To standardize an analytical method we use standards containing known amounts of analyte. The accuracy of a standardization, therefore, depends on the quality of the reagents and glassware used to prepare these standards. For example, in an acid–base titration the stoichiometry of the acid–base reaction defines the relationship between the moles of analyte and the moles of titrant. In turn, the moles of titrant is the product of the titrant’s concentration and the volume of titrant needed to reach the equivalence point. The accuracy of a titrimetric analysis, therefore, can be no better than the accuracy to which we know the titrant’s concentration.
See Chapter 9 for a thorough discussion of titrimetric methods of analysis.
5.1.1 Primary and Secondary Standards
We divide analytical standards into two categories: primary standards and secondary standards. A primary standard is a reagent for which we can dispense an accurately known amount of analyte. For example, a 0.1250-g sample of K2Cr2O7 contains 4.249 × 10–4 moles of K2Cr2O7. If we place this sample in a 250-mL volumetric flask and dilute to volume, the concentration of the resulting solution is 1.700 × 10–3 M. A primary standard must have a known stoichiometry, a known purity (or assay), and it must be stable during long-term storage. Because of the difficulty in establishing the degree of hydration, even after drying, a hydrated reagent usually is not a primary standard.
Reagents that do not meet these criteria are secondary standards. The concentration of a secondary standard must be determined relative to a primary standard. Lists of acceptable primary standards are available.2 Appendix 8 provides examples of some common primary standards.
The base NaOH is an example of a secondary standard. Commercially available NaOH contains impurities of NaCl, Na2CO3, and Na2SO4, and readily absorbs H2O from the atmosphere. To determine the concentration of NaOH in a solution, it is titrated against a primary standard weak acid, such as potassium hydrogen phthalate, KHC8H4O4.
5.1.2 Other Reagents
Preparing a standard often requires additional reagents that are not primary standards or secondary standards. Preparing a standard solution, for example, requires a suitable solvent, and additional reagents may be need to adjust the standard’s matrix. These solvents and reagents are potential sources of additional analyte, which, if not accounted for, produce a determinate error in the standardization. If available, reagent grade chemicals conforming to standards set by the American Chemical Society should be used.3 The label on the bottle of a reagent grade chemical (Figure 5.1) lists either the limits for specific impurities, or provides an assay for the impurities. We can improve the quality of a reagent grade chemical by purifying it, or by conducting a more accurate assay. As discussed later in the chapter, we can correct for contributions to Stotal from reagents used in an analysis by including an appropriate blank determination in the analytical procedure.
5.1.3 Preparing Standard Solutions
It is often necessary to prepare a series of standards, each with a different concentration of analyte. We can prepare these standards in two ways. If the range of concentrations is limited to one or two orders of magnitude, then each solution is best prepared by transferring a known mass or volume of the pure standard to a volumetric flask and diluting to volume.
When working with larger ranges of concentration, particularly those extending over more than three orders of magnitude, standards are best prepared by a serial dilution from a single stock solution. In a serial dilution we prepare the most concentrated standard and then dilute a portion of it to prepare the next most concentrated standard. Next, we dilute a portion of the second standard to prepare a third standard, continuing this process until all we have prepared all of our standards. Serial dilutions must be prepared with extra care because an error in preparing one standard is passed on to all succeeding standards.
Figure 5.1 Examples of typical packaging labels for reagent grade chemicals. Label (a) provides the manufacturer’s assay for the reagent, NaBr. Note that potassium is flagged with an asterisk (*) because its assay exceeds the limits established by the American Chemical Society (ACS). Label (b) does not provide an assay for impurities, but indicates that the reagent meets ACS specifications. An assay for the reagent, NaHCO3 is provided.