JULY, 1947
DETECTION OF ACETATE ION BY THE REACTION WITH ARSENIOUS OXIDE TO FORM CACODYL OXIDE L. R. BRANTLEY, T . M. CROMWELL, and I. F . MEAD Occidental College, Los Angeles, California
IT
BAS bden the experience of the authors in teaching qualitative analysis that the tests usually given for the detection of acetate ion are unsatisfactory. Detection by the formation of cacodyl oxide, easily recognized by its characteristic odor, although not a new reaction, is mentioned only in an occasional qualitative analysis laboratory manual.' The following procedure for detection of acetate ion has been developed and successfullyused by the authors. To a few mg. of solid or the solid residue left from the evaporation of a few ml. of solution, approximately 1 mg. of solid arsenious oxide is added. Upon gentle heating, the presence of acetate is indicated by the formation of cacodyl oxide which is easily detected by its characteristic disagreeable odor. The reaction for the formation of the cacodyl oxide is AszOl + 4KCzHaOl (CHa)aAs-4-As(CH& 2C02 2KsC08
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+
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S i x anions-tartrate, sulfide, thiocyanate, chlorate, nitrate, and nitrite-interfere with the test due to the
formation of decomposition products or to reaction with the Asz03. The treatment of interfering ions would depend on which of these were suspected. The tartrate ion can be removed from the unknown by the addition of calcium or barium hydroxide solution until precipitation ceases, then filtering. Likewise, the sulfide and thiocyanate ions can be removed by the addition of mercuric chloride. If chlorate, nitrate, or nitrite are suspected, they can be eliminated by adding several mg. of Zn dust and several ml. of concentrated HC1 to a few ml. of the unknown solution. This is boiled for several minutes, cooled, and neutralized with dilute NaOH solution. The precipitate is removed and the filtrate treated as above in testing for the acetate ion. Students should be warned to smell the cacodyl oxide fumes cautiously since they are poisonous as well as unpleasant. Under ordinary conditions this arsenious test is sensitive to about 0.1 mg. of the acetate ion. ' MCALPINE, R. K., AND B. A. SOULE,"Preseott and Johnson's QualitativeChemical Anal.ysis,"D. VanNostrand Cornpsnv. Inc.. New York, 1933, p. 451,
