Radiolysis of Hydrogen Cyanide in an Aqueous System*, (III)

Transcription

1 Radiolysis of Hydrogen Cyanide in an Aqueous System*, (III) ph Dependence of Radiolysis Hajime OGURA**, Takashi FUJIMURA**, Sohichi MUROZONO**, Katsuhiko HIRANO** and Masaharu KONDO** Received October 25, 1971 polymeric HCN is inhibited by decreasing ph. This can be explained as the result of the degradation from dimeric to monomeric form undergone by HCN, which is known to be an intermediate compound. Based on experiments on the effect of adding Cl- as OH radical scavenger, it is considered that formaldehyde and a part of ammonia are produced through methylenimine as intermediate. The radiolysis of aqueous KCN is also studied for making a comparison of the radiolysis behavior between cyanide ions and undissociated HCN. I. INTRODUCTION In the past ten years, many studies have been successfully carried out on the determination of the kinetic behavior and radiolytic yield in the radiolysis of many compounds in aqueous solution. However little attention has been paid to the radiolysis of HCN in aqueous solution in spite of its high reactivity observed in many chemical reactions and its importance as an intermediate compound in the chemical evolution taking place in the prebiotic earth, in which ionizing radiation is presumed to participate to a great extent(1)(2). This situation has urged us to study the radiolysis of this compound. The present work concerns the ph dependence of radiolysis and the reaction mechanism leading to the formation of formalde- has been well confirmed. Since CN- was found to interfere with the determination of ammonia by aqueous HCN, and it constitutes an extention of our earlier studies on the radiolysis of HCN in aqueous system(3)(4).. IIEXPERIMENTAL The general procedure for the preparation of the sample have been described previously(3)(4). Gamma irradiation was carried out with a 60Co on Fricke dosimetry, taking 15.5 for G(Fe3+). All solutions were irradiated under deaerated state. The methods used for the determination of the H2O2, H2 produced and of the HCN, KCN radiolyzed have been described previously(3). The use of chromotropic acid yielded good results for the determination of formaldehyde without any interference of CN- when acidic solutions (ph<3) were irradiated. A mixture of 1ml aliquot of irradiated solution and 3ml of H2SO4 containing 0.3% chromotropic acid was kept in a boiling water bath for exactly 30min. After the solution was cooled to room temperature, H2SO4 was added to the 5ml mark. The resulting colored solution was applied to formaldehyde in the present experiment means of Nessler reagent, Conway's microdiffusion technique(5) was applied to separate ammonia from the irradiated solutions containing cyanide. One half ml of Nessler reagent was added to the same volume of diffusate, and the total volume of the solution was kept at 5ml by adding a 1.25% * Previous articles are published in Refs. (3) and (4). ** Department of Chemistry, Faculty of Science, Tokyo Metropolitan University, Setagaya-ku, Tokyo.

2 J. Nucl. Sci. Technol., NaOH aqueous solution. The resulting colored 20min. The analysis of CO2 from irradiated solution was carried out by a setup (Hitachi-Horiba Ltd.) for CO2 analysis III. using IR absorption. RESULTS AND DISCUSSION Figure 1 shows the radiolytic yields of HCN and KCN decomposed as a function of initial concentration of these solutes at various ph. The G values were obtained from the slope of the yield-dose plots presented in Fig. 2, which illustrates the decrease in solute concentration with dose in KCN and HCN solutions. The ph value the dissociation of HCN into CN- is negligible in the solution at natural ph. The value of G(-HCN) appears to decrease with decreasing ph of the solution, and is relatively insensitive to the initial concentration of HCN in the ph In order to elucidate the kinetic behavior of cyanide ions as compared with those of HCN, a deaerated solution of KCN was irradiated, in which CN- was in large excess over HCN. A value of 4.1 was observed for G(-CN-), and which was almost independent of the initial concentration of KCN in the range of concentration below 10mM, as shown in Fig. 1. As seen in Fig. 2, G(-CN-) increases by the presence of 20mm N2O. The observed increase in G(-CN-) is about 3, which is very close to, G-eaq or G(N2) in the radiolysis of deaerated water containing N2O. The present finding indicates that OH radicals resulting from the following reaction may react with CN-, giving a higher G(-CN-) of nearly 7, in the presence of N2O. Fig. 1 Dependence of radiolytic yields of HCN and KCN on initial concentration Fig. 2 Yield-dose plot of KCN and HCN solutions If the reaction of e-aq with CN- in the irradiated aqueous KCN solution is primarily responsible for the radiolysis of CN-, the observed increase of G(-CN-) due to the presence of N2O can not be explained. The radiolytic yield of hydrogen, G(H2), from aqueous KCN solution was found to be about 0.4, which appears very close to the molecular yield of hydrogen from irradiated water. This finding indicates that e-aq will react with some intermediates produced during the course of radiolysis; otherwise, the observed G(H2) should have to be expected to be much higher than that of GH2, the molecular yield of hydrogen, due to the recombination of hydrated electrons. Thus there is the possibility of reaction of an OH radical with CN- which leads to the radiolysis of CN- as in reaction (2) below. It should be quite evident from Fig. 2 that the HCN molecule reacts with e-aq as well as with OH, as observed in the previous work(4). The reaction(2) below is of a form of electron transfer in aqueous solution, and would appear quite similar to that of OH with halide ions or with thiocyanate ions in the same medium:

3 Vol. 9, No. 6 (June 1972) This reaction is also plausible from thermochemical considerations. If the hydration energies of the two radicals are assumed to be the same, then kcal/mole(6). Trumbore et al.(7) have shown that the rate of reaction of the OH radical with CN- when using p-nitrosodimethylaniline, a specific OH radical scavenger. And it is also known that the reaction of e-aq with CN- is not so important(8). These data provide a clear explanation for our present results. As to the fate of CN radical from reaction (2), one can only speculate that it may react with CN- to form a radical anion, (CN)2-, for which a some evidence is provided from pulse radiolysis to form (CN)2-, which is of isoelectronic structure with these dimer anions. As was reported in the previous work(4), the HCN (6mM) yielded mainly ammonia (G=1.9), formaldehyde (G=1.1), carbon dioxide (G=0.8) and polymeric products (G(-HCN, as polymeric following material balances have been observed to hold with these products: In Fig. 3 are shown as function of ph, the yields of major radiolytic products and of HCN decomposed in the irradiated solution of 6mM HCN. It seen from Fig. 3 that at lower values of ph below 3, G(-HCN) appears insensitive to differences in ph, but beyond ph 3 it increases with ph to attain a plateau value (nearly 5.8). The UV absorption due to the polymeric products from irradiated aqueous HCN is markedly lowered by decreasing ph over the whole range of wave lengths investigated, as shown in Fig. 4(A). On the contrary, other product yields are scarcely affected by ph of the irradiated solution. One Fig. 3 Dependence of radiolysis on ph in 6mM HCN can thus explain the observed decrease in G(-HCN) with ph of acidic solution in terms of the material balance (I) along with the present findings. In KCN solution, an absorption peak appears the case of HCN solution (Fig. 4(A)). Ammonia (G=2.3) and CO2 (G=1.1) are observed as major radiolytic products in irradiated aqueous KCN, while formaldehyde is detected only in trace amount*. Although a complete analysis of the radiolytic products from irradiated aqueous KCN has not yet been carried out, one may reasonably expect a different pattern of radiolytic products from that of aqueous HCN because the major substrate is CN- in the KCN solution. * Since cyanohydrin resulting from the reaction of CN- with formaldehyde in the present KCN solution dissociates into CN- and HCHO in alkaline medium, formaldehyde can be determined by chromotropic acid.

4 J. Nucl. Sci. Technol., As observed in Fig. 4, the radiolytic yields of polymeric product decrease with decreasing ph of the solution, while those of the low molecular products are little affected by ph. In order to gain further information on the radiolysis of aqueous HCN, the following experiments were carried out using Cl- as a potential scavenger of OH. Figure 5 shows the yields of several products and of HCN decomposed, as a function of Cl- concentration in 6 mm HCN acidified by H2SO4 (ph=2). It is seen that the addition of Cl- decreases G(NH3), G(CO2) and G(-HCN) but does not affect G(HCHO). The observed radiolytic yields of H2O2 and H2 in 6mm HCN in ph=2 are found to be equal to the molecular yield of the respective species from irradiated water up 10-2M of Cl-. (4). The results presented in Fig. 3 show that product yields are almost independent of ph in polymeric products decreases with decreasing ph, as shown in Fig. 4. This finding indicates that in the presence of H2SO4 the degradation of dimeric HCN* into HCN molecules takes place more predominantly, as in (9) below, than the formation of more stable polymeric products as in (8 ), to which UV absorption is attributed. Thus, the observed G(-HCN) and UV absorbance in the presence of acid are seen to be lower than those observed in neutral solution. It is generally known that the primary radical yields from irradiated water increase with hydrogen ion concentration (ph<3). Such being the case, one can expect that the product yields increase with decreasing ph of the solution in conformity with the general reaction mechanism described above. Our present result shows very little dependence of ph on the product yields. While a definitive explanation for the present result cannot yet be presented, it might be surmised that H2CNH and HOCN, which are considered to be the precursors of final radiolytic products, are formed through reaction of H2O with (HCN)2 appearing in reaction (6a): Fig. 5 Variation of yields as function of Clconcentration on radiolysis of 6mM HCN at ph=2 The following sequence of reactions has been previously presented as a general mechanism of the radiolysis of aqueous HCN at ph ca. 5. 1H+ Reaction (11) will compete with reaction (5) for OH radicals, which serves to lower G(-HCN) to a steady value of 1.7 at 10-2M Cl-, where almost all OH radicals are scavenged and the H(OH)CN radical is no longer important. The observed In acidic solution in the range of interest for the present study (ph<3), all hydrated electrons by any investigator, polymerization of HCN has been usually considered to propagate via successive reaction (3) can be ignored relative to reaction In acidic solution, reaction (9) is likely to occur in parallel with reaction (10), giving no net increase in the yields of the final products despite the increase in the yields of radicals initially produced from the radiolyzed water. By addition of Cl- to the system at ph=2, the following reaction is most likely to occur(11) and the resulting Cl atoms become Cl2- at higher concentrations of Cl-. H+ Cl- * Though dimeric HCN has not yet been isolated

