Inorganic Chemicals for
IJohn R. Moody, Robert R. Greenberg, Kemeth W. Pial, and Theodore C. Rains Inorganic Analytical Research Division Center for Analytical Chemistry National Institute of Standards and Technology (NET) (Formerly National Bureau of Standards) Gaithersburg. MD 20899
All analytical techniques depend on the use of calibration chemicals to relate analyte concentration to an instrumental parameter. A fundamental component ip the preparation of calibration solutions is the weighing of a pure chemical or metal before preparing a solution standard. The analyst must be assured that the purity, stoichiometry, and assay of the chemical are known. These terms have different meanings, and each has an important influence. This REPORT is intended to assist the analyst in the selection and use of chemical standards for instrumental calibration. Purity, stoichiometry, and preparation of solutions for different purposes are discussed, and a critical evaluation of the best materials available for each element is presented for use in preparing solutions or calibration standards. Information on the
chemical form, source, purity, drying, and appropriate precautions is given in Table I. In some cases, multiple sources or chemical forms are available. Certain radioactive elements, the transuranic elements, and the noble gases are not considered. There is a subtle difference between the two twesof calibration solutionsthose foiassay standards and those for matrix matching-commonly used in the laboratory. Assay solutions can be less expensive to prepare than matrixmatched calibration solutions; the latter often require the highest available purity for the matrix component in order to avoid uncertainty in the analyte concentration. Because high-purity materials may be expensive, the analyst must balance the cost against attainable accuracy. Occasionally, there is no way to prepare a calibration solution with the required accuracy. By reviewing the principles discussed in this REPORT, analysts can better evaluate the accuracy of their calibration solutions. Interpretationof purity claims There are two ways to establish the elemental assay for a compound. The first way is to obtain the assay of each element directly by a suitable method. Usually these assays are not much
This article not subject to US.copyright Published 1988 American Chemical Society
more accurate than -0.1%. Accurate assays of elements in compounds are not easily obtained. The second method is to determine the assay of one or more elements and infer the concentration of the last element by subtraction from 100%.The uncertainty in the concentration of the last element is increased because of the uncertainty from each component element. Subtraction from 100% also does not account for species that are not determined (e.g., HzO, Con, and Sod. Therefore, the actual assay of an element in a high-purity compound can be lower than that calculated by subtract-
r----r ing all other constituents from 100%. For metals, the easiest route is to use a spectroscopic or other multielement technique to determine the impurities in the metal. The results are expressed as “total metallic impurities” or “total impurities” and usually are given as an upper limit (e.g.,
