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SCIENTIFIC EDITION26 1 however, if they are not present in excessive amounts, no apparent interference results. If the solution contains a relatively large amount of copper, the copper is removed with dithizone before completing the test.Occasionally a small amount of a precipitate, light green in color, may be observed suspended in the butyl acetate layer which is used to dissolve the tungsten-dithiol. The precipitate collects on the filter paper during the filtration of the butyl acetate layer. The determination of tungsten should be repeated if a green precipitate is observed on the filter paper, because this precipitate is apparently a tungsten compound.Mplybdenum appears to form a more stable compound than does tungsten. Also, the molybdenum compound forms much more readily and is not greatly affected by moderately high concentrations of acids. However, the pH of 2.5 should not be exceeded since high pH values adversely affect the test. CONCLUSIONSDetailed methods and also modifications of such methods are described to estimate tungsten and molybdenum using the reagent dithiol. Di-thiol is perhaps the most satisfactory reagent t o use routinely for the colorimetric estimation of tungsten and molybdenum in the range 0 t o 50 micrograms. The estimation of tungsten and molybdenum in the presence of each other is described. Citric acid prevents the precipitation of tungsten with dithiol sufficiently to permit a satisfactory determination of molybdenum.Spectrophotometric data, which support the described meth,ods, are presented. REFERENCES(1) Hamence J. H. A n~l y s t 65, 152(1940). (2) Clark R.'E. The authdrs ar; indebted' to Dr. A. A. White, Jr., for tory of Ex-Lax Inc. Brooklyn N. Y. editorial assistance. * All melting points are corrected. The carbon and hydrogen figures are the average of 3 to 8 analyses. Elementary analyses were made by Arlington Laboratories, William Saschek, and Carl Tiedke.