Distillation of Hydrocyanic Acid from Sulfuric Acid Solutions Quantitative Deter mination S&EL MORRISAND VIRGILGREENELILLY Department of Chemistry, Division of Industrial Sciences, West Virginia University, Morgantown, W. Va.
P
AGEL and Pagel and CarlC a r l s o n (9) son’s apparatus w a s recently pubmodified as shown in lished a method for Figure 1. the accurate determination of h y d r o ADSORPTION OF HYc y a n i c a c i d in the DROCYANIC ACID presence of various BY STOPPERS salts (including chlor i d e s ) showing that A quantitative the quantitative discomparison was made tillation of h y d r o between determinacyanic acid from tions in w h i c h t h e dilute sulfuric a c i d stopper was covered solution takes place with t i n f o i l and in within 15 m i n u t e s which it was not, and under specified conthe results are shown ditions. R i c h a r d s in Table I. The data FIGURE 1. DIAGRAM OF APPARATUS and Singer (10) are averages of two A , 500-ml. Pyrex distilling flask with sealed-in capillary D , absorption apparatua claimed t h a t m a n y or more determinaB , long-stemmed funnel through which acid is added E , flowmeter hours’ boiling (8 to C, special Pyrex condenser F,tinfoil-covered rubber stopper tions. With tinfoil10) is necessary to recovered stomers the move all the hydrocyanic acid from solutions containing ap- individual determinations agreed within 0.03 ml. preciable amounts of chlorides, the retardation being ascribed OF HYDROCYANIC ACID BY STOPPERS to the formation of a complex between the chloride ion and TABLEI. ABSORPTION EXPBRI- C Y A N I D B ~ DIBTILLAhydrocyanic acid. Richards and Singer found that a much MINT UaBD TION KINDOF STOPPER ERROR shorter time (less than 2 hours) suffices to remove hydroMin. % cyanic acid from solution in the absence of more than a trace 1 NaCN 15 Rubber -0.14 NaCN 30 Rubber -0.12 2 of chloride ion. 3 NaCN 15 Rubber cov-
__
METHODAND APPARATUS When an attempt was made to determine hydrocyanic acid quantitatively by the method of Pagel and Carlson (9) certain difficulties were encountered which were traced to adsorption of hydrocyanic acid in the rubber stopper of the distilling flask and to the presence of ferrocyanide in the cyanides used. The optimum conditions reported in their paper were maintained. The excess sulfuric acid was calculated on the basis of the amount of cyanide used and was 0.35 N (8). Stock solutions of approximately 0.1 N alkali cyanides were used. The cyanide content was determined according to Liebig’s method (6),with potassium iodide as the indicator. Titers of the stock solution were determined just before use. An enhanced Tyndall effect was obtained by using a focusing arc light (1). To insure the same conditions in all experiments the flow of gas to the burner was regulated by a manometer and a flowmeter was used to regulate the flow of air through the capillary. The same pipet and buret were used throughout. Fifty milliliters of stock solution were used in all experiments. The solution in the distilling flask was heated to boiling as quickly as possible. Time of distillation was 15 minutes unless otherwise noted. Rate of distillation was 75 ml. per 15 minutes. Fifteen milliliters of 6 N sulfuric acid were used unless otherwise stated.
ered with tinfoil -0.02 4 NaCN 15 Cork -0.28 a Sodium cyanide free from ferrocyanide, prepared by distilling hydrocyanic acid into sodium hydroxide.
I n two runs, when the stoppers were not protected by tinfoil, distillation was continued after the initial 15 minutes. Cuts were taken a t the end of 30 and 45 minutes, and were redistilled once according to the procedure of Morris, Callaghan, and Dunlap (7). The distillate was tested for hydrocyanic acid by the Prussian blue method of Viehoever and Jones (18). All cuts gave the Prussian blue test, showing that adsorbed hydrocyanic acid was given up by the rubber stoppers.
INTERFERENCE OF FERROCYANIDE According to Roe (11) simple cyanides cannot be determined by distillation in the presence of complex cyanides. I n carrying out these determinations many of the commercial brands of c. P. potassium cyanide were tested for ferrocyanide by the method of Krauch (4). Ferrocyanide was present in all cases tested; in but two cases was the presence admitted on the label. According to Sharwood (16)the presence of ferrocyanide causes high results when Liebig’s method is used. This was confirmed by adding potassium ferrocyanide to sodium cyanide and titrating in the usual manner.
407
ANALYTICAL
408
Ferrocyanide gave high results in all the cases where i t was present. Feld (2) reports that the simple cyanides may be determined in the presence of complex cyanides by distilling the mixture with magnesium chloride. The magnesium chloride yields sufficient hydrochloric acid to displace the hydrocyanic acid from the simple cyanides, but not from the complex cyanides. Tartaric acid is frequently recommended in cyanide distillations, especially in forensic examinations. The results obtained with these various reagents upon different cyanides are reported in Table 11. TABLE11. DISTILL~TION WITH E X P ~ R I ~ ~ X CYAN I N T ID^ USZD
VARIOUS
REAGENTS
R B A Q ~ USH~D NT
KCNa KCNb NaCNC NaCNO NaONC KCN" NaCN; NaCN a Well-known brand of c. P. KCN. b Second well-known brand of c. P. KCN. C Ferrocyanide-free sodium cyanide. d 60 ml. of 3 N MgCla. e 16 ml. of 3 N MgCIz. I 16 ml. of 6 N tartaric acid. u 1 to 6 grams of NaCl were added during these runs.
