4,5-Diamino-6-thiopyrimidine as an analytical reagent—ISpectrophotometric determination of selenium

A number of organic compounds have been screened as reagents for the spectrophotometric determination of small amounts of selenium. 4,5-Diamino-6-thiopyrimidine shows promise for this determination. In ammoniacal solution, even on long standing, this reagent is colourless in the presence of quadrivalent selenium. However, at pH 1.5–2.5 or lower, 4,5-diamino-6-thiopyrimidine forms a yellow colour with quadrivalent selenium, and this has a strong absorption peak at 380 mμ when compared with a reagent blank of the same concentration. Beer's Law holds when the selenium concentration is between 0.1–2.5 μg per ml. The conditions for the quantitative determination of selenium have been investigated. When selenium in solution is in the quadrivalent state, measurement of absorption should be carried out within 30 min of the addition of the reagent. On longer standing, the reagent gives a slight yellow colour with the formation of elemental selenium. The formation of elemental selenium is slow in dilute solutions of selenium.