THEGeilmann (2) color reaction for rhenium is brought about by adding hydrochloric acid, stannous chloride, and potassium thiocyanate to a solution of a perrhenate. The stannous chloride presumably reduces the rhenium to the hexavalent state where it reacts with the thiocyanate to yield an intensely colored complex. The actual compound formed is said to be ReO(CNS)< (1, 9). Geilmann and coworkers have found that the lower limit of sensitivity lies somewhere in the neighborhood of . per 10 ml. (1 = 0.001 mg.). The reaction has been adapted to the quantitative determination of rhenium in much the same manner as the corresponding molybdenum reaction has been utilized in the Maag and McCollum (7) determination. Although molybdenum interferes when the Geilmann determination is applied directly to the analysis of minerals and concentrates, the method represents, with slight modification and when used in conjunction with a preliminary separation, the best available to date.The procedure recommended by the original authors and similar to the analogous molybdenum method was as follows:To the nearly neutral solution of rhenium as the perrhenate were added 10 mi. of 20 per cent hydrochloric acid and 2 ml. of 10 per cent potassium thiocyanate. This was diluted to 50 ml. and treated with 10 ml. of 2 per cent stannous chloride. After shaking for 0.5 minute, 20 ml. of ether were added and the yellow complex was extracted. Residual traces of the complex were removed by a second extraction. It was early recognized that the intensity of color produced was dependent upon a number of factors. Chief among these were concentrations of reagents and

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