THE HYDROLYSIS OF SODIUM CYANIDE. 1057 XCIII.-The Hydrolysis of Sodium Cyanide. By FREDERICK PALLISER WORLEY and VERE ROCHELLE BROWNE. IN the extraction of gold by the cyanide process the degree of hydrolysis of the sodium cyanide a t the dilution employed is con-siderable and any investigation of the chemical actions which occur in this process involves an accurate knowledge of the degree of hydrolysis of the cyanide in solutions of different concentra-tions. I n an investigation of the chemical actions occurring in the dissolution of gold by solutions of sodium cyanide the results of which will be published in a subsequent pap”’ it has been shown that the degree of hydrolysis of the cyanide is a most important factor in the rate of dissolution of the gold. The present paper deals with the degree of hydrolysis of sodium cyanide in solutions of widely different concentrations a t tempera-tures from Oo t o 30°.The method employed is so simple that it would form a useful exercise for students of physical cheniistry. It consists in com-paring the concentrations of hydrogen cyanide vapour above solutions of sodium cyanide and of hydrocyanic acid by drawing the vapour through a solution containing 0.2 per cent. of picric acid and 2 per cent. of sodium carbonate. The intensity of the reddish-brown colour produced in the indicator solution varies VOL. 0x1. u 1058 WORLEY AND BROWNE : rapidly with the concentration of the hydrogen cyanide in the vapour and by comparing the colours produced when a stream of air passes through a series of solutions of sodium cyanide of one concentration alternating with solutions of hydrocyanic acid of different concentrations the &strength of hydrogen cyanide solu-tion can be found which has the same hydrogen cyanide vapour pressure as the sodium cyanide solution.E X P E R I M E N TAL. The sodium cyanide was freshly prepared and in order to ascertain that no appreciable amount of free sodium hydroxide was present a sample was dissolved in water and titrated for cyanide with a standard solution of silver nitrate and for alkali with standard hydrochloric acid using methyl-orange as indicator. The hydrocyanic acid was prepared immediately before us0 and FIG. 1. 9 i NkOH NCN Indicator --of the strength required by adding an equivalent amount of hydrochloric acid to a solution of sodium cyanide.I n each experiment t,wo concentrations of hydrocyanic acid were employed one of which had a higher hydrogen cyanide vapour pressure than the hydrocyanic acid solution .and the other a lower pressure. The solutions of cyanide and of hydrocyanic acid were contained in 300 C.C. flasks and the indicator solutions in test-tubes 10 C.C. of the indicator solutions being used and about 100 C.C. of the other solutions. The flasks and test-tubes were arranged as in Fig. 1 a test-tube containing sodium hydr-oxide solution being placed a t the beginning in order to remove carbon dioxide. I n a preliminary experiment, the diff ereiice in concentration between the two acid solutions was considerable thereby giving an indication of the approximate strength required in order t THE HYDROLYSIS OF SODIUM CYANIDE.1059 match the cyanide solution. In subsequent experiments the difference in concentration could be made very much smaller still allowing the intensity of colour produced by the cyanide to be intermediate between those produced by the two acid solutions or to coincide with one of them. Thus the concentration of hydrocyanic acid which had the same hydrogen cyanide pressure as the cyanide solution was determined within very narrow limits. Air was drawn through the flasks a t the rate of about one bubble per second. A t the higher temperatures the apparatus was kept in a constant-temperature air-chamber electrically heated and electric-ally controlled and a t the lower temperatures in water a t constant temperature or in melting ice.The temperatures employed were Oo 5O loo 1 5 O 20° 25 and 30°. Possible Sources of Error.-(l) By placing two indicator tubes together in series it was found that the whole of the hydrogen cyanide was absorbed in the first no perceptible change of colour being produced in the second. Thus one indicator tube after each flask was sufficient. (2) The amount of hydrogen cyanide removed from the solu-tion was found to be too small to affect the concentration or degree of hydrolysis of the solution. This was shown by measuring the degree of hydrolysis of the same solution of cyanide a second time, using fresh solutions of hydrocyanic acid the result in the second case being the same as in the first.