ensure the uniformity and required accuracy of the measurement results of pollution indicators.
In the multifaceted and complex work to ensure the uniformity of measurements in the country, the most important place is given to the development and certification of measurement techniques (MVI). This is quite clearly evidenced by the fact that the Law of the Russian Federation “On Ensuring the Uniformity of Measurements” includes a separate article 9, which states: “Measurements must be carried out in accordance with duly certified measurement techniques.”
In connection with the introduction of GOST R ISO 5725-2002, changes have been made to the state standard of the Russian Federation GOST R 8.563-96 "GSI. Measurement methods", which defines the procedure for the development and certification of measurement methods, including methods of quantitative chemical analysis (QCA). According to the requirements of this standard, organizations must have lists of documents for QCA methods used in the areas of state metrological control and supervision in a given organization, as well as plans for the cancellation and revision of documents for QCA methods that do not meet the requirements of the standard. In addition, these plans should provide for certification and, if necessary, standardization of CA techniques.
The six GOST R ISO 5725-2002 standards set out in detail and specifically (with examples) the basic provisions and definitions of accuracy indicators of measurement methods (MMI) and measurement results, methods for experimental evaluation of accuracy indicators and the use of accuracy values in practice. You should pay attention to the new terminology presented in the GOST R ISO 5725 standard.
In accordance with GOST R 5725-1-2002 - 5725-6-2002, three terms are used to describe the accuracy of chemical analysis: precision, correctness and accuracy.
Precision is the degree of closeness to each other of independent measurement results obtained under specific established conditions. This characteristic depends only on random factors and is not related to the true value or the accepted reference value.
Accuracy is the degree of closeness of the analysis result to the true or accepted reference value.
A reference value is a value that serves as a consistent value. The following can be taken as a reference value:
· theoretical or scientifically established value;
· certified CO value;
· certified mixture value (AC);
· mathematical expectation of the measured characteristic, i.e. the average value of a given set of analysis results.
The variability of the result of a chemical analysis can be influenced by various factors: time (time interval between measurements), calibration, operator, equipment, environmental parameters.
Depending on the influencing factors, the precision of the analysis results includes:
· precision of analysis under repeatability conditions - conditions under which the results of analysis are obtained using the same method in the same laboratory, by the same operator using the same equipment, almost simultaneously (parallel determinations);
· precision of analysis under conditions of reproducibility - conditions under which the results of analysis are obtained using the same method in different laboratories, varying by various factors (different time, operator, environmental conditions);
· intra-laboratory precision of analysis - conditions under which analysis results are obtained using the same method in the same laboratory with variations in various factors (time, operator, different batches of reagents, etc.).
A measure of precision is the standard deviation (RMS):
r - standard deviation of repeatability;
R - standard deviation of reproducibility;
Rl - standard deviation of intra-laboratory precision).
The standard deviation characterizes the spread of any result from a series of observations relative to the average analysis result () and is denoted by S.
Sample S is calculated using the formula:
where i is the result of i - definition;
- arithmetic mean of the results of parallel determinations;
N is the number of parallel definitions.
The assessment is made using the sample standard deviation S ~ S,
where is the general set of measurement results.
Qualitative characteristics of methods and analysis results are: accuracy, repeatability, intra-laboratory precision, reproducibility, correctness.
It is important for the laboratory to evaluate the quality of analytical results obtained using the technique over a long period of time. When accumulating statistical material based on the results of intra-laboratory control, it is possible, in accordance with GOST R ISO 5725-6, RMG 76-2004, to monitor the stability of the standard deviation (RMSD) of repeatability, the standard deviation (RMSD) of intermediate precision, the accuracy indicator using Shewhart cards. Stability control is carried out for each composition indicator analyzed in the laboratory in accordance with the methodology used. Moreover, control of the stability of accuracy is carried out only for those indicators for which there are control means that are sufficiently stable over time in the form of GSO, OSO, SOP, AS or calibration solutions.
In accordance with the selected algorithm for carrying out control procedures, the results of control measurements are obtained and control procedures are formed. It is permissible to construct control charts closer to the beginning, middle and end of the range of measured concentrations.
The stability of the standard deviation of repeatability, the standard deviation of intermediate precision, and the accuracy indicator is assessed by comparing the discrepancies obtained over a certain period of the results of the analysis of the controlled indicator in the sample with those calculated when constructing control charts with warning and action limits. The results of stability control using Shewhart control charts are given in GOST R ISO 5725-6.
