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Conversion of Ammonium Cyanate Solution at 293.15 K

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Conversion of Ammonium Cyanate Solution at 293.15 K JournalJournal of Chemical of Chemical Technology Technology and Metallurgy,and Metallurgy, 53, 5, 53, 2018, 5, 2018 872-877 CONVERSION OF AMMONIUM CYANATE SOLUTION AT 293.15 K Mladen E. Lilov, Plamen P. Kirilov University of Chemical Technology and Metallurgy Received 30 April 2018 8 Kliment Ohridski, Sofia 1756, Bulgaria Accepted 15 June 2018 E-mail: [email protected] ABSTRACT Conversion of 0.064 mol L-1 ammonium cyanate solution has been studied at 298.15 K over a time period of 90 days. The data is obtained by periodic measurement of the electric conductivity, pH and concentration of the main species of the solution such as cyanate, urea, ammonim carbonate and ammonium hydrogencarbonate. The distribution of the species in the solution during this period is presented in graphic mode. The pH value increases gradually due to the formation of NH4HCO3 and (NH4)2CO3. The spontaneous conversion of the ammonium cyanate ions to a neutral substance, urea, is accompanied by a sharp conductance decrease. Ammonium cyanate hydrolysis to NH4HCO3 and (NH4)2CO3 proceeds to a much smaller extent than that of urea conversion. During 90 days stor- age of the solution at 298.15 K the degree of overall transformation of ammonium cyanate is found equal to 96.3 %. Keywords: ammonium cyanate, transformation, urea, kinetics, mechanism. INTRODUCTION a modified form, is supported by E.E. Walker [5] and Werner [7]. Moelwyn-Hughes [11] assumes that the rate Since Wöhler’s discovery [1] that ammonium cy- depends upon collisions between unionized molecules anate undergoes a spontaneous transformation to urea the of ammonium cyanate. mechanism and kinetics of this conversion has been dis- The rate of conversion of ammonium cyanate into cussed for many years. The topic is not yet exhausted [2]. urea is studied in several isodielectric solvent-water The earliest investigations [3 - 11] show that the reaction mixtures over a wide dielectric constant range [12 - 18]. is bimolecular, that the reaction is reversible and that it The calculations based on the experimental data are is not complete in an aqueous solution. Furthermore, a compared with those obtained from existing solution partial hydrolysis of ammonim cyanate to ammonium kinetic theories. carbonate occurs, while the rate of conversion is higher According to Wyatt and Kornberg [19] the reaction in mixtures of water with alcohols, sugars, acetone, etc., NH4OCN ↔ CO(NH2)2 in unbuffered aqueous solution than that in pure water. at 343,15 K is accompanied by the formation of larger Concerning the exact nature of the bimolecular amounts of carbonate than is formerly supposed. This leads mechanism, there is a considerable controversy. Some to a modification of the rate constants of urea reaction. authors [3, 4, 8 - 10] indicate that the reaction rate de- The simultaneous formation of urea and carbonate pends upon collisions between ammonium and cyanate in aqueous solutions of ammonium, barium, or sodium ions (an ionic mechanism). Chattaway [6] argues against cyanate is studied at 333,15 and 353,15 K [20]. The the ionic mechanism and assumes that the rate depends results of Kemp and Kohnstam fully confirm Wyatt and upon collisions between molecules of ammonia and Kornberg’s observation [19] that carbonate formation isocianic acid (a non-ionic mechanism). This view, in is not negligible in the decomposition of ammonium 872 Mladen E. Lilov, Plamen P. Kirilov cyanate. A general rate equation describing urea and the rate of the process under rather different conditions carbonate formation is derived. It is generally accepted is accumulated. The aim of the present study is quite that cyanates decompose in aqueous solutions according different, namely to investigate the change of the compo- to the stoichiometric equations: sition of ammonium cyanate solution during prolonged - 2- + OCN + 2H2O → CO3 + NH4 (1) staying at ambient conditions. It is a continuation of our - + OCN + NH4 ↔ CO(NH2)2 (2) investigation of the reverse reaction, decomposition of The rate of conversion of ammonium cyanate into urea in a solution, presented in a previous paper [26]. urea is determined in the presence of added foreign salts [9, 21]. It is observed that the reaction rate decreases EXPERIMENTAL by salt addition. The negative catalysis becomes better expressed with the increase of added salt concentra- Materials tion. The higher the valency of one of the component AgNO3 and NH4Cl (p.a.) and potassium cyanate for ions of the added salt, the more striking the retarding synthesis KOCN ≥ 97.0 % obtained from Merck were influence is. used in this investigation. The transformation of ammonium cyanate to urea can take place both in a solution and in a solid state. Two Procedures years after Wöhler’s discovery [1] Liebig and Wöhler Ammonim cyanate solution was prepared from silver [22] succeed to prepare ammonium cyanate as a fluffy, cyanate and ammonium chloride solution. Silver cyanate white powder by the direct union of dry ammonia and was precipitated by mixing solutions of silver nitrate and cyanic acid vapour. The powder can be kept essentially potassium cyanate, filtered, washed with distilled water unchanged in a sealed vessel under ammonia. When and dried. A suspension of silver cyanate in an aqueous exposed to air, the cyanate is converted into urea in solution of equivalent amount of ammonium chloride was the course of a few days, ammonia being continually agitated for 1 h until the solution was free of chloride ions. evolved during the transformation. Walker and Wood Silver cyanate had a very low solubility and the exchange [23] study the influence of the temperature on the rate into ammonium cyanate took place slowly, leaving behind of transformation of dry ammonium cyanate. An ex- solid silver chloride. After 50 min - 60 min the exchange periment with ordinary dry cyanate shows that 80 % of was complete (AgOCN + NH4Cl = NH4OCN + AgCl), the cyanate is converted into urea in the course of two and after filtering, the solution of ammonium cyanate hours at 333,15 K. Crystalline ammonium cyanate is was used. Solutions of ammonium cyanate were known synthesized by Dunitz et al. [24] by an ion exchange to undergo the isomeric change into urea at an appreciable between tetraethylammonium cyanate and ammonium rate, even at room temperature [27]. For this reason the thiocyanate. The authors determine that microcrystalline solution was analyzed within the next half an hour and ammonium cyanate transforms completely to micro- distributed in bottles. 100-ml samples of the solution were crystalline urea in the course of about 6 h at 323,15 K. transferred, by a pipette, to 120-ml plastic bottles which Tsipis and Karipidis [25] report a comprehensive were sealed and maintained at a constant temperature at study of the transformation of ammonium cyanate into 298.15 ± 0.5 K for 90 days. The bottles were removed at urea in a vacuum by means of first-principles quantum regular time intervals, and the contents analyzed by the chemical calculations. The synthesis of urea is predicted methods described in the sequel. Measurements of specific to proceed in two steps; the first step involves the de- conductivity, pH, and concentration of the species such composition of the ammonium cyanate to ammonia and as cyanate, urea, ammonim carbonate and ammonium isocyanic or cyanic acid, while the second one, which hydrogencarbonate in the solutions were carried. All data is the rate determining step, involves the interaction of reported here were mean values from 3 measurements. NH3 with either isocyanic or cyanic acid. The literature review indicates that a controversy Methods and confusion concerning the mechanism of the trans- The electrolytic conductivity and pH were measured formation reaction of ammonium cyanate into urea in using a BOECO model conductometer and Mettler To- a solution still exists although much information about ledo pH meter FE 20. 873 Journal of Chemical Technology and Metallurgy, 53, 5, 2018 The cyanate in the solution was analyzed by pre- cipitation as AgOCN [28]. For this purpose a solution of 0.1 M AgNO3 was added in excess to the sample for analysis. The hydrogen ion concentration was adjusted to pH 5.0 by adding dilute HNO3. pH meter was used. Under these conditions, all the silver carbonate (and carbamate, if any) was dissolved, while silver cyanate remained. The suspension was kept for 1 h at 278 K for decreasing the solubility of AgOCN. The precipitated silver cyanate was filtered off with Gooch crucible G4, washed 2 - 3 times with cold distilled water, dried for 1 h at 373 K, cooled in a desiccator and weighed. A spectrophotometric method for the determina- tion of urea concentration in the solution, described in Knorst’s work [29], was applied. The method was based on the development of a yellow color when an ethanolic Fig. 1. pH of 0.064 M ammonium cyanate solution as a solution of p-dimethylaminobenzaldehyde with dilute function of time at 298.15 K. mineral acid was added to urea. The concentrations of NH4HCO3 and (NH4)2CO3 in the solution were determined by the direct alkalinity titration method [30]. The hydroxide alkalinity (OH-) was negligible in all analyzed samples. The amount of (NH4)2CO3 was determined by titration with 0.01 M HCl to pH of 8.3. The end point was detected with phenol- phthalein. The amount of NH4HCO3 was determined by S/cm further titration to pH of 4.5. The end point was detected s, m with mixed bromcresol green-methyl red indicator. RESULTS AND DISCUSSION The experimental data are presented in a graphic mode in Figs. 1 - 7. Fig. 1 illustrates pH change in the solution within the 90-day period at 298.15 K. The pH Fig. 2. Electrical conductivity of 0.064 M ammonium value increases gradually within the first 20 days fol- cyanate solution as a function of time at 298.15 K.
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