Radiation-induced isomerization of thiourea into ammonium thiocyanate

Abstract

The products of γ-radiolysis of solutions of thiourea are reported to be ammonia, sulfur, cyanamide, dicyanamide, and possibly guanidine sulfate. Ammonium thiocyanate, the product of isomerization, has not been observed although it is found to be one of the products in the themal decomposition of aqueous solutions of thiourea over the temperature range of 100–180°C. Hence, the observation of a close to 1% yield of the isomerization product in a 6.25% (0.82M) solution of thiourea, at neutral pH on exposure to γ-radiation from a⁶⁰Co suurce at room temperature, would appear to be a result of interest in radiation chemistry. This paper present results on the variation of the yield of the isomerization product with the γ-dose. The experiments consisted in irradiating 2 ml of 0.82M aqueous solution of thiourea at neutral pH and under normal atmospheric conditions, i.e., at room temperature and with dissolved air present. The source of radiation was a 2.5 kCi (nominal)⁶⁰Co source. The method used to detect and measure the isomerization product, ammonium thiocyanate was based on the well-known color reaction between CNS⁻ and Fe⁺³ monitored spectrophotometrically at 450 nm, after obtaining the calibration curve of absorbance and concentration of NH₄CNS. The yield of NH₄CNS, expressed as$G_{NH_4 CNS}$ was 0.083 molecules of NH₄CNS for 100 eV absorbed. A suitable mechanism for the process of isomerization is suggested involving the hydrated electrone _aq, the major primary product of water radiolysis. The mechanism is consistent with the observed effect of the addition of acetone, a well-known electron scavenger. This suppressed the yield NH₄CNS by about 25%.