The measurement technique is considered as a set of operations and rules, the implementation of which ensures obtaining measurement results with a known error. The guarantee of measurement error is the main, decisive feature of MVI. Previously, in accordance with the requirements of regulatory documents, each analysis result was assigned an error calculated during a metrological study of the method and assigned to the method during its certification. GOST R ISO 5725-2002 introduces an additional concept - laboratory error. Thus, the laboratory has the right to assess its error for each MVI, and it should not exceed the assigned one and, in accordance with RMG 76-2004, draw up a protocol of established indicators of the quality of analysis results when implementing the analysis technique in the laboratory.
In addition, previously, to assess the metrological characteristics of analytical measurements of the content of a component in the objects under study, it was enough to conduct an in-laboratory experiment. Modern regulations for the certification of methods of quantitative chemical analysis require an interlaboratory experiment with the participation of at least eight laboratories under identical measurement conditions (same methods, homogeneous materials). Only in metrological studies of methods that require unique equipment is statistical processing of the results of an in-laboratory experiment allowed.
The method must necessarily indicate the characteristics of the error and the values of the repeatability limits (if the method provides for parallel determinations) and reproducibility. In the most extreme case, at least one of the components of the error, or the total error, must be indicated. If this is not the case, then the methodology cannot be applied and references to it are not allowed.
But at the same time, in accordance with the requirements of RMG 61-2003, if it is impossible to organize an experiment in different laboratories, it is allowed to obtain experimental data in one laboratory under conditions of intra-laboratory precision, varying as many different factors as possible. In this case, the reproducibility indicator of the analysis technique in the form of standard deviation is calculated using the formula:
R = k·S Rл,
where SRl is the sample standard deviation of the analysis results obtained under conditions of intra-laboratory precision;
k is a coefficient that can take values from 1.2 to 2.0.
In accordance with GOST R 8.563-2009, methods that are intended for use in the dissemination of state metrological control and supervision must be certified and entered into the Federal Register. Institutions eligible for certification are:
All-Russian Research Institute of Metrology and Certification (VNIIMS),
Ural Research Institute of Metrology (UNIIM),
All-Russian Research Institute of Metrology (VNIIM) named after. Mendeleev (Center for Research and Control of Water Quality (CIKV, St. Petersburg),
Hydrochemical Institute of the Federal Service for Hydrometeorology and Environmental Monitoring, JSC "ROSA" (Moscow).
The All-Russian Scientific Research Institute of Metrology and Certification (VNIIMS) is responsible for the state registration of certified methods and for compliance with the copyright of the developing organization.
Methods not used in the areas of state metrological control and supervision are certified in the manner established at the enterprise. If the metrological service of an enterprise is accredited to carry out certification of methods, then it can carry out metrological examination of methods that are used in the field of dissemination of state metrological control and supervision.
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Certificates guaranteeing high quality of services
- Certificate of accreditation for environmental audit EAO No. N-12-094
- SRO Certificate No. 1806.00-2013-7719608182-P-177
- Certificate of application of the legal information system "EKOYURS" No. EYUS-10309/12
- Certificate of auditor Evgeniy Valerievich Tyutyunchenko No. N-10-03-12-1000
The regulatory document for the measurement method must regulate how many (one or several) single observations must be made, the methods of their averaging (arithmetic mean value of the results of multiple observations, median or standard deviation) and methods of presentation as a measurement result (or test result). It may be necessary to introduce standard corrections (for example, such as bringing the volume of gas to normal temperature and pressure). Thus, the result of measurements (tests) can be presented as a result calculated from several observed values. In the simplest case, the result of measurements (tests) is the actual observed value).
According to “PMG 96-2009 GSI. Results and quality characteristics of measurements. Forms of presentation”, the measurement result is presented as a named or unnamed number. Together with the measurement result, the characteristics of its error or their statistical estimates are presented. The presentation of measurement results obtained as the arithmetic mean of the results of multiple observations is accompanied by an indication of the number of observations and the time interval during which they were carried out.
Accuracy of chemical analysis results. Standards for monitoring the accuracy of the measurement result of the content of the controlled component in a sample of the analyzed substance, procedures and frequency of monitoring
According to GOST R ISO 5725-1-2002 Accuracy (correctness and precision) of measurement methods and results. Part 1. Basic provisions and definitions":
accuracy –With the degree of closeness of the measurement result to the accepted reference value.
accepted reference value - a value that serves as a matched value for comparison and is obtained as:
a) theoretical or established value based on scientific principles;
b) an assigned or certified value based on experimental work by any national or international organization;
c) an agreed or qualified value based on joint experimental work under the direction of a scientific or engineering team;
d) mathematical expectation of the measured characteristic, that is, the average value of a given set of measurement results - only when a), b) and c) are not available.