5 Vol. 9, No. 6 (June 1972) decrease of G(-HCN) and of the product yields with increase of Cl- concentration can thus be explained. The H2CN radical will disproportionate to form methylenimine, H2CNH, and HCN, as in reaction (12). The former may be hydrolyzed to form HCHO and NH3 as final products: mainn interest of our present work is in its con- tribution to the study of the effects of ionizing radiations in the chemical evolution. Moore et al.(12) have provided evidence of H2CNH as an intermediate produced by photolysis of ACKNOWLEDGMENT diazomethane dimer in solid nitrogen matrix at very low temperatures. If reaction (12) represents We express our sincere thanks to Dr. T. Masuda the sole fate of H2CN radical, then one should of Tokyo Metropolitan University for his deep find G(-HCN) equal to G(HCHO) or G(NH3) interest throughout this work. We are also indebted to Mr. K. Kurita for his collaboration in and to 1/2GH. The steady value for G(-HCN) the preliminary stages of this work. `1/2GH=1.7 from deaerated water at ph=2, as REFERENCES seen in Fig. 5. However G(HCHO) and G(NH3) are all smaller than G(-HCN), which indicates that the disappearance of the H2CN radical has been brought about by a procedure other than reaction (12) and should therefore give rise to other products. A possible product among such (3) OGURA, H.: J. Radiat. Res., 8, 93 (1967). is polymeric HCN, because there was observed (4) OGURA, H.: Bull. Chem. Soc. Jap., 41, 2871 (1968). in the present case an absorption in the UV region similar to the case of Cl- free solution. As for the participation of the Cl2- radical in the present system, the following reaction with HCN or H2CN radical can be expected: acids(1)(2). Our results have well demonstrated that HCHO and NH3 are the main radiolytic ing to the Strecker mechanism, amino acids are synthesized from these radiolytic compounds. The (5) CONWAY, E. J.: "Microdiffusion Analysis and Volumetric Error", (Rev. ed.), (1947), Crosby Lockwood. (6) AIREY, P. L., DAINTON, F. S.: Proc. Roy. Soc. (London), Ser. A, 291, 340 (1966). (7) KRALJIC, I., TRUMBORE, C. N.: J. Amer. Chem. Soc., 87, 2547 (1965). (8) ANBAR, M.: Quart. Rev., 22, 578 (1968). In the case where reaction (13) occurs, most CN radicals would react with water, thus regenerating (9) DRAGANIC, I. G., DRAGANIC, Z. D., HOLROYD, R. A. : J. Phys. Chem., 75, 608 (1971). HCN and OH radicals. If reaction (14) occurs, (10) ROOT, K. D. J., SYMONS, M. C. R.: J. Chem. Soc., A, 21 (1968). G(-HCN) will be much lower than the observed (11) ANBAR, M., THOMAS, J. K.: J. Phys. Chem., 68, G(-HCN)=1/2GH=1.7. Moreover, we have seen 3829 (1964). that H2 and H2O2 from irradiated water remained (12) THOMAS, J. K.: Trans. Faraday Soc., 61, 702 in the solution without further reaction. These (1965). results rule out the participation of Cl2- radicals in the radiolysis. The essential feature of the system with Cl- as OH scavenger is the occurrence of the disproportionation reaction of the (14) VOLKER, TH.: Angew. Chem., 72, 379 (1960). H2CN radical, as in reaction (12), giving methyleneimine as the precursor of HCHO and NH3. (15) HUMMEL, D., JANNSEN, O.: Z. Phys. Chem., 31, 111 (1962). (16) MATTHEWS, C. N., MOSER, R. E.: Proc. Nat. Acad. It has already been established that HCN can Sci., 56, 1087 (1966). be obtained from simpler compounds under the (17) SERRE, J., SCHNEIDER, F.: J. Chim. Phys., 61, influence of high energy radiations and constitutes 1655 (1964). an intermediate of primary importance, leading to several primordial organic compounds like amino (18) MOORE, C. B., PIMENTEL, G. C.: 43, 63 (1965). J. Chem. Phys.,