JAMES C. RAMSEY, 11.' University of Florida, Gainesville
MOST students,
graduate as well as undergraduate, are generally unfamiliar with the various grade labels of chemicals, and the significance of the various grades. Terms such as Reagent Grade, Chemically Pure, Technical Grade, etc., are loosely used. That this uncertainty should exist is not surprising, for this topir is not covered a t any point in the chemistry or chemical engineering curriculum. In this article an attempt has been made to present a summary of the preseutday grades of chemicals, in a form suitable for easy comprehension by students. The highest grade of chemicals is considered to be that of Primary Standard. Interestingly enough, there are no specifications for this purity. The name is generally considered to indicate a chemical of such purity that i t can be weighed out directly, in making solutions of known strengths or in standardizing solutions of unknown strengths. Primary standard grade is commonly considered to mean purity of 99.95% or greater. Purities are usually given to four significant figures, rather than the usual three for reagent grades. The best known primary standards are those offered 1
Department of Chemical Engineering.
by the National Bureau of Standards: acid potassium phthalate and benzoic acid for acidimetric values, sodium oxalate, arsenic trioxide, and potassium dichromate for oxidimetric values, benzoic acid for calorimetric value, sucrose for saccharimetric value, and dextrose for reducing value. I t ill be noted that these are not listed or labeled as primary standards, but as standard samples. Each such sample is furnished with a certificate of analysis. Various commercial companies also offer primary standards, consisting of specially selected lots of chemicals, which are purer or more carefully analyzed than their usual reagent grades. Reagent Grade chemicals are next to primary standards in purity. They are not always labeled vith the words "Reagent Grade" but may bear easily recognized synonyms: Mallinckrodt "Analytical Reagent," Fisher "Certified" Reagent, J. T. Baker "Baker Analyzed" Reagent, Merck "Reagent," E & A "Tested Purity," etc. These names are usually registered trademarks of the companies concerned. Reagent grade chemicals normally have lists of limits of impurities on the labels, by which they may be recognized in the absence of other criteria.
JOURNAL OF CHEMICAL EDUCATION
A committee of the American Chemical Society has established standards for many reagent chemicals, which it has published in the book entitled "Reagent Chemicals, American Chemical Society Specifications." The latest edition appeared in 1955 and contains methods of testing and limits of impurities for almost 200 chemicals. A specification is based in general on "the highest level of purity (of the reagent chemical to which it applies) that is competitively available in the United States." Chemicals which meet these specifications may bear on the label, in addition to the usual legend, the words: "Meets A.C.S. Specifications." This system does not necessarily result in exceptionally pure chemicals, but in those of known limits of impurities; for example, potassium hydroxide, A.C.S. grade, must contain not less than 85% potassium hydroxide and not more than 2y0 potassium carbonate, most of the rest of the impurity being water. For reagent grade chemicals not listed in the American Chemical Society book, there are no official American standards. The conscience of the manufacturer is the only limit on impurities. It is to the credit of the companies concerned that they have been so conscientious in the discharge of this responsibility. Companies manufacturing such reagents are governed by a complicated balance between manufacturing costs and desire for sales. A higher percentage of purity increases costs but also increases sales and builds up the reputation of the company. Limits of impurities are usually, but not always, shown on the labels. Tests are believed to be generally of the same type as those of the A.C.S., and limits of impurities are of the same order of magnitude. Some companies actually test for the amounts of impurities present, rather than for the limits present, and so indicate on the labels. This does not necessarily mean that the chemicals from these companies are purer than others, but that the testing results are made quantitative rather than limiting, probably by using standards of varying strengths in the tests. U.S.P. Chemicals are those which meet or exceed the standards of the United States Pharmac~peia.~ Similar to these are the N.F. chemicals, which meet or exceed the standards of the National F ~ r m u l a r y . ~ These two grades may or may not be as pure as reagent grades of the same chemicals. More attention is paid to impurities which would make the substance unfit for medicinal use, and less attention to other impurities. U.S.P. or N.F. chemicals are, however, often used as reagents, especially in solutions which are to be standardized after making, such as sodium thiosulfate. The U.S.P. and N.F. committees work closely together, the U.S.P. in each edition including the medicinals most favored from the standpoint of good medicinal practice, and the N.F. including those for which standards are needed because of continuing extensive use. A chemical moving from one of these books to the other usually does so as a complete monograph, with assays, - . tests, and limits of im~urities unchanged. The tests on the A.C.S., U.S.P.: and N.F. grades are not as specific as would he expected for high% "Pharmsco~eiaof the United States," 15th revision. United States Pharma&peial Convention, 1955.' a "National Formulary," 10th ed., American Pharmxeer~ticill As~aristion,1955.
VOLUME 35, NO. 10, NOVEMBER, 1958
purity chemicals. Usually the assay, if one is specified, is made by testing for only one component. For example, the assay for potassium chloride U.S.P. consists of a Volhard determination of the chloride content, while the A.C.S. specifications for the same chemical do not include an assay. Tests for impurities are almost without except,ion limit tests; these do not show the actual amount of any impurity present. The sample is compared, usually calorimetrically, with a standard containing the limit of the impurity in question; if the sample shows less of the impurity than the standard, the sample passes the test. Impurities are usually present in quantities too small to measure accurately by separating and weighing. The methods are standardized tests, which actually compare the sample with previously tested samples which have been found satisfactory for the intended use. Therefore the directions must he followed exactly and in detail. Setting the limits of impurities is a delicate task. Too much impurity may make a substance useless for the intended purposes, while too stringent specifications may make the chemical uneconomical or too troublesome to manufacture, thereby removing it from the market. Supplements to the United States Pharmacopeia and National Formulary are issued from time to time to change any specifications found to be unrealistic. There are also many unofficial grades, for which no standards exist. Purified, Highest Purity, Recrystallized, etc., have no meaning per se, hut usually mean chemicals of greater than usual commercial purity. Actual meanings vary from company to company. "Typical" or "Guaranteed" analyses for these may he obtained from manufacturers as needed, but no one analysis will necessarily apply to the same chemical, under the same grade name, from another manufacturer. The term "Chemically Pure," for years denoting a very high purity, is now considered to be largely meaningless, and is falling into disuse. Technical Grade chemicals are the usual commercial chemicals. They may have almost any composition, depending on company policy, sources of raw material, and ease of purification. For example, recent tests of samples of potassium chloride, technical grade, from several sources, showed all to be equivalent to A.C.S. reagent grade. However, future lots from the same companies might not be as pure as these. Other compounds, such as sodium hydroxide, might have a much lower per cent of the labeled material than is characteristic of the A.C.S. reagent grade. Special grades are set up as needed or desired by dealers or customers. These may he for use as phosphors, spectrographic solvents, transistors, or other uses where high general purity or low content of certain impurities may be important. There is little likelihood that the confusion in grading mill be substantially reduced. Various companies have spent much time, effort, and advertising on their own products, and are not now likely to change present nomenclature. The number of chemicals in the A.C.S. reagent chemicals list will probably continue to increase, thus reducing the confusion somewhat. In other grades, the present ambiguous terms will probably he with us for many years.