The term "accuracy", when referring to a series of measurement (test) results, includes a combination of random components and an overall systematic error.
right – the degree of closeness of the average value obtained on the basis of a large series of measurement results (or test results) to the accepted reference value. Notes: The indicator of correctness is usually the value of systematic error.
systematic error – the difference between the mathematical expectation of the measurement results and the true (or in its absence, the accepted reference) value. Notes: The true value of the quantity is unknown; it is used only in theoretical studies.
As components of the systematic measurement error, the non-excluded systematic error is distinguished, the component of the systematic measurement error due to the imperfection of the implementation of the accepted measurement principle, the calibration error of the measuring instrument used), etc.
precision – the degree of closeness to each other of independent measurement results obtained repeatedly under specific regulated conditions. Notes: Precision depends only on random errors and has no relation to the true or established value of the measured quantity. A measure of precision is usually expressed in terms of uncertainty and is calculated as the standard deviation of the measurement results. Less precision corresponds to a larger standard deviation. “Independent results of measurements (or tests)” are results obtained in a manner that is not influenced by any previous result obtained by testing the same or a similar object. The quantitative values of precision measures depend significantly on the regulated conditions. Extreme cases of sets of such conditions are repeatability conditions and reproducibility conditions.
repeatability (synonym convergence) – precision under repeatability conditions.
repeatability (convergence) conditions– conditions under which independent measurement (or test) results are obtained repeatedly by the same method on identical test objects, in the same laboratory, by the same operator, using the same equipment, within a short period of time .
reproducibility – precision under reproducibility conditions.
reproducibility conditions – conditions under which measurement (or test) results are obtained repeatedly the same method, on identical test objects, at different times, in different laboratories, by different operators, using different equipment, but brought to the same measurement conditions (temperature, pressure, humidity, etc.).
Standards for monitoring the accuracy of a measurement result are indicators of repeatability (convergence), reproducibility and correctness of the measurement result.
Chapter 4. Quantitative chemical analysis
Titrimetric analysis
Quantitative analysis of a substance is experimental determination (measurement) of the content of chemical elements, compounds or their forms in the analyzed substance, expressed in numerical form. The purpose of quantitative analysis is to determine the content (concentration) of components in a sample. It can be carried out using various methods: chemical, physico-chemical, physical, biological.
Chemical methods include gravimetric (weight) and titrimetric or volumetric types of analysis .
Gravimetric methods based on accurate mass measurement the component being determined, or a compound quantitatively related to it with a precisely known composition.
Under titrimetric analysis understand the determination of the content of a substance by an accurately measured amount of a reagent (mass or volume) that reacted with the component being determined in an equivalent amount.
Methods of quantitative chemical analysis do not require complex equipment and have good accuracy and reproducibility. Since the error of many titrimetric methods does not exceed ± 0.5 ¸ 0.1%, and gravimetric methods - no more than 0.1%, these methods are still used as metrological when conducting certification of analysis methods. However, they have a number of disadvantages. The most significant are insufficient selectivity and sensitivity, which requires careful preparation of the sample and the reagents used.
To carry out chemical analysis, reagents of the following qualifications are used: h.(clean), ch.d.a.– clean for analysis; reagent grade– chemically pure; o.s.h.- especially clean. Reagents of the brand have the lowest content of impurities o.s.h. And ch.d.a., whereas the reagents are qualified h.h..(pure) and below are not always suitable for quantitative determinations and require additional purification.
The quality of the results obtained is largely determined by the correct selection of dishes and equipment. To carry out quantitative analysis, a wide variety of laboratory glassware and scales are used. According to its purpose it is classified into:
Ø special purpose utensils – used to perform a narrow range of operations. This various types of pycnometers, hydrometers, refrigerators, round-bottom flasks, Kjeldahl flasks;
Ø general purpose cookware – most often used in a variety of types of work: boiling, titration, filtration, etc. This test tubes, funnels, beakers, flat-bottomed round and conical Erlenmeyer flasks, crystallizers, Petri dishes, bottles, desiccators(Fig. 4.1 and 4.2);
Figure 4.1 – General purpose laboratory glassware used in various analytical methods.
Figure 4.2 – general purpose utensils: a) glass bottles with lids for weighing and storing hygroscopic substances; b) various types of washer for rinsing dishes.
Ø measuring cups – used to measure liquid volumes. It is divided into dishes precise measurement : pipettes (Mohr and graduated), burettes, Mohr volumetric flasks (Fig. 4.3) and inaccurate measuring utensils: graduated cylinders, beakers, beakers, graduated flasks, graduated test tubes: cylindrical and conical or finger-shaped (Fig. 4.4).
