Reduction of Selenious Acid bv Thiocvanic, Acid J
J
WILLIAM T. HALL, Massachusetts Institute of Technology, Cambridge, M a s s .
sc-s-s
R
ECEXTLY Ljung (20) proposed a new qualitative test for selenium. Ammonium thiocyanate is added to the boiling solution which is about 6 N in hydrochloric acid, t o precipitate any selenium present as selenious acid, and it is claimed that one part of selenium can be detected in 20,000,000 parts of solution. The statement is made that there “is an exact stoichiometric relationship between the reacting ions” but it is admitted that the equation given Se03-BCNS6H+ +Se 2CN2s-3Hn0
+
+
+
+
+
does not represent the correct ratio between the weights of thiocyanate and selenite that take part in the reaction. (1) The equation does not balance; the sum of the electron charges on the left is +2 and on the right is -6 which means that eight electrons have been created, whereas in a properly balanced ionic equation the sum of the charges on the right must equal the sum of the charges on the left. (2) The reaction represents a reduction of quadrivalent selenium to the neutral state. If the thiocyanate ion accomplishes this reduction it must itself undergo oxidation; a reduction can never take place without a n equivalent oxidation. The formation of CiY- and S-- from CNS- cannot be regarded as representing an oxidation. (3) Since the reaction takes place in the presence of a large excess of hydrochloric acid, it is impossible that an appreciable quantity of either CN- or S-can be formed, for in acid solutions these ions will unite with H+ to form undissociated hydrocyanic acid and hydrogen sulfide. A study of the literature shows that Ivanov (10) used thiocyanate for the quantitative precipitation of selenium from acid solutions. He mixed together fairly strong solutions of selenious acid and ammonium thiocyanate and then, upon the addition of considerable hydrochloric acid, a yellow compound was precipitated which proved to be (HCSS)z H2SeO3. The compound was fairly stable in the cold and Ivanov was able to establish its constitution by analysis. H e was unable to prepare a similar compound with tellurium and thought that the reaction might be used to separate selenium from tellurium. As decomposition took place in hot solutions, Ivanov diluted the solution, heated it for 12 hours on the water bath, and allowed it to stand overnight. Under these conditions, he was able to accomplish complete precipitation of all selenium (0.1 to 0.4 gram) but the precipitates were always contaminated with sulfur. Ivanov’s conditions were very different from those recommended by Ljung. Ljung boiled his solutions only a few minutes, a t a slightly higher temperature than Ivanov. Ivanov did not attempt to write a n equation to represent this decomposition. I t is difficult to do this because thiocyanic acid itself, in the presence of hot hydrochloric acid, is very unstable. The instability of thiocyanic