J. Cosmet. Sci., 65, 187–195 (May/June 2014) 187 Stability of urea in solution and pharmaceutical preparations NATTAKAN PANYACHARIWAT and HARTWIG STECKEL, Department of Pharmaceutics and Biopharmaceutics, Christian Albrecht University of Kiel, 24118 Kiel, Germany. Accepted for publication March 10, 2014. Synopsis The stability of urea in solution and pharmaceutical preparations was analyzed as a function of temperature (25°–60°C), pH (3.11–9.67), and initial urea concentration (2.5%–20%). This study was undertaken to (i) obtain more extensive, quantitative information relative to the degradation of urea in both aqueous and non- aqueous solutions and in pharmaceutical preparations, and (ii) test the effects of initial urea concentration, pH, buffer, and temperature values on urea degradation. The stability analysis shows that urea is more stable at the pH range of 4–8 and the stability of urea decreases by increase in temperature for all pH values. Within the experimental range of temperature and initial urea concentration values, the lowest urea degradation was found with lactate buffer pH 6.0. The urea decomposition rate in solution and pharmaceutical preparations shows the dependence of the initial urea concentrations. At higher initial urea concentrations, the rate of degradation is a decreasing function with time. This suggests that the reverse reaction is a factor in the deg- radation of concentrated urea solution. For non-aqueous solvents, isopropanol showed the best effort in re- tarding the decomposition of urea. Since the losses in urea is directly infl uenced by its stability at a given temperature and pH, the stability analysis of urea by the proposed model can be used to prevent the loss and optimize the operating condition for urea-containing pharmaceutical preparations. INTRODUCTION Being widely used in pharmaceutical and cosmetic products, urea plays a vital role in maintaining the skin’s moisture balance and suppleness. Reduced levels of urea, repre- senting 7% of the natural moisturizing factors in the stratum corneum (skin-building layer), lead to a lower water-binding capacity within the skin, which in turn, results in roughness, tightness, fl aking, and irritation of the skin. Urea preparations typically range in strength from 3 to 20, in specifi c preparations up to 40%, and can take many forms, including creams, gels, shampoos, deodorants, foundation, and even toothpaste. Ever since the decomposition of urea was fi rst presented by Wöhler in 1829 (1), the under- standing of its products, by-products, and reaction pathways has been extensively the sub- ject of several studies over the past century (2–11), but little information exists relative to the stability of urea in non-aqueous solutions and pharmaceutical preparations. Urea Address all correspondence to N. Panyachariwat at npanyachariwat@pharmazie.uni-kiel.de and H. Steckel at hsteckel@pharmazie.uni-kiel.de.

JOURNAL OF COSMETIC SCIENCE 188 decomposition yields ammonium ions (NH4+) and cyanate (CNO−), further readily under- going conversion to carbon dioxide (CO2) and ammonia (NH3). In aqueous solution, an elimination mechanism for urea decomposition appears to be operative. In contrast, when catalyzed by ureases, urea is generally believed to undergo hydrolysis rather than ammonia elimination (2–6). Although the earlier workers agreed that ammonium cyanate is an inter- mediate in the decomposition of urea, the numerical magnitudes of the rate constant and the order of reaction reported were quite different. Bull et al. (7) reported that urea degrada- tion followed fi rst-order kinetics in both dilute and concentrated solutions as well as Shaw and Bordeaux (8). On the other hand, the hydrolysis of urea was observed by several inves- tigators as a reversible reaction in some specifi c conditions (9–11). Many direct approaches for urea determination involving the reactions with urea to form colored products have been described, for instance, the well-known Fearon reaction (12), the reaction with o-phathaldehyde, and the reaction with p-dimethylaminobenzaldehyde (p-DMAB). Because of the use of corrosive reagents or incubation temperature and other disadvantages in diacetylmonoxime and o-phathaldehyde assays, in this experiment, the reaction of urea with p-DMAB was used. Impressed by Knorst’s work (13), we then ap- plied the protocol using p-DMAB for the kinetic study of urea degradation both in aque- ous solution, non-aqueous, and in pharmaceutical preparations with a broad range of pH and temperature. MATERIALS AND METHODS MATERIALS AND REAGENTS Urea and sulfuric acid were obtained from Merck (Darmstadt, Germany). p-DMAB was obtained from Sigma-Aldrich (Steinheim, Germany). Chemicals and solvents were of re- agent grade and were used without further purifi cation. Lactate buffers pH 4.5 and 6.0, phosphate buffer pH 6.0, and citrate pH 6.0 were prepared according to European Pharmacopoeia (14). The UV absorption spectra were recorded against a reagent blank using a ThermoScientifi c (Waltham, MA) Helios Omega spectrophotometer. DETERMINATION OF UREA DEGRADATION RATE CONSTANTS IN AQUEOUS SOLUTIONS The various concentrations of urea solutions (2.5%, 5%, and 10% [w/v]) were prepared and subjected to investigate the effect on urea degradation rate constant. 1 M NaOH, 1 M HCl, and different buffers were used to prepare solutions at different pH intervals between 3.0 and 10.0. At pH values between 3.11 and 4.19, 1 M HCl pH values be- tween 8.40 and 9.67, 1 M NaOH pH values 4.5 and 6.0, lactate buffer pH values 6.0, phosphate buffer and for pH values 6.0, citrate buffer solutions were used. The solutions of urea at different pH values were incubated at 25°, 40°, and 60°C, respectively. The residual urea concentration values at a defi ned pH and temperature values and different time intervals of 3, 7, and 14 days were determined as per the following procedure. A solution (0.5 ml) containing 4% (w/v) of p-DMAB and 4% (v/v) sulfuric acid in 99% ethanol was added to the mixture of 0.05 ml of urea solution and 9.95 ml of water. After