(3) The possibility of error due ta the slight pressure gradient in the series of flasks was avoided by placing the cyanide solution between the two acid solutions. By reversing the order of the two acid solutions placing the stronger after the cyanide in one experiment' and before it in another (the same strength of solu-tion being used) no perceptible difference was observed showing that the slight pressure gradient was negligible. (4) I n order to ascertain whether the effect of the non-hydro lysed sodium cyanide on the vapour pressure of the free hydrogen cyanide was of importance experiments were carried out in which the hydi.ogen cyanide vapour pressure of hydrocyanic acid solu-tions containing sodium chloride was compared with that of solu-tions containing no salt.The presence of even large quantities of salt had no perceptible effect on the amount of hydrogen cyanide removed by the air and i t was consequently judged that the effect of the sodium cyanide was negligible. Results.-From the equation expressing the hydrolysis of sodium cyanide, NaCN + H,O NaOH + HCN, u u 1060 WORLEY AND BROWNE : it is obvious that in the presence of a large excess of water the relationship between the concentrations of the various compounds should be expressed by the mass-action equation “aOHICHCN1 _hT. LHCNI and also that this relationship should hold in the presence of excess of either sodium hydroxide or of hydrocyanic acid. I n the P I G . 2. Cube roots of concentrations. absence of excess of alkali or acid if C is the concentration of the total sodium cyanide and P the percentage hydrolysed then p2c - K.Experiments were carried out a t 2 5 O on the hydrolysis of sodium cyanide a t a number of concentrations from 5.2 t o 0.00725 gram-molecules per litre and a t a concentration of 0.0435 in the presence of different amounts of sodium hydroxide and of hydrochloric acid. I n the latter case it is obvious that an excess of hydro-cyaiiic acid equivalent to the hydrochloric acid was liberated. The results of these experiments are shown in tables I and 11. 100(100 - P THE HYDROLYSIS OF SODIUM CYANIDE. 106 1 Gram-mols. of NaCN per litre (C). 5-20 2.60 1.00 0-325 0.163 0.0813 0.0435 0.0290 0.0145 0.0073 TABLE I. Tcrnperature 2 5 O .Percentage hydrolysed P (PI* I< x lo4. (calculated). 0.30 0.47 0.303 0.425 0.48 0.429 0.70 0.49 0.692 1-25 0.50 1.21 1-75 0.49 1-70 2.5 0.50 2.40 3.3 0.49 3.27 4-0 0.48 3.99 5.5 0.47 5-59 7.75 0.47 7-82 Mean 0,484 TABLE 11. 0.0435 Gram-mols. of NaCN per litre NROH per gram - Percentage Gram-mols. of mol. of NaCN. hydrolysed. K X lo4. 1.00 0.11 0.48 0.50 0.223; 0.60 0.25 0.45 0.50 0.01 2.8 0-47 -Gram-mols. of HCI. per gram-innl. of NaCN. 0.4 40.175 0.49 0.3 30.25 0.47 0.2 20.45 0.48 0.1 10.9 0.47 -Mean 0.483 I n the last coluinn of table I is given the percentage of hydro-lysis calculated from the mean value of the constant. It is obvious that there is no perceptible variation in K beyond that due t o experimental error and thatt there is remarkably little difference between the degree of hydrolysis found by experiment and that calculated from the mean value of I .. I n Fig. 2 the curve expresses the calculated degrees of hydro-lysis and the circles the experimental. values the abscissze being the cube roots of the concentrations in order t o produce a flatter curve. The effect of temperature on the value of the constant was found from experiments carried out a t intervals of 5 O from Oo to 30°. Fewer concentrations were used than at 2 5 O a t some temperatures only one concentration being employed. The results are given in table 111 the relationsliip between the tempera 1062 THE HYDROLYSTS OF SODIUM CYANIDE. ture and the mean values of K being shown in the form of a curve in Fig. 3. TABLE 111. C. P. K x 104. (Mean) Temperature. 0" 0.550 0.50 0.14 0.135 - 0.0435 1.75 0.13 -5 0.550 0.53 0.15 0.15 10 0.550 0-54 0.16 0.16 11 0.550 0.60 0.20 0-20 20 0.325 0.27 -0.0435 0.28 0.0145 4.25 0.27 0.27 0.0036 8.0 0.25 - - - - 0.484 25 Table 1 -30 0.325 1.5 0-74 -0.0435 4.0 0.72 0-72 I 0.0145 6.75 0.71 -FIG. 3. -Temperature. From these results it is possible t o calculate the degree of hydrolysis of solutions of sodium cyanide of concentrations up to 5.2N a t any temperature between Oo and 35O. UNIVERSITY COLLEGE, AUCRLANn [Received September 26th 1917.