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A Simple Qualitative Detection Test for Perchlorate Contamination in Hoods

Wayne C. Wolsey, Ph.D., Associate Professor of Chemistry,
Macalester College, St. Paul, Minn. 55105
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.

An examination of the analytical chemical literature for perchlorates shows that there are few tests which are specific for perchlorates or perchloric acid. This is not too surprising, considering the relative chemical "inertness" of perchlorates under normal conditions. The desirable characteristics of a test for perchlorates would include: formation of a colored substance, minimal interference by other similar species, the use of a readily available reagent, and easy interpretation of the test. An analytical reagent which comes close to satisfying these criteria is methylene blue. The use of methylene blue in analytical chemistry has been treated thoroughly by Welcher (4). It has been frequently used for both the qualitative and quantitative determination of perchlorates. It was used in the early part of the century to detect small amounts of naturally-occurring perchlorates in the Chilean nitrate deposits.

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

a adapted from Welcher (4)

Methylene blue forms a violet precipitate with perchlorate ion. Other substances which form similar appearing precipitates are shown in Table I. In most cases, knowledge of the composition of the chemical system being investigated will eliminate the majority of these species.

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).

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