Table II The values of the rate constants, kin h−1calculated from the experimental data of the residual urea concentrations with various initial concentrations (2.5%, 5%, and 10%) as a function of pH and temperature k Value at 25° k Value at 40° k Value at 60° 2.50% 5% 10% 2.50% 5% 10% 2.50% 5% 10% 3.11–3.36 3.51 × 10−6 3.34 × 106 − 3.33 × 106 − 4.88 × 10−5 4.84 × 105 − 4.67 × 10−5 8.18 × 10−4 8.17 × 104 − 8.14 × 10−4 4.08–4.19 3.09 × 10−6 3.07 × 106 − 3.05 × 106 − 4.34 × 10−5 4.28 × 105 − 4.01 × 10−5 8.06 × 10−4 7.95 × 104 − 7.89 × 10−4 6.43–7.36 3.03 × 10−6 2.95 × 10−6 2.90 × 10−6 4.29 × 10−5 4.27 × 10−5 3.98 × 10−5 7.97 × 10−4 7.92 × 10−4 7.81 × 10−4 8.40–8.59 3.11 × 10−6 3.10 × 10−6 3.07 × 10−6 4.37 × 10−5 4.28 × 10−5 4.07 × 10−5 6.13 × 10−4 8.01 × 10−4 7.90 × 10−4 9.40–9.67 3.92 × 10−6 3.86 × 106 − 3.83 × 106 − 5.27 × 10−5 5.02 × 105 − 4.95 × 10−5 8.20 × 10−4 8.17 × 104 − 8.17 × 10−4 6.00 (lactic buffer) 2.95 × 10−6 2.79 × 106 − 2.67 × 106 − 4.24 × 10−5 4.08 × 105 − 3.84 × 10−5 6.00 (phosphate buffer) 2.95 × 10−6 2.83 × 10−6 2.73 × 10−6 4.26 × 10−5 4.14 × 10−5 3.85 × 10−5 6.00 (citric buffer) 2.99 × 10−6 2.94 × 10−6 2.82 × 10−6 4.32 × 10−5 4.22 × 10−5 3.88 × 10−5 4.50 (lactic buffer) 3.06 × 10−6 3.07 × 106 − 2.84 × 106 − 4.30 × 10−5 4.27 × 105 − 3.96 × 10−5 STABILITY OF UREA IN SOLUTION AND PHARMACEUTICAL PREPARATIONS 191

JOURNAL OF COSMETIC SCIENCE 192 range. In acid, a rapid quantitative conversion of the cyanate ion (CNO−) to ammonium ion (NH4+) occurs as shown in equation 1. This reaction is complete at room temperature in suf- fi ciently concentrated acid solution. These fi ndings are in complete agreement with those of Warner (9). The absence of occurrence of reverse reactions in acid media by the rapid hydroly- sis of cyanate promotes the increase in urea degradation rate constant in strong acid condition. l - 2 4 2 3 CNO 2H 2H O NH H CO + + + + + (1) The reversibility is also limited in alkaline solution by the decomposition of cyanate ion in basic environment as depicted in equation 2. This agrees with the increase in urea degradation rate constant in strong basic condition in our experiment. l - - 2 4 3 CNO OH 2H O NH OH CO2- + + + (2) Increase in temperature as well as working out of the range of 4–8 for pH increased the decomposition of urea. The calculated values of the decomposition free energies as a function of pH and tem- perature are shown in Table III and can be used for stability analysis of urea. The higher value of the free energy means the more the stability of urea at a given condition (tem- perature and pH). Therefore, the maximum value of ∆Gi or the maximum stability of urea was obtained at 25°C, the highest initial urea concentration (10%) and pH 6.0 with lactate buffer for the temperature and pH ranges under study. THE KINETICS OF UREA DEGRADATION IN NON-AQUEOUS SOLVENTS A variety of non-aqueous solvents used mainly in pharmaceutical preparations (propylene glycol, glycerol, N-methylpyrrolidone, dimethylisosorbide, ethanol, isopropanol, pentylene Table III The values of the free energies of decomposition, ∆Gi, in kJ/gmol calculated from the experimental data of rate constant with various initial urea concentrations (2.5%, 5%, and 10%) as a function of pH and temperature pH ∆Gi at 25°C (kJ/gmol) ∆Gi at 40°C (kJ/gmol) ∆Gi at 60°C (kJ/gmol) 2.50% 5% 10% 2.50% 5% 10% 2.50% 5% 10% 3.11–3.36 104.16 104.28 104.29 102.67 102.7 102.79 101.59 101.6 101.61 4.08–4.19 104.47 104.49 104.51 102.98 103.02 103.18 101.63 101.67 101.69 6.43–7.36 104.52 104.59 104.63 103.01 103.02 103.2 101.67 101.68 101.72 8.40–8.59 104.46 104.47 104.49 102.96 103.02 103.15 102.39 101.65 101.69 9.40–9.67 103.88 103.92 103.94 102.47 102.6 102.64 101.59 101.6 101.6 6.00 (lactic buffer) 104.59 104.73 104.84 103.04 103.14 103.3 6.00 (phosphate buffer) 104.59 104.69 104.78 103.03 103.1 103.29 6.00 (citric buffer) 104.56 104.6 104.7 102.99 103.05 103.27 4.50 (lactic buffer) 104.5 104.49 104.68 103 103.02 103.22