Interpretation of Specifications - American Chemical Society

Definition pubs.acs.org/doi/book/10.1021/acsreagents

Interpretation of Specifications Part 1, Introduction and Definitions eISBN: 9780841230460 Tom Tyner Chair, ACS Committee on Analytical Reagents James Francis Secretary, ACS Committee on Analytical Reagents

Downloaded by 80.82.77.83 on June 15, 2017 | http://pubs.acs.org Publication Date (Web): February 28, 2017 | doi: 10.1021/acsreagents.1003

ABSTRACT The specifications of reagent chemicals can be divided into two main classes: an assay or quantitative determination of the principal or active constituent and the determination of the impurities or minor constituents. The specifications of standard-grade reference materials are divided into identity and assay sections. In some cases, physical properties are specified.

Note: A sample reagent monograph appears in Figure 1-1 with notation describing the features used throughout this book.

© 2017 American Chemical Society

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ACS Reagent Chemicals ACS Reagent Chemicals; American Chemical Society: Washington, DC, 2017.

DOI:10.1021/acsreagents.1003 ACS Reagent Chemicals, Part 1

ACS Reagent Chemicals

Definition


pubs.acs.org/doi/book/10.1021/acsreagents

Figure 1-1. Features of a typical reagent monograph

PHYSICAL PROPERTIES OF REAGENTS In addition to the specifications and tests for each reagent, ACS Reagent Chemicals contains a section for each reagent monograph called “General Description.” This section provides physical properties for each reagent, including typical appearance, representative analytical use, change in state, aqueous solubility, density, and pKa. Note: The information under “General Description” is not to be used as a measurable reagent specification under any circumstances whatsoever.

Information under “General Description” is provided for the analyst’s ease of reference. The following points should be kept in mind. For the sources of information, see the bibliography on Physical Properties, beginning in [Part 6: List of Bibliographies; Physical Properties].


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• Melting points and boiling points have been approximated, even though more exact data may be available. • Densities for the reagent chemicals have been approximated because exact numbers require exact conditions, and the reagents described may not be in the exact form necessary to duplicate literature densities. Small variations in moisture or physical properties have a great effect on the density of both solid and fluid materials. • Where possible, the solubility of reagents has been approximated based on literature that is available. Some solubility will be described according to a general formula given in Table 1-1.


Table 1-1. Classification of Terms Describing Solubility

Descriptive Term

Parts of Solvent Required for 1 Part of Solute

Very soluble

Less than 1

Freely soluble

From 1 to 10

Soluble

From 10 to 30

Sparingly soluble

From 30 to 100

Slightly soluble

From 100 to 1000

Very slightly soluble

From 1000 to 10,000

Practically insoluble or insoluble

More than 10,000

Source: Reprinted with permission from National Research Council, 1981. Copyright 1981 National Academies Press.

CALCULATION OF RESULTS Because different techniques can be used to obtain results for assays and some secondary tests, the calculation of these results traditionally has been left to the analyst to derive. In order to clarify these calculations, to demonstrate the chemistry employed, and to eliminate ambiguity, the Committee has ensured that all calculations for assay determinations are presented in the Eleventh Edition. These calculations follow the general forms, with necessary modifications, that are described in standard works on quantitative analysis (see the bibliography on Analytical Chemistry, [Part 6: List of Bibliographies; Analytical Chemistry]). As much as possible, these calculations will not contain condensed factors or constants that may be confusing to some. The Committee also has included throughout the Eleventh Edition the historic titer method of giving the milliequivalent weight in the statement “one milliliter of 1 N [titrant] corresponds to 0.xxxx g of [sample].” This method allows analysts who already have programmed the calculation to continue doing so and provides a means for new users to do the same.

ASSAY SPECIFICATIONS Assay specifications are included for most of the reagent chemicals and all of the standard-grade reference materials in this book. An assay value, in the sense used herein, is the content or concentration of a stated major component in the reagent. Unless otherwise specified, assay specifications are on an as-is basis (i.e., without drying, ignition, or other pretreatment of the sample). Unless described in great detail and carried out with exceptional skill, available assay methods seldom are accurate enough to permit using a weighed quantity of a reagent, which was assayed in an exacting stoichiometric operation. This use of reagent chemicals should be limited to those designated as standards (for example, acidimetric or reductometric standards) because exacting assay methods are provided for such reagents. Except in the case of standards, assays, through their minimum and maximum limits, mainly serve to ensure acceptable consistency of the strength of reagents offered in the marketplace. They are particularly useful, for example, in the requirements for acid-water systems to control strength, for alkalis to limit the content of water and carbonate, for oxidizing and reducing substances that may change strength during storage, and for hydrates to control, within reasonable limits, deviations in the amount of water from that indicated in the formula. If, however, it should be necessary to use such reagents