Figure 4.3 - utensils for precise volume measurement used for
aliquot selection, standard solution preparation and titration.
Figure 4.4 - Utensils used for inaccurate volume measurement
for the preparation of solutions and reagents subject to standardization
in qualitative analysis.
To take aliquots in titrimetry, during quantitative precipitation from solutions, as well as when preparing standard solutions for various purposes, always use only precision measuring utensils and analytical balances! Dishes for imprecise volume measurement And technochemical scales used: for cooking standardized solutions, measuring the volumes of solutions used to maintain the acidity of the medium (buffers), performing precipitation and titration of aliquots. When working with measuring utensils, especially accurate , it is necessary to maintain its cleanliness. For this purpose, the dishes before use always rinse with distilled water and dry. Accurate The dishes are dried in air using ether or alcohol, and inaccurate And general purpose– on heated dryers or in a drying cabinet. To eliminate errors when selecting aliquots and working with burettes, they are additionally rinsed with the solution being measured.
A change in the temperature of the medium leads to a measurement error: an overestimation or underestimation of the determined volume, and therefore the calculated concentration. Therefore, all measuring utensils have a stamp indicating their volume at 20ºС, and dishes of precise measurement - additionally calibrated with distilled water using an analytical balance and correcting for the density of water at a given temperature. Sometimes there are additional markings indicating heat resistance and chemical resistance. The heat resistance of glass is indicated matte square or circle. In such dishes, liquids are heated and boiled on stoves and gas burners.
Scales. Devices used to determine the mass of bodies are called scales . In chemical analysis, two types of balances are used: technical and analytical. They can be either mechanical or electronic; have one cup (quadrant mechanical and electronic) or two (cup and damper scales). Under weighing understand comparison of the mass of a given object with the mass of calibrated weights (weights) or measurement of the pressure that the object exerts on the scale pan in terms of units of mass. Weights are necessary when working on damper or pan scales, and in quadrant and electronic single pan scales the scales are already graduated in mass units.
Scales vary in accuracy class and measurement limits. Technical scales – the least accurate and are used for weighing relatively large samples. For chemical purposes, quadrant or cup technical scales of 0.2 - 1 kg (sometimes up to 5 kg) are usually used. Their accuracy does not exceed 0.01 - 20 g. Technical scales with an accuracy of 0.1 - 0.01 g called technochemical and used in the laboratory to take samples from 1 to 500 g . In modern electronic technical scales the measurement accuracy can be even higher: with a maximum object weight of 500 g, it varies from 0.001 g to 0.2 g.
Analytical balances are used to accurately determine the mass of a sample when preparing standard solutions, carrying out gravimetric measurements, etc. The accuracy of damper scales is ± 2 × 10 - 4 - 2 × 10 - 5 g, and electronic ones - up to 2 × 10 - 6 g. On average, such scales are designed for a maximum mass of an object of 50 - 200 g, but scales with increased accuracy are also produced for a maximum mass of a sample of 1 - 20 g, which are used in some types of instrumental analysis, for example, spectral.
When working on scales, you must strictly follow the rules for handling them. Improper installation or careless handling can result in unreliable results and damage the scale. This is especially important to remember when using electronic and analytical damper balances.
Indicators and their selection
To detect the equivalence point in titrimetric analysis, use indicators(from lat. indicare show, reveal). Indicators are reagents that can contrastly change their color depending on changes in the properties of the medium. Most often these are organic substances with reversibly changing color(exception – precipitation indicators).
Not every substance that changes its color depending on the properties of the medium is suitable as a titration indicator. Moreover, the indicators change their color regardless of whether the equivalence point has been reached or not yet reached: The determining factor is only the environmental parameters. Therefore it is important choose the right indicator . TO necessary requirements When selecting an indicator, the following include:
Ø titration index pT (indicator color transition interval) must be located in the jump area and be as close as possible to the equivalence point, and the value of the indicator error cannot exceed 0.5%;
Ø indicator color- very intense and clearly visible in the solution even with strong dilution (for 1 - 2 drops of indicator);
Ø sensitivity of the indicator substance to changes in the properties of the environment– high, so that the color change occurs with a minimal excess of titrant in the solution (from 1 to 2 drops of titrant);
Ø transition interval- narrow and high-contrast;
Ø the indicator should be stable- do not decompose in air and in solution;
Ø indicator substance- indifferent to the titrated solution or titration products, i.e., reactions that affect the course of the titration curve should not occur between them.
