PRIMARY STANDARDS their evaluation and manufacture
The color illustration on the preceding page is an unretouched photo of a rectangular, tabular, prismatic crystal of the triclinic system. This 30-gram crystal was grown from Mallinckrodt Potassium Dichromate AR Primary Standard by Dr. L. A. Patterson of Mallinckrodt's research staff in St. Louis.
During the 50 years since Mr. H. V. Farr of Mallinckrodt manufactured the first chemical primary standard issued by the National Bureau of Standards, Quality Control people at Mallinckrodt—now over 100 strong and led by Drs. A. Q. Butler and S. M. Tuthill—have been dedicated to and engaged in the cause of extreme purity. Many of the exacting techniques developed for the manufacture of primary standards have been adapted to production of the full line of Mallinckrodt AR " laboratory chemicals. As a result, purities that were not available at one time are today routinely available to the analytical chemist in Stormor" bottles carrying the AR label. Specify AR's on your next order—most chemists do.
Mallinckrodt® MALLINCKRODT CHEMICAL WORKS St. Louis · New Y o r k · Montreal
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ANALYTICAL CHEMISTRY
PRIMARY STANDARDS their evaluation and manufacture The National Bureau of Standards, early in its existence, recognized its opportunity to make available certain chemical compounds as p r i m a r y standards. The first certified sample issued by the Bureau was sodium oxalate . . . made by Mallinckrodt. During the 50 years since then, Mallinckrodt has continued to develop and offer new and improved primary standards for the chemist. Many of the techniques developed for the exacting manufacture of p r i m a r y standards have been adapted for the manufacture of other analytical reagents. As a result, purities t h a t once were not available from any source are expected as a m a t t e r of course today.
This article is based on published papers and interviews with Dr. Tuthill and members of his staff at Mallinckrodt.
Today's commonly accepted primary standards are sodium carbonate, benzoic acid, arsenic t r i oxide, potassium dichromate, sodium oxalate, potassium acid phthalate, calcium carbonate and potassium iodate. In addition to these, Mallinckrodt offers standard grades of iron wire and sodium t a r t r a t e dihydrate (used as a \vater standard for standardizing Karl Fischer r e a g e n t ) . A primary standard is selected by a critical evaluation of the properties of the chemical itself. In general it must have the following properties: It must be a stable substance of definite composition. It must be a substance t h a t can be dried in the course of preparation, preferably at 105110°C, without change in composition. It must be a substance t h a t can be analyzed with accuracy. It should have a high equivalent weight so t h a t weighing errors will have a small relative effect. Desired reactions should occur according to single, well-defined, rapid and essentially complete processes. Having passed this critical evaluation, p r i m a r y standards must be manufactured to the ultimate in purity. Even small amounts of water are equally as objectionable as any chemical impurity. Most chemicals occlude some of the mother liquor during crystallization and in many cases resist all
attempts to remove it by simple drying, even at elevated temperatures. Therefore, in addition to a careful separation of chemical impurities, a primary standard must be crystallized or precipitated under conditions t h a t avoid occlusion. Successful purification to this degree depends upon rigid adherence to techniques t h a t will produce this result. No one method is adequate for all problems. Procedures must be adapted to the nature of the chemical compound and the impurities that must be removed. A typical example of the specific problems t h a t arise and the specialized methods by which they are solved is benzoic acid. Benzoic acid is purified (freed from chemical contaminants) by recrystallizing from water. However, adequate drying thereafter is a problem because benzoic acid sublimes at 100°C. Therefore it was necessary to develop the commercial scale equivalent of the laboratory procedure of drying in a desiccator. Purified by this method, benzoic acid is quite acceptable as a primary acidimétrie standard. However, when benzoic acid was proposed as a fixed point in thermometry, it became obvious that even this high degree of purity must be improved upon. F r a c t i o n a l f r e e z i n g w a s employed to achieve such ultra-high purity. Even the physical form of a primary standard is important. Small, fine crystals are preferable because they are free flowing, easy to handle and easy to dry. Very fine powders tend to stick to spatulas and weighing bottles ; needlelike crystals tend to mat together; large crystals are difficult to dry and weigh out in precise amounts. Thus, conditions of crystallization must be rigidly controlled to produce the best form of crystals as well as reduce chemical impurities and water occlusion. Naturally, contamination must be avoided at all stages of manufacturing, blending, subdividing and storage. The finished product must then be analyzed for impurities and assayed for purity as a guarantee that a suitable primary standard has indeed resulted from all these critical precautions. Analytical chemistry has made increasingly sophisticated demands of its primary standards and other analytical reagents. As production methods are further refined and improved, even p u r e r chemicals will be made available. As needs for new standards arise, they will be met.
Circle No. 189 on Readers' Service Card
VOL. 35, NO. 7, JUNE 1963
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