A MODIFICATION OF THE BETTENDORF TEST AS A CONFIRMATORY TEST FOR ARSENIC IN QUALITATIVE ANALYSIS JACOB CORNOG University of Iowa, Iowa City, Iowa
STUDENTS in elementary qualitative analysis often fail to get decisive confirmatory tests for arsenic by use of reagents such as silver nitrate, magnesium salts, or ammonium molybdate. The modified Bettendorf test, described here, requires less manipulative skill than the tests by the reagents just named and has been found quite satisfactory for the use of freshmen students a t this institution. To use this test, separate arsenic from antimony and tin by any usual method which leaves arsenic in the form of a sulfide precipitate. Transfer the precipitate to a beaker, add 10 ml. of 6 M perchloric acid and heat till dense white fumes are copiously emitted. Then add 10 ml. of 12 M hydrochloric a d d and 5 ml. of 0.5 M stannous chloride, heat the mixture to boiling, and allow i t to stand one minute without fnrther heating. The formation of a dark brown solution indicates the presence of arsenic. The coloration is due to the presence of free arsenic. The fnrther manipulation described in this paragraph was suggested by King and Brown1 and should be nsed when no brown coloration appears in the solntion after i t has stood one minute. In a separate vessel thoroughly mix 1 ml. of 0.25 M mercuric chloride with 9 ml. of water, add 1 ml. or less of this mixture to the solution being tested for arsenic, stir, and allow the final mixture to stand for ten minutes. If the solntion fid. Eng. C h . ,Analyt. Ed., 5, 188 (1933).
is colorless a t the end of ten minutes, arsenic is absent. Although this test is very delicate and certain, the reactions are often slow, and time is required for the formation of the brown coloration. According to King and Brown2 mercuric chloride catalyzes the reactions and thus hastens the appearance of the brown coloration. The following remarks will indicate the particular utility of the manipulation to persons lacking recent experience with the Bettendorf test. The Bettendorf test consists of the reduction of dissolved arsenite or arsenate to free arsenic by stannous chloride in concentrated hydrochloric acid solntion. Boiling temperatures and high hydrochloric acid concentrations favor reaction. Although the Bettendorf test is one of the more simple and easily executed arsenic tests, it has not been nsed as a confirmatory test in qualitative analysis because oxidizing agents, other than arsenite or arsenate, must be absent, and nitric acid, an oxidizing agent, is the only reagent commonly nsed to dissolve arsenic sulfide before making a confxmatory test. Perchloric acid has the useful quality of acting as an oxidizing agent in the hot fuming condition and of not being an oxidizing agent when somewhat diluted. Hence, perchloric acid may be nsed to dissolve arsenic sulfide, and later, when somewhat diluted, perchloric acid does not interfere with the Bettendorf test.