Depending on their properties, indicators are classified by number transitions (single and multi-junction) and by area of application . TO unijunction refers to phenolphthalein (raspberry - colorless), and to multi-junction– methyl orange (yellow – orange and orange – pink). Examples of other multi-transition indicators are: a-Naphtholbenzein - two transitions: green - yellow (pH = 0 - 1) and yellow - blue (pH = 8.4 - 10); Methyl violet - three transitions (yellow - green, green - blue, blue - violet); Cresol red - two transitions (red - yellow and yellow - magenta). Multi-transition indicators also include universal indicators. Sometimes multi-transition indicators in titration are used as single-transition indicators if the color change of not all transitions occurs in a relatively narrow range of values or they are not clearly recorded.
By Areas of use The following groups of indicators are distinguished:
1. Acid - basic.
2. Redox indicators (redox indicators).
3. Metallochromic (complexing agents).
4. Precipitating.
5. Adsorption.
6. Specific.
7. Mixed.
8. Luminescent (fluorescent) and metal fluorescent.
9. Extraction.
10. Shielding.
This division is quite arbitrary, since during titration several parameters that correlate with each other often naturally change simultaneously. For example, pH and system potential E, pH and PR value (solubility product). There is also a more complete classification of indicators, taking into account both their chemical structure and the mechanism of color change, but such a classification is quite complex and will not be considered by us.
Chromophore theory (CT)
The change in the color of the indicator by CT is associated with reversible structural processes (isomerization) occurring due to intramolecular rearrangements of individual functional groups in the molecule. Each of the structural forms ( tautomers) is stable only in a certain range of pH values or other environmental parameters, therefore the addition or removal of a proton leads to a restructuring of the indicator molecule, as a result of which new functional groups responsible for color (chromophores) appear or disappear. These features explain why the color change of a number of indicators does not occur instantly, but is extended over time, since tautomeric transformations are intramolecular rearrangements, which, unlike ionic reactions (dissociation), occur more slowly.
Functional groups responsible for the color of the indicator substance, got the name chromomorphic(chromo – color). These include: nitro group (O = N –); azo group (– N = N –), several carbonyl groups located close to each other (>C=O).
Functional groups, enhancing or stabilizing color indicator are called auxochromic. Similar properties are possessed by: amino groups (–NH 2) and amine derivatives; oxygen- and nitrogen-containing compounds (–O–CH 3 ; –N(CH 3) 2 ; –N(C 2 H 5) 2), hydroxo groups (electron donor). The color of the indicator appears brighter if the substance contains, in addition to auxochromic groups, also antiauxochromic(electrophilic) groups that provide a shift in electron density in the molecule. For example, some oxygen-containing radicals (-NO 2, -NO, -COCH 3) have electrophilic properties. As an example, we give the structural formulas of tautomeric isomers of a one-transition indicator p-nitrophenol(Fig. 4.8)
 |
Figure 4.8 – Structure of tautomeric forms of the indicator substance
(p-nitrophenol), containing chromophore and auxochrome groups.
The chromophore theory also has a number of disadvantages, in particular:
Ø does not explain why color changes and tautomeric transformations depend on the pH value of the medium;
Ø how the color of most indicators having chromophore groups changes almost instantly, which contradicts the mechanism of intramolecular rearrangement;
Ø and finally, the chromophore theory cannot be described quantitatively.
Ion-chromophoric theory.
This theory combined the ideas of ionic (dissociative) and chromophore theories. According to ion-chromophoric theory, acid-base indicators are weak acids and bases, and neutral molecules and their ionized forms contain different chromophoric groups. In an aqueous solution, an indicator molecule is capable of either donating hydrogen ions (weak acid) or accepting them (weak base), while undergoing tautomeric transformations according to the scheme:
HInd Û H + + Ind - Û H + + Ind - B,
Where Hind- non-ionized indicator molecule (weak acid, tautomeric form I); Ind-B- anion of a strong acid having a tautomeric form II in a dissociated state (basic form II).
When the pH decreases (acidification of the solution), the equilibrium in the system shifts to the left towards the non-ionized form Hind. As soon as it begins to dominate, the solution takes on its color.
If the solution is alkalized (pH increases and the concentration of H + decreases), the equilibrium in the system shifts to the right and the dominant form becomes Ind-B, which gives the solution a different color, characteristic of the main form II. Thus, the acidic form of phenolphthalein (pH = 8.2) is colorless, and upon transition to an alkaline medium, an anion of the tautomeric basic form (pH = 10) is formed, colored red-crimson. Between these forms there is a range of pH values (from 8.2 to 10), corresponding to a gradual change in the color of the indicator.
