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Chapter 5 Calibrations, Standardizations, and Blank Corrections 107

same sample is placed in a 250-mL volumetric flask and diluted to volume, the con-
centration of the resulting solution is exactly 1.700 ´10 –3 M. A primary reagent
must have a known stoichiometry, a known purity (or assay), and be stable during
long-term storage both in solid and solution form. Because of the difficulty in es-
tablishing the degree of hydration, even after drying, hydrated materials usually are
not considered primary reagents. Reagents not meeting these criteria are called sec- secondary reagent
ondary reagents. The purity of a secondary reagent in solid form or the concentra- A reagent whose purity must be
tion of a standard prepared from a secondary reagent must be determined relative established relative to a primary reagent.
4
to a primary reagent. Lists of acceptable primary reagents are available. Appendix 2
contains a selected listing of primary standards.
Other Reagents Preparing a standard often requires additional substances that are
not primary or secondary reagents. When a standard is prepared in solution, for ex-
ample, a suitable solvent and solution matrix must be used. Each of these solvents
and reagents is a potential source of additional analyte that, if unaccounted for,
leads to a determinate error. If available, reagent grade chemicals conforming to reagent grade
5
standards set by the American Chemical Society should be used. The packaging Reagents conforming to standards set by
label included with a reagent grade chemical (Figure 5.1) lists either the maximum the American Chemical Society.
allowed limit for specific impurities or provides the actual assayed values for the im-
purities as reported by the manufacturer. The purity of a reagent grade chemical
can be improved by purification or by conducting a more accurate assay. As dis-
cussed later in the chapter, contributions to S meas from impurities in the sample ma-
trix can be compensated for by including an appropriate blank determination in the
analytical procedure.

Figure 5.1
Examples of typical packaging labels from
reagent grade chemicals. Label (a) provides
the actual lot assay for the reagent as
determined by the manufacturer. Note that
potassium has been flagged with an asterisk
(*) because its assay exceeds the maximum
limit established by the American Chemical
Society (ACS). Label (b) does not provide
assayed values, but indicates that the
reagent meets the specifications of the ACS
for the listed impurities. An assay for the
reagent also is provided.
(a) (b) © David Harvey/Marilyn Culler, photographer.