1.1 Spot tests

Spot tests (spot analysis) are simple chemical procedures that uniquely identify a substance. They can be performed on small samples, even microscopic samples of matter with no preliminary separation. The first report of a spot test was in 1859 by Hugo Shiff for the detection of uric acid.

In a typical spot test, a drop of chemical reagent is added to a drop of an unknown mixture. If the substance under study is present, it produces a chemical reaction characterized by one or more unique observables, e.g., a color change.

Detection of chlorine

A typical example of a spot test is the detection of chlorine in the gas phase by the exposure to paper impregnated with 0.1% 4-4’bis-dimethylamino-thiobenzophenone (thio-Michler’s ketone) dissolved in benzene. In the presence of chlorine the paper will change from yellow to blue. The mechanism involves the Zwitter ionic form of the thioketone, [link] , undergoing an oxidation reaction and subsequent disulfide coupling, [link] .

Bibliography

• L. Ben-Dor and E. Jungreis, Microchimica Acta , 1964, 52 , 100.
• F. Feigl, Spot Tests in Organic Analysis , 7 ^th Ed. Elsevier, New York, 2012
• N. MacInnes, A. R. Barron, R. S. Soman, and T. R. Gilbert, J. Am. Ceram. Soc. , 1990, 73 , 3696.
• H. Schiff, Ann. Chim. Acta , 1859, 109 , 67.