The human eye is capable of perceiving the color of only one of the two forms in a mixture, provided that their color intensity is the same, if the concentration of one of these forms is approximately 10 times higher than the second.
Indicators.
1. Acid-basic indicators – These are weak organic acids or bases. The color of the indicators is reversible and is determined by the pH value of the medium. The transition interval is calculated using the dissociation constant:
pH ind. = – logK a ± 1, where K a is the dissociation constant of the indicator.
Let's look at an example. Indicator dissociation constant alizarin yellow K a = 10 -11. Let's determine the transition interval of the indicator DрН ind:
pH ind. = – log (10 -11)± 1 =11 ±1 Þ DрН ind [(11-1) ¸ (11+1)] = .
Indicator transition interval DрН ind = 10 ¸ 12.
2. Redox indicators– organic substances exhibiting the properties of weak oxidizing or reducing agents. They can be either reversible (diphenylamine) or irreversible, the color of which is destroyed (methyl red, methyl orange, they are also known as acid-base indicators). A change in the color of the indicator corresponds to a reversible reaction: Ind + + ne Û Ind; Where Ind +- oxidized (Ox), and Ind- restored (Red) form of the indicator, n- number of electrons in a given half-reaction . Change redox potential (indicator transition interval) calculated using the Nernst equation: DE = E 0 ± 0.059/n,
where E 0 is the standard redox potential for the indicator; n is the number of electrons in the half-reaction.
For example: Redox indicator diphenylamine has E 0 = + 0.76 V and n = 2. Let us determine the interval of its transition.
According to the formula: DE = 0.76 ± 0.059/2 = 0.76 ± 0.0295 Þ DE = (0,76 –0,0295) ¸ (0.76 + 0.295) = 0.73 ¸ 0.79 (V).
3. Metallochromic (metal indicators)- these are organic dyes (weak acids) that have their own chromophore groups and reversibly change their color upon the formation of a complex salt with metal cations. They are used mainly in complexometry, for example, eriochrome black T. For these indicators, the following condition must additionally be met: The stability of the complex of the titrated substance with the titrant is higher than that of the complexes it forms with the indicator in solution. Transition interval calculated by the formula:
DрМе = – logK set. ± 1, where Kst is the stability constant of the complex formed by this indicator with the titrated substance.
4. Precipitation indicators The group of indicators is insignificant in composition, since a colored precipitate should form in the solution immediately after almost complete precipitation of the substance being determined (residual concentration less than 10 –6 mol/dm 3), and there are few such substances.
The transition interval of the indicator is determined by the value of the solubility product (SP) of the precipitate formed by it:Dp(PR) = – logPR. ± 1.
Adsorption indicators- these are organic substances , exhibiting the properties of weak acids or bases, such as eosin or fluorescein.
The mechanism of action of the adsorption indicator is shown in the diagram (Fig. 4.9). As can be seen from Figure 4.9, the appearance of color occurs as a result changes in the composition of ions on the surface of the dispersed phase(precipitate or colloidal particle) due to processes of adsorption or desorption of indicator ions. This phenomenon is explained by the change in the sign of the electrostatic charge on the surface of the sediment particles during titration. The reason for this is that in an under-titrated solution, the surface of the precipitate predominantly sorbs the titratable ions that are part of its composition (the AgCl precipitate sorbs un-titrated Cl - ions) and acquires their charge. As a result, sorption of indicator ions becomes impossible.
Figure 4.9 – Schematic representation of the structure of the sorbed layer on the surface of the AgCl deposit formed during the titration of Cl ions with a solution of AgNO 3.
A - to the point of equivalence(Cl - ions are sorbed by the surface, and Ind - indicator ions remain in solution);
b – after the equivalence point(the surface sorbs Ag + titrant ions, which attract Ind - indicator ions).
As soon as the equivalence point is reached, the solution will appear excess of oppositely charged titrant ions, which will also begin to accumulate near the surface of the sediment, attracting indicator ions from the solution. The resulting substance colors the surface of the sediment.
5. Specific indicators – A relatively small group of indicators, since their use is based on specific reactions with the titrated substance. A starch solution has these properties in relation to J 2 molecules: the formation of a blue compound.
Titration methods.
Since not every substance can be analyzed directly by reaction with a titrant, especially if it is unstable in air, several methods have been developed to solve such problems. techniques (ways) carrying out analysis. They allow you to replace unstable, under these conditions connections, by an equivalent amount more stable, which does not undergo hydrolysis or oxidation. The following main ones are known methods of titrimetric analysis:
Ø direct titration;
Ø reversible;
Ø back titration or titration by residue;
Ø indirect titration or by substitution (by substituent).
