Distillation of Hydrocyanic Acid from Sulfuric Acid Solutions

Quantitative Deter mination

S&EL MORRIS AND VIRGIL GREENE LILLY Department of Chemistry, Division of Industrial Sciences, West Virginia University, Morgantown, W. Va.

A G E L a n d C a r l s o n (9) P recently pub-

lished a method for the accurate deter- mination of hydro - c y a n i c ac id in the presence of various salts (including chlo- r ides ) showing that the quantitative dis- tillation of hydro - c y a n i c a c i d f r o m dilute sulfuric ac id solution takes place within 15 m i n u t e s under specified con- ditions. R i c h a r d s a n d S i n g e r ( 1 0 ) claimed t h a t m a n y hours’ boiling (8 to 10) is necessary to re-

Page l a n d C a r l - son’s apparatus was modified as shown in Figure 1.

FIGURE 1. DIAGRAM OF APPARATUS A , 500-ml. Pyrex distilling flask with sealed-in capillary B , long-stemmed funnel through which acid is added C, special Pyrex condenser

D , absorption apparatua E , flowmeter F, tinfoil-covered rubber stopper

move all the hydrocyanic acid from solutions containing ap- preciable amounts of chlorides, the retardation being ascribed to the formation of a complex between the chloride ion and hydrocyanic acid. Richards and Singer found that a much shorter time (less than 2 hours) suffices to remove hydro- cyanic acid from solution in the absence of more than a trace of chloride ion.

METHOD AND 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 calcu- lated 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

ADSORPTION OF HY- DROCYANIC ACID

BY STOPPERS

A q u a n t i t a t i v e comparison was made between determina- tions in wh ich t h e stopper was covered with t i n f o i l and in which it was not, and the results are shown in Table I. The data are averages of two or more determina- tions. With tinfoil- covered stomers the _ _

individual determinations agreed within 0.03 ml.

TABLE I. ABSORPTION OF HYDROCYANIC ACID BY STOPPERS EXPBRI- CYANIDB~ DIBTILLA-

MINT UaBD TION KIND OF STOPPER ERROR Min. %

1 NaCN 15 Rubber -0.14 2 NaCN 30 Rubber -0.12 3 NaCN 15 Rubber cov-

ered with tinfoil -0.02

4 NaCN 15 Cork -0.28 a Sodium cyanide free from ferrocyanide, prepared by distilling hydro-

cyanic acid into sodium hydroxide.

In 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, Calla- ghan, and Dunlap (7). The distillate was tested for hydro- cyanic 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 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 According to Roe (11) simple cyanides cannot be deter- of gas to the burner was regulated by a manometer and a mined by distillation in the presence of complex cyanides. flowmeter was used to regulate the flow of air through the In carrying out these determinations many of the commercial capillary. The same pipet and buret were used throughout. brands of c. P. potassium cyanide were tested for ferrocyanide Fifty milliliters of stock solution were used in all experiments. by the method of Krauch (4). Ferrocyanide was present in The solution in the distilling flask was heated to boiling as all cases tested; in but two cases was the presence admitted quickly as possible. Time of distillation was 15 minutes on the label. According to Sharwood (16) the presence of unless otherwise noted. Rate of distillation was 75 ml. per ferrocyanide causes high results when Liebig’s method is 15 minutes. Fifteen milliliters of 6 N sulfuric acid were used. This was confirmed by adding potassium ferro- used unless otherwise stated. cyanide to sodium cyanide and titrating in the usual manner.

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408 A N A L Y T I C A L E D I T I O N Vol. 5 , No. 6

Ferrocyanide gave high results in all the cases where i t was present.

Feld (2) reports that the simple cyanides may be deter- mined in the presence of complex cyanides by distilling the mixture with magnesium chloride. The magnesium chloride yields sufficient hydrochloric acid to displace the hydro- cyanic acid from the simple cyanides, but not from the com- plex 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.

TABLE 11. DISTILL~TION WITH VARIOUS REAGENTS E X P ~ R I ~ ~ X I N T CYAN ID^ USZD RBAQ~NT USH~D ERROR

% KCNa KCNb NaCNC NaCNO NaONC KCN" NaCN; NaCN

+0.69 $0.72 -0.02 -0.08 -0.04 -0.22 -0.07 -0.04

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.

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 am- monium formate. In 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. In 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 cor- rection for this hydrolysis, the final error of the determina- tion was f0.008 per cent. In 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 In order to avoid loss of hydrocyanic acid by adsorption,

rubber stoppers should be covered with tinfoil, or ground- glass 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 dis- tillate. In order to avoid excessive hydrolysis, the con- centration 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

University of Iowa, Iowa City,

Iowa

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. Un- protected rubber stoppers and corks were found to adsorb hydrocyanic acid and very slowly give it up again. Ferro- cyanide has been found to be a common impurity in com- mercial 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 stop- pers, 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,

(4) Krauch, C., and Merok, E., "Chemical Reagents, Their Purity

(5) Krieble, V. K., and Peiker, A. L., J. Am. Chem. Soc., 55, 2326-

(6) Liebig, J. von, Ann., 77, 102-7 (1851). (7) Morris, S., Callaghan, E. B., and Dunlap, L., J. Am, Chem.

(8) Pagel, H. A., private communication. (9) Pagel, H. A., and Carlson, W., J . Ana. Chem. Soc., 54, 4487-9

(10) Richards, T. W., and Singer, 9. K., Am. Chem. J., 27, 205-9

(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.

80-1 (1933).

and Tests," p. 231, Van Nostrand. 1907.

31 (1933).

SOC., 52, 2415-17 (1930).

(1932).

(1902).

A Convenient Weighing Buret JACOB CORNOG AND ROBERT CORNOG

The accompany- ing illustration repre- sents the cross section of a weighing buret that has been found to be of good design and c o n v e n i e n t to use. Such a b u r e t , hav- ing a graduated barrel, may be made from an old-buret . RECIIVH~D August 26, 1933.

Y LO cm.

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