Reagents Used as Standards
The accuracy of a standardization depends on the quality of the reagents and glass- ware used to prepare standards. For example, in an acid–base titration, the amount of analyte is related to the absolute amount of titrant used in the analysis by the stoichiometry of the chemical reaction between the analyte and the titrant. The amount of titrant used is the product of the signal (which is the volume of titrant) and the titrant’s concentration. Thus, the accuracy of a titrimetric analysis can be no better than the accuracy to which the titrant’s concentration is known.
Reagents used as standards are divided into primary reagents and secondary reagents. A primary reagent can be used to prepare a standard con- taining an accurately known amount of analyte. For example, an accurately weighed sample of 0.1250 g K2Cr2O7 contains exactly 4.249 x 10–4 mol of K2Cr2O7. If this 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 x 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- ondary reagents. The purity of a secondary reagent in solid form or the concentra- tion of a standard prepared from a secondary reagent must be determined relative to a primary reagent. Lists of acceptable primary reagents are available.4 Appendix 2 contains a selected listing of primary standards.
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 standards set by the American Chemical Society should be used. The packaging label included with a reagent grade chemical (Figure 5.1) lists either the maximum 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, contributions to Smeas from impurities in the sample ma- trix can be compensated for by including an appropriate blank determination in the analytical procedure.
Solutions of primary standards generally are pre- pared in class A volumetric glassware to minimize determinate errors. Even so, the relative error in preparing a primary standard is typically ±0.1%. The relative error can be improved if the glassware is first calibrated as described in Example 5.1. It also is possible to prepare standards gravimetrically by taking a known mass of stan- dard, dissolving it in a solvent, and weighing the resulting solution. Relative errors of ±0.01% can typically be achieved in this fashion.
It is often necessary to prepare a series of standard solutions, each with a differ- ent concentration of analyte. Such solutions may be prepared in two ways. If the range of concentrations is limited to only one or two orders of magnitude, the solu- tions are best prepared by transferring a known mass or volume of the pure stan- dard to a volumetric flask and diluting to volume. When working with larger con- centration ranges, 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 a volume of a concentrated stock solution, which is the first standard, is diluted to prepare a second standard. A portion of the second standard is then diluted to prepare a third standard, and the process is repeated until all nec- essary standards have been prepared. Serial dilutions must be prepared with extra care because a determinate error in the preparation of any single standard is passed on to all succeeding standards.