Table 4.1 shows the applications of different methods depending on the type of titration.
Table 4.1 - Application of various types and methods of titration.
| method name | private method name; (working solution) | substances determined by titration | ||
| direct | reverse | indirect | ||
| Protolithometry | Acidimetry (acids: HCl) | grounds; salts formed by a strong base and a weak acid | salts of weak bases and strong acids; organic compounds | - |
| Alkalimetry (alkalis: NaOH) | acids; salts formed by a weak base and a strong acid | - | - | |
| Redoximetry | Permanganatometry () | reducing agents | oxidizing agents | substances that react with reducing agents |
| Iodometry (and) | reducing agents | reducing agents | oxidizing agents; acids | |
| Complexometry | Complexometry (EDTA) | cations that form complexes with EDTA | cations in water-insoluble compounds; cations for which there is no indicator | cations that form a more stable complex with EDTA than with |
| Sedimentation method | Argentometry () | Anions that form a precipitate | cations that form a slightly soluble precipitate with halogen ions: , , ; , | - |
Let us consider in more detail the essence of various titration methods.
1. Direct titration consists in the direct interaction of the titrant and the titrated substance. During the titration process, a titrant solution is gradually added to an aliquot or weighed portion of the substance, the volume of which is accurately recorded in T.E. A working solution of known concentration is used as a titrant. Calculation of the substance content in the sample is carried out according to the law of equivalents:
= (4.1)
where is the number of mole equivalents of the analyte in the titrated sample; A - the number of mole equivalents of the titrant that reacted with the component being determined A.
Component concentration A in solution is calculated using the formula:
(4.2)
where is the molar concentration of the equivalent (normality) of the titrated solution (component being determined), mol-equiv/l; – volume of an aliquot of the titrated solution, ml; – concentration and – titrant volume at the equivalence point. During titration method of individual samples formula (4.2) is transformed into expression (4.3):
(4.3)
The method is used in all cases where there are no restrictions. For example, when analyzing acids, determining water hardness.
2. Reverse titration – This is a type of direct titration, when the working and titrated solutions are swapped. In this case, we select for analysis aliquots of the working solution, and in T.E. measure spent titration volume of the analyzed solution. Calculations are carried out in the same way as in direct titration, using formulas (4.2) or (4.3). The method makes it possible to limit the surface area of a solution in contact with air while standardizing relatively unstable compounds, such as NaOH.
Titration by substituent (indirect) and titration by residue (reverse) based on usage auxiliary solution interacting with the component being determined. This technique allows you to analyze chemically unstable objects or in the absence of a suitable indicator.
In indirect titrationfirst carry out the reaction of the analyte A with auxiliary solution IN, and then titrated equivalent amount of reaction product formed WITH(deputy). This method can be represented as a diagram: A + B C + (t-t), based on which we write the expression for the law of equivalents:
= = . (4.4)
From equality (4.4) it follows that = and the calculation can also be performed using formulas (4.2) and (4.3), used for direct titration. To complete the reaction, the auxiliary solution is always taken in slight excess. This titration method is implemented in iodometry.
Back titration Also First, a reaction occurs between the substance being determined A and the auxiliary solution taken in excess IN, but then titrated the remainder of the unreacted auxiliary solution . Therefore it is necessary to know exactly concentration auxiliary solution IN and him volume, taken for analysis. Component Definition A performed according to the scheme: A + B B ost + (t-t). Based on the titration conditions, the law of equivalents can be written as:
– = . (4.5)
Where do we get:
= - . (4.6)
If all substances are taken in the form of solutions, then formula (4.6) will take the form
(4.7)
If at least one of the substances is taken in dry form (its mass is known), then you should use expression (4.6) and write down the value for each of the substances individually.
And ways to prepare them.
In titrimetry, solutions are used the concentration of which is determined by some method with a high degree of accuracy. Such solutions are called standard titrated or simply titrated . Solutions are classified by purpose and by the method of establishing their concentration.
By purpose they are conventionally divided into working solutions and solutions standards (primary and secondary).
Workers These are solutions that are used directly in analysis to determine the content of a substance. If the working solution is not standard, then it must be standardized immediately before performing the analysis, since the concentration could change significantly during storage. The exact concentration of the working solution is found by titration standard solution or setting substances (accurate weighing method). This applies, for example, to such working solutions as: NaOH, Na 2 S 2 O 3 × 5H 2 O.
Under standard solution understand a titrated solution that stably maintains its concentration during long-term storage. Main purpose of standard solutions - determination of the exact concentration of working and other solutions used in titration.
