Rate constants for cyanate hydrolysis to urea: A Raman study

Abstract

Raman spectra of aqueous solutions of potassium cyanate have been obtained at suitable time intervals. It was found that the peaks attributed to cyanate became less intense, while the peaks attributed to urea, carbamate and carbonate, centred at 1003, 1034, and 1065 cm−1, respectively, enhanced greatly with the passage of time. Further Raman spectroscopic studies revealed that the cyanate ion hydrolysed slowly and spontaneously at room temperature. Urea, carbamate, and carbonate were formed even without additional ammonium. Raman intensity measurements were used to monitor species concentrations as a function of time. The results suggested a complicated hydrolysis process: the hydrolysis of cyanate ion to form carbonate and ammonium, a rearrangement type reaction of aqueous cyanate ion with aqueous ammonium to formed urea, and an equilibrium reaction of carbonate and ammonium to form carbamate. The initial hydrolysis of cyanate with pure water was found to be first order with rate constant k1 = (2.67 ± 0.53) × 10−4 min−1 at 22 °C. The reaction of cyanate with aqueous ammonium was found to be second order with rate constant k2 = (4.64 ± 0.93) × 10−4 mol−1•L•min−1. The equilibrium reaction of carbonate and ammonium to form carbamate was very fast. Urea and carbamate were formed in parallel reactions. It was not possible to convert urea to carbamate or carbamate to urea at room temperature.