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THE ORCBS > Chemical Safety > Resources & Links > A Simple Qualitative Detection Test for Perchlorate Contamination in HoodsWayne C. Wolsey,
Ph.D., Associate Professor of Chemistry,
The use of perchloric
acid in chemical analysis frequently brings questions of chemical
safety to the minds of chemists and safety engineers. Precautions
for the safe handling of perchloric acid and perchlorates have been
summarized in various sources (1, 2, 3). The problem
sometimes arises concerning whether perchloric deposits are present,
such as in hoods or whether perchloric acid may be leaking from an
apparatus. It would be highly desirable to have a simple means of
detecting perchlorate available in such cases.
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| Table
I Colored Precipitates formed with Methylene Bluea | |
|---|---|
| Salt | Color of Precipitate |
| ZnCl2 | Blue |
| Ti(NO3)4 | Blue |
| Vanadate | Blue |
| Tungstate | Blue |
| FeSO2 | Brown Red |
| Hg2Cl2 | Violet-red |
| PtCl62 - | Violet-blue |
| Molybdate | Violet-blue |
| Chromate | Brown Red |
| Thiocyanate | Green |
| Ferricyanide | Dark Green |
| Ferrocyanide | Dark Blue |
| Perchlorate | Violet |
It has been reported (6) that the detection limit of perchlorate with methylene blue is 0.003 mg., but investigations in the author's laboratories has shown that a more practical limit in a finite time is 0.001 M perchlorate. We have found that the use of a 0.3% aqueous solution of methylene blue gives satisfactory results.
It should be noted however, that concentrated perchloric acid, such as the 70% HClO4 of commerce, destroys the reagent. These solutions will give the expected results, however, if the acid is diluted.
A practical application of the test was carried out recently by the author in investigating a Technicon Continuous Digestor which uses a perchloric acid mixture for the digestion of blood samples for protein-bound iodine. The apparatus had been setting for a number of years under a metal hood. The design of the Continuous Digestor is such that the perchloric acid mixture is heated to 350° C. to decompose the perchloric acid before the effluent is vented up the hood or down a drain.
The hood was washed down with water and a few drops of methylene blue reagent applied to the washings. A violet precipitate formed immediately, indicative of the presence of perchlorate salt deposits in the hood. Further investigation showed that small amounts of perchloric acid was in the effluent from the digestor. These findings were subsequently confirmed by the Technicon Corporation which has issued a precautionary bulletin.
Literature Cited
(1) K. Everett and F. A. Graf, Jr., in "Handbook of Laboratory Safety," 2nd ed., N. V. Steere, Ed., Chemical Rubber Company, Cleveland, Ohio, 1971, pp. 265-276.
(2) L. A. Muse, J. Chem. Education, 49, A463 (1972).
(3) W. C. Wolsey, J. Chem. Education, 50, A335 (1973).
(4) F. J. Welcher, "Organic Analytical Reagents," D. Van Nostrand Co., 1948, Vol IV, pp. 517-531.
(5) F. L. Hahn, Z. Angew. Chem., 39, 451-4 (1926).
(6) D. Kruger and E. Tschirch, Z. Anal. Chem., 85, 171-6 (1931).