The process of establishing the exact concentration of a solution by titrating it against a standard is called standardization.
By method of determining concentration differentiate primary standards And standardized solutions .
Standardized solutions - These are solutions whose concentration is established according to a standard and cannot be accurately determined in advance. These include solutions of acids, alkalis, hydrolyzable and hygroscopic salts, as well as substances that can react with atmospheric oxygen and carbon dioxide. There are many known methods for preparing standardized solutions. Most often used for this purpose are: preparation using an approximate sample (alkali, salts), methods of diluting or mixing solutions (acids, salts), methods of ion exchange (salt solutions).
Standard solutions are classified by the method of determining their concentration . There are: primary standards or solutions with prepared titer And secondary standards - solutions with a set titer.
Primary Standards- these are solutions that are prepared either by precise weighing of the substance(Fig. 4.10), or by diluting specially prepared standardized reagents - fixans(Fig. 4.11). Fixanal is a sealed glass ampoule produced by industry and containing a strictly standardized amount of reagent, usually calculated for 1 liter of 0.1 N. solution.
Preparation of the solution by precise hitching begin by calculating its mass based on a given concentration (titer or normality) and volume of the flask. A sample of the standard substance is weighed on an analytical balance with an accuracy of 1×10 -4 g and transferred quantitatively into a volumetric flask, where it is dissolved with stirring (Fig. 4.10).
Figure 4.10 – Procedure for preparing the primary solution
standard for precise weighing: 1 – Mohr volumetric flask; 2 – funnel;
3 – bottle with a sample of substance; 4 – rinsing with distilled water;
5 – pipette or dropper.
a – transfer of a sample of the substance into a volumetric flask; b – rinsing the funnel;
c – bringing the volume of the standard solution to the mark.
This method is usually used to prepare solutions of salts such as borax (Na 2 B 4 O 7 × 10H 2 O), K 2 Cr 2 O 7. The amount of a substance in a solution is found or by value accurately taken sample weight(when transferring it, you must thoroughly rinse the bottle), or calculate difference method, defining the exact weight of the weighing bottle, first with a hitch, and then empty, after transferring the substance into the flask. If necessary, the concentration of the solution is recalculated taking into account the actual mass of the sample.
Procedure for preparing the solution by dilution method from fixanal shown in Figure 4.11. In order for the standard obtained by this method to be of high quality and meet all requirements, it is necessary to eliminate the loss of the substance when opening the ampoule and transferring it to the flask, and also to ensure that fragments of the ampoule do not fall into the solution. This largely depends on the correct handling of the ampoule.
Figure 4.11 – Method for preparing solutions of the primary standard
dilution method from fixanal: 1 – Mohr’s 1L volumetric flask;
2 – lower striker; 3 – funnel; 4 – fixanal ampoule; 5 – upper striker.
Before use, the ampoule should be rinsed with distilled water and only then opened with a special striker. Immediately after transferring the substance into the flask, you need to thoroughly rinse the ampoule with distilled water, no less than 6 times its volume. This method of preparing a primary standard is simpler than using precise samples, but is inferior in accuracy. It is used not only to obtain solutions of salts, but also of various acids.
Since for cooking primary standard solution only suitable precision measuring utensils And analytical balances, then and to substances used for this purpose are subject to a number of mandatory requirements. Only reagents that are characterized by:
Ø high purity(usually no worse than 99.99 – 99.999% - analytical grade and special grade qualifications);
Ø exact compliance with the formula composition and relatively high molecular weight;
Ø Stability during storage both in solid form and in solution(absence of hydration, hydrolysis, oxidation and carbonization processes);
Ø easy to prepare and good solubility;
Ø irreversibility of the reaction during standardization, selectivity;
Ø the possibility of accurately fixing T.E. by any method.
Secondary standard These standardized solutions are called, which are shelf stable and can be used to standardize other solutions.
Secondary standards are prepared as solutions approximate concentration by any known method, and before use - determine their exact concentration by standardizing against a primary standard. Therefore, when preparing secondary standards, high accuracy in measuring the mass of a substance or volume of a solution is not required, as is the case with primary standards. Quite suitable for this purpose technochemical scales And imprecise measuring utensils(cylinders, beakers, graduated test tubes).
An example of a solution with the properties secondary standard , is hydrochloric acid. Its diluted solutions can be stored for a long time, up to 1 month or more, without a noticeable change in concentration. Borax, used in protolitometry to standardize HCl, refers to primary standards and is prepared according to precise weighing. Whereas NaOH working solution– does not have the properties of the standard at all and its the concentration must be reset each time you use it.
And their application in analysis
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