During a recent course of instruction in the Microscopical Identification of White-Powder Unknowns, a question asked was, “How can you tell the difference between sucrose and dextrose?”  Another was, “How can you determine whether an alum is the potassium alum, the sodium alum, or the ammonium alum?”  Questions such as these constantly come up during any microscopical identification course:  “Is there a quick way to tell if aspirin is present in this analgesic?”  “Is boron present in this sample of dentifrice?”  “Does the paper-coating consist of cooked starch?”  Microscopists trained in classical methods realize that all of these questions can be answered quickly and economically through the use of simple microchemical tests which have been developed over the last 180 years: elaborate and costly instrumentation is not necessary, even though such equipment may be available.  Knowledge of these microchemical tests may be obtained from numerous reference books, and acquisition of such references should be an on-going process for the laboratory or for one’s personal professional library.

In this article I would like to survey the books that contain information on the performance of microchemical tests, including spot testing; illustrations and/or annotations are offered for each reference.  The presentation will be more or less chronological so that the history of the entire branch of this microscopical science will be apparent.  We will start with some definitions, and set boundaries.

DEFINITION OF CLASSICAL MICROCHEMISTRY

In 1975, the American Chemical Society initiated a project to prepare a history of the Society and of the various branches of chemistry, to be published in the Society's centennial year, 1976.  At the same time, the executive committee of the Division of Analytical Chemistry decided that in addition to supplying information for such a volume, it would undertake a centennial history of its own, and thus A History of Analytical Chemistry [Laitinen, H.A. and G.W. Ewing (1977)] was produced.  The history of the development of microchemistry and microanalysis as a special branch of analytical chemistry for this book was prepared by Herbert K. Alber, who studied under the founders of microanalytical chemistry at the Technical University of Graz, Austria.  One of these founders was Fritz Pregl (1860-1930), who was awarded the Nobel Prize in Chemistry in 1923.  Alber defines the "Classical Period of Microanalysis" as the years 1900 to about 1945, and states (p. 26) "It is generally agreed that Friedrich Emich (1860-1940) at the Technical University in Graz, Austria, was the founder of classical microchemistry."  He defines the years prior to 1900 as "The Early Microchemical Period."  In this period, he states (p. 25) that François-Vincent Raspail (1794-1878) "established himself as the founder of chemical microscopy, and he probably can be recognized as the first microchemist."

In the Preface to the first edition of his Elementary Chemical Microscopy, É.M. Chamot (1915) stated, "...the earliest comprehensive work dealing with microchemical methods was from the pen of an American—Theodore G. Wormley—whose classic, The Microchemistry of Poisons, appeared in 1867."  [Wormley (1867)]

Some historians attribute the first microchemical analysis to Pliny (23-79 A.D.) who mentions an iron sulfate test in his 37-volume Historia Naturalis, the work in which he described the entire natural world as far as it was known in his time, but it is doubtful that the test was actually performed on very small samples, and a microscope was certainly not in use.

As a microscopist and microchemist, I define classical microchemistry as the period from the first application of chemical tests conducted while actually viewing the results with a microscope, to the general introduction of non-microscopical instrumental analysis.  For me that means from Raspail's earliest experiments (early 1820’s), through, say, the 1960's - and even to the present day, for those few practicing in the classical manner.

Spot tests, or spot-testing, is understood to mean color and/or precipitate reactions resulting from mixing an unknown and a test reagent in a “spot plate”.  Spot plates are small rectangular or square tiles with ~1 cm diameter depressions in which the chemical reactions are conducted; the spot plate may be made from white, blue, black, or black-and-white ceramic, or from clear glass or plastic (which may be laid on paper of any color, for contrast enhancement), and may have from one to twelve or more smooth concave depressions.  A well-slide used in microscopy can be used for the same purpose.

“Chemical Microscopy” was defined by Émile Chamot in the Introduction to his Elementary Chemical Microscopy as “the application of the microscope to the solution of chemical problems”, for the purpose of differentiating it from “microchemical” methods and tests.  He went on to explain that “Microchemistry implies chemistry on a small scale,” and, indeed, “all highly sensitive identification reactions and all quantitative methods, which permit the use of samples smaller than are commonly necessary in ‘standard methods’”.  Under the term “chemical microscopy” Chamot included “those methods, principles, and phenomena of chemistry which may be studied particularly advantageously by means of the microscope….”

Microchemistry and spot tests are, therefore, vital methods in the practice of chemical microscopy.