Primary standards for

Should there be more than one known material, a weighted average of the individual differences (x) should be taken. The value of 5- should be based on the combined estimate from the two or more materials (perhaps different primary standards for bases). Should the materials differ markedly in composition, a plot of the individual constant errors against composition should be made. If the constant error appear to depend upon the composition, they should not be pooled in a weighted average. 

Table 11.27 Primary Standards for Aqueous Acid-Base Titrations Table 11.28 Titrimetric (Volumetric) Factors...

Selected Primary Standards for the Standardization of Strong Acid and Strong Base Titrants... 

National Institute of Standards and Technology (NIST). The NIST is the source of many of the standards used in chemical and physical analyses in the United States and throughout the world. The standards prepared and distributed by the NIST are used to caUbrate measurement systems and to provide a central basis for uniformity and accuracy of measurement. At present, over 1200 Standard Reference Materials (SRMs) are available and are described by the NIST (15). Included are many steels, nonferrous alloys, high purity metals, primary standards for use in volumetric analysis, microchemical standards, clinical laboratory standards, biological material certified for trace elements, environmental standards, trace element standards, ion-activity standards (for pH and ion-selective electrodes), freezing and melting point standards, colorimetry standards, optical standards, radioactivity standards, particle-size standards, and density standards. Certificates are issued with the standard reference materials showing values for the parameters that have been determined. 

FeSO (NH 2S04 6H20, is used as a primary standard for iron. It is soluble in water but insoluble in alcohols. Both the soHd and its solution are more stable to oxidation than iron(II) sulfate. 

Potassium hydrogen phthalate has many uses in analytical chemistry. It is a primary standard for standardization of bases in aqueous solutions. Its equivalent weight is 204.2. It also is a primary standard for acids in anhydrous acetic acid. Other applications are as a buffer in pH determinations and as a reference standard for chemical oxygen demand (COD). The theoretical COD of a Img/L potassium hydrogen phthalate is 1.176mg O2. 

Constant-boiling aqueous HCI can be used as a primary standard for acid-base titrations. When 20 wt% HQ (FM 36.461) is distilled, the composition of the distillate varies in a regular manner with the barometric pressure ... 

Potassium bromate, KBr03, is a primary standard for the generation of Br2 in acidic solution ... 

The table in this appendix recommends primary standards for many elements. An elemental assay standard must contain a known amount of the desired element. A matrix matching standard must contain extremely low concentrations of undesired impurities, such as the analyte. If you want to prepare 10 ppm Fe in 10% aqueous NaCl, the NaCl must not contain significant Fe impurity, because the impurity would then have a higher concentration than the deliberately added Fe. 

Action levels for decisions related to drinking water quality are the Maximum Contaminants Levels of the Safe Drinking Water Act (SDWA). The MCLs are the maximum permissible contaminant concentrations in the drinking water that is delivered to the user through a public water system. First enacted in the USA in 1974 and reauthorized in 1996, the SDWA protects drinking water and groundwater resources. This law establishes two kinds of standards for drinking water quality primary standards for the contaminants that pose a risk to human health (EPA, 1985), and secondary standards for the contaminants that affect the physical characteristics of water (odor, taste, and appearance). 

The network at the top of the traceability system providing the primary standards for chemical measurements consists at present of four institutes as shown in Fig. 2. 

In this scheme, the primary reference material is defined as a chemical substance of the highest (and known) purity, or a well-characterized substance in a matrix, This classification of materials is, however, fairly arbitrary. It is ideal when used in connection with standards characterized in terms of biological activity. Primary standards are thus the International Reference Preparations (IRP) produced by the World Health Organization (WHO). In this case the primary standard for a particular antibiotic is the WHO reference preparation which constitutes the unit of that antibiotic. When people wish to use it they have to prepare a large batch of samples calibrated to the primary. This is then called a secondary standard. However, for well-defined chemical parameters, the term certified reference material is preferred. 

Liquid HF is miscible with H20 in all proportions. The other hydrogen halides are very soluble and form constant boiling mixtures with water. Therefore, constant boiling HC1 has been used as a primary standard for analytical purposes. At 760 torr, the compositions and boiling points of the constant boiling mixtures are as follows ... 

P. R. Mussini,T. Mussini, and S. Rondinini, Reference value standards and primary standards for pH measurements in D2O and aqueous-organic solvent mixtures New accessions and assessments. Pure Appl. Chem. 69 (1997), 1007-1014. 

Measurement of the electrocapillary curve consists of changing the potential stepwise and determining the pressure required to return the mercury meniscus to the same location in the fine capillary. A plot of this pressure as a function of potential is nothing but the electro-capillary curve, within a constant, as seen from Eq. 53H. The best way to determine the magnitude of this constant is by calibration with a known system. This requires, of course, one accurate determination of y by an independent method. Very careful experiments were performed by Gouy around the turn of the century, and these results are used even now as the primary standard for electrocapillary measurements. 

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