Edward F. Ferrand, JI. The
Cooper Union, Cooper Square New York 3, New York
Improved Confirmatory Test for Chromium
A test in the qualitative analysis scheme which has always given trouble is the confirmatory test for chromium. This test involves production of CrOs by oxidation with hydrogen peroxide in nitric acid. The instability of the Cr05 in nitric acid is circumvented somewhat by extracting the blue Cr06 with ethyl ether, in which its stability is a little greater. The new test has been successfully used in routine qualitative analysis work by freshman students. The modified test also removes the necessity of having ethyl ether in the freshman laboratory with the accompanying hazard of fire and explosion. The work of the students consisted of investigating various related organic solvents as possible substitutes for ethyl ether. Ethylene glycol mono-butyl ether was found to be a much better solvent than ethyl ether. Conditions and concentrations were varied until a practical test was devised. SORUM,C. H., "Introduction t o Semimicro Qualitative Analysis," 2nd ed., Prentice-Hall, Inc., New York, 1953.
The test is performed as follows. After standard qualitative analysis procedure' has yielded a precipitate suspected to be barium chromate, 3 drops of 3 M HNOa (stir), 10 drops of water, 10 drops of ethylene glycol mono-butyl ether, and 1 drop of HzOzare added in that order. The blue color indicating CrO5 is quite intense and lasts about 20 minutes when the test is run on the precipitate obtained from 6 drops of an AI-Ni group test solution containing all ions a t 0.1 M concentration. The blue color which results when ethyl ether is used under similar conditions lasts only a matter of seconds and sometimes does not appear if the concentrations of HNOa and HzOzare too great or the temperature is too high. Students have found that they are able to detect very low concentrations of dichromate ion using this method. At room temperature they have reported a positive test from a 0.00005 M solution of potassium dichromate a t a pH of 7.
Volume 37, Number 8, Augusf 1960
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