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in stoichiometric operations, the user should ascertain that the assay values produced are of sufficient accuracy and precision for such use. Assayed values for standard-grade reference materials are sufficient for the intended use described in this book. Assay specifications for all standard-grade reference materials include a minimum of two assay methods. Additional assay methods (such as thin-layer chromatography) may include a specification of “Passes test”. These secondary methods are intended to support the primary numerical assay methods.


IMPURITY SPECIFICATIONS Specifications for impurities are expressed in the following ways: (1) as numerical limits, (2) in terms of the expression “Passes test” with an accompanying approximate numerical limit, or (3) in terms of the expression “Passes test” without an approximate numerical limit. The distinction among these forms of expression is based on the Committee’s opinion as to the relative quantitative significance of the prescribed test methods. The methods given for determining conformity to specifications of the first type are considered to yield, in competent hands, what are usually thought of as “quantitative” results, whereas those of the second type can be expected to yield only approximate values. Those in the third category give definitions that cannot be expressed in numbers. It is obvious, however, that these distinctions as to quantitative significance cannot be sharp and that even the numerically expressed specifications are not all defined with equal accuracy. The final and essential definition of any specification, therefore, must reside in the described test method rather than in its numerical expression. If a test method yields results that are adequately reproducible on repeated trials in different laboratories, it offers a satisfactory definition of the content of an impurity whether or not the result can be expressed by a number. Although the Committee has endeavored to base specifications, so far as possible, on validated test methods that meet this criterion, a considerable number are based on essentially undefined statements such as “no turbidity”, “no color”, or “the color shall not be completely discharged in x minutes”. Although some of the specifications of this kind could be replaced by others based on quantitative comparisons or measurements, to do so would require more costly or time-consuming procedures than appear at this time to be justified. The approach to an ultimate goal of replacing in every instance the word “none” or its equivalent by the expression “maximum allowable…”, therefore, is limited both by deficiencies of knowledge and by practical considerations of expediency.

UNLISTED IMPURITIES The primary objective of the Committee in preparing reagent or standard specifications is to assure the user of the strength, quality, and purity of the material. It is, however, manifestly impossible to include in each specification a test for every impurity and contaminant that may be present. The Committee recognizes that for certain uses more stringent or additional requirements may be appropriate, and for such uses, additional testing beyond that described in the specifications should be employed by the user. The Committee’s intent in establishing the specifications is to recognize the common uses for which the reagent or standard is employed and to establish requirements that are consistent both with these uses and with the manufacturing processes and quality of the available reagents. The presence of moisture, either as water of crystallization or as an impurity, falls within the purview of contamination unless permitted by an applicable specification. Although tests for foreign particulate matter are not usually included in the specifications for solid reagents, such matter constitutes contamination. Similarly, although tests for clarity are not usually included in the specifications for liquid reagents or for solutions of solid reagents, the presence of haze, turbidity, or foreign particulate matter also constitutes contamination. In some instances, residual amounts of substances that have been added as aids in the process of purification may be present. An example is the use of complexing agents to keep certain metal ions in solution during recrystallization. These substances not only are impurities but may interfere with the tests. Certain reagents, such as desiccants and indicators, have requirements that ensure suitability for their intended use. Such reagents may contain impurities that do not interfere with their intended use but that may make these reagents unsuitable for other uses. When the Committee becomes aware of an unlisted impurity that affects adversely the known or specified uses of reagents or standards, a new specification is added, provided a suitable test method is available. Users of reagents can protect themselves against the effects of unlisted impurities on a specific analytical procedure by applying, ad hoc, an appropriate suitability test.


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Definition


IDENTITY SPECIFICATIONS Identity specifications and tests are not included in the monographs for reagent chemicals. If there is any question as to the identity of a chemical, identity can be ascertained by appropriate analytical methods. Identity specifications are included for standard-grade reference materials. For each material, a minimum of one identity method is required. Specifications are designed to unequivocally prove identity. Many of the methods rely on comparisons of data to standardized special libraries, such as the National Institute of Standards and Technology (NIST) infrared and mass spectral libraries. In most instances, a combination of complementary techniques is used.

ACCURACY OF MEASUREMENTS


In specifying the weight or volume of sample to be used in the individual test procedures, it is intended, unless otherwise specified in the individual procedure, that the accuracy of measurement be such that the amount of sample used is within 2.0% of the stated amount. Thus, where a 10 g (or mL) sample is specified, the amount actually taken for analysis must be between 9.8 and 10.2 g (or mL). Similarly, where a test procedure directs that a solution be diluted to a specific volume or that a specified volume of solution be used, it is intended that the volume actually be within 2.0% of the stated amount. When the term “weigh accurately” is used, this means weigh to 0.1 mg. Where the term “pipette” or “burette” is used as a verb, it is intended that the specified volume be taken in a volumetric pipette or burette conforming to the tolerances accepted by NIST, Class A (NIST, 1974). Commercially available, certified, mechanical pipettes may be used with proper calibration and maintenance. The addition of small volumes of liquid reagents is generally stated to the nearest 0.05 mL (0.05 mL, 0.10 mL, 0.15 mL, etc.). Use of the term “drop” to represent 0.05 mL is avoided.

ROUNDING PROCEDURES For comparison of analytical results with specifications for assays and impurities, the observed or calculated values are rounded to the number of digits carried in the requirement. The method and the rounding procedure are in accord with those in the United States Pharmacopeia. When rounding is required, consider only one digit in the place to the right of the last digit in the limit expression. If this digit is smaller than 5, it is eliminated, and the preceding digit is unchanged. If this digit is greater than 5, it is eliminated, and the preceding digit is increased by one. If this digit equals 5, the 5 is eliminated and the preceding digit is increased by one. This rounding procedure is illustrated in Table 1-2. The foregoing procedure conforms to the common electronic calculator and computer procedure of rounding up when the digit to be dropped is 5 or 5 followed by zeros. The rounded value is obtained in a single step by direct rounding of the most precise value available and not in two or more steps of successive rounding. For example, 97.5487 rounds to 97.5 against a requirement of 97.6 and not in two possible steps of 97.55 and then 97.6. Table 1-2. Rounding Procedures

Requirement

Observed Value

Rounded Value

Pass/Fail

Not less than 98%

97.6

98

pass

Not less than 98.0%

Not more than 0.01%

Not more than 0.02%

97.5

98

pass

97.4

97

fail

97.95

98.0

pass

97.94

97.9

fail

0.014

0.01

pass

0.015

0.02

fail

0.016

0.02

fail

0.015

0.02

pass

0.025

0.03

fail

0.026

0.03

fail


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Definition


Throughout the monographs, the formula weights are rounded to two decimal places.

NEW ATOMIC WEIGHTS The formula weights and factors for computing results are based on the 2011 Atomic Weights published by the IUPAC and are shown in Part 6 of this book. There are several changes from the weights published in 2001 that were used in the last edition. There are a few elements for which the atomic weight has changed slightly, possibly enough to change formula weights and calculations in ACS Reagent Chemicals. There are also 12 elements for which a range is now given rather than a single value. The rationale for this is not a new idea but has been known and debated within the IUPAC for decades. These 12 elements have significant isotopic variations in nature, thus precluding an exact atomic weight. The range gives the reader an idea of the known variations in nature. F o r c a l c u l a t i o n s , h o w e v e r, a s p e c i fi ficc s i n g l e v a l u e w i t h n o u n c e r t a i n t y h a s b e e n a s s i g n e d t o t h e s e e l e m e n t s b e c a u s e a r t i c l e s o f c o m m e r c e n e e d a s p e c i fi fie ed n u m b e r t o u s e . For very exacting calculations, it may be necessary to determine the exact atomic weight of the elements in the particular sample at hand.


The elements with defined-for-commerce values are given below and in [Part 6: Atomic Weights of the Elements 2011]. Boron Bromine Carbon Chlorine Hydrogen Lithium Magnesium Nitrogen Oxygen Silicon Sulfur Thallium

B Br C Cl H Li Mg N O Si S Tl

10.81 79.904 12.011 35.45 1.008 6.94 24.305 14.007 15.999 28.085 32.06 204.38


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Interpretation of Specifications - American Chemical Society

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