ERROR % +0.69 $0.72 -0.02 -0.08
-0.04 -0.22 -0.07 -0.04
The data are averages of two or more determinations. Tinfoil-covered stoppers were used throughout.
HYDROLYSIS OF HYDROCYANIC ACID It will be noted that the error in all the determinations on ferrocyanide-free sodium cyanide is a negative one. Part of this loss is due to hydrolysis of hydrocyanic acid to ammonium formate. I n a few cases the amount of hydrolysis during distillation was determined by a modification of the method of Gales and Pensa (3). After distilling off the hydrocyanic acid for a stated time (15 minutes) sodium hydroxide solution was added to the distilling flask and the ammonia was distilled off and determined in the distillate by nesslerization. A blank determination was made on the reagents and this value subtracted from the total amount of ammonia found. It is impossible, of course, to determine the amount of ammonia present in the cyanide solution directly by nesslerization; so a freshly prepared solution of sodium cyanide was used. I n one case the amount of hydrolysis under the usual conditions was determined. The average error of four determinations was -0.06 per cent. An average of 0.04 mg. of nitrogen was found in the distillate after the addition of the sodium hydroxide. Making correction for this hydrolysis, the final error of the determination was f0.008 per cent. I n another series (error -0.02 per cent) after correction for hydrolysis the final error was +0.04 per cent. Since Krieble and Peiker (6) have shown that the rate of hydrolysis is dependent upon the acid concentration, it is advantageous to keep the concentration of the acid low. RECOMMENDED PROCEDURE I n order to avoid loss of hydrocyanic acid by adsorption, rubber stoppers should be covered with tinfoil, or groundglass joints should be used. By modifying the condenser as shown in Figure 1 a good joint can be made with heavy rubber tubing, which exposes very little rubber to the distillate. I n order to avoid excessive hydrolysis, the concentration of acid should be kept low; the excess acid should be kept within 0.35 N . Either sulfuric or tartaric acid may be used to liberate the hydrocyanic acid. Fifteen minutes' distillation, when the total volume is 250 ml., is sufficient to remove all the hydrocyanic acid from solution when the rate
Vol. 5 , No. 6
EDITION
of distillation is 300 ml. per hour. A slight vacuum should be maintained on the distilling flask at all times. SUMMARY No evidence was found that the distillation of hydrocyanic acid from sulfuric acid solutions was retarded by the presence of the chloride ion; this supports the findings of Pagel and Carlson rather than those of Richards and Singer. It was found that the removal of hydrocyanic acid was rapid and complete when stoppers were protected with tinfoil. Unprotected rubber stoppers and corks were found to adsorb hydrocyanic acid and very slowly give it up again. Ferrocyanide has been found to be a common impurity in commercial c. P. alkali cyanides; and when such cyanides are determined by distillation they give high results, owing to the decomposition of ferrocyanide. The method of Feld permits the determination of simple cyanides in the presence of complex cyanides, but the results are rather low. With ferrocyanide-free cyanides and using protected rubber stoppers, the method of Pagel and Carlson is accurate within 0.05 per cent. The rate of hydrolysis of hydrocyanic acid is a function of acid concentration. LITERATURE CITED (1) Cupples, H. L., IND. ENG.CHEN., Anal. Ed., 5, 50-2 (1933). (2) Feld, W., Chem. Zentr., 1903,11, 1398-1400. (3) Gales, N., and Pensa, A. J., IND. ENQ.CHEM.,Anal. Ed., 5, 80-1 (1933). (4) Krauch, C., and Merok, E., "Chemical Reagents, Their Purity and Tests," p. 231, Van Nostrand. 1907. (5) Krieble, V. K., and Peiker, A. L., J. Am. Chem. Soc., 55, 232631 (1933). (6) Liebig, J. von, Ann., 77, 102-7 (1851). (7) Morris, S., Callaghan, E. B., and Dunlap, L., J. Am, Chem. SOC.,52, 2415-17 (1930). (8) Pagel, H. A., private communication. (9) Pagel, H. A., and Carlson, W., J . Ana. Chem. Soc., 54, 4487-9 (1932). (10) Richards, T. W., and Singer, 9. K., Am. Chem. J., 27, 205-9 (1902). (11) Roe, J., J. Am. Chem. SOC.,45, 1878-83 (1923). (12) Sharwood, W. J., Ibid., 19,400-35 (1897). (13) Viehoaver, A.. and Jones, C. O., Ibid., 37, 601-7 (1915). RECEIYBD June 23, 1933. Contribution 91, Department of Chemistry, Division of Industrial Sciences, West Virginia University.
A Convenient Weighing Buret JACOB CORNOG AND ROBERT CORNOG University of Iowa, Iowa City, Iowa The a c c o m p a n y ing illustration represents the cross section of a w e i g h i n g buret that has been found to be of good design and c o n v e n i e n t to use. S u c h a b u r e t , having a graduated barrel, may be made from an old-buret
.
RECIIVH~D August 26, 1933.
Y
A
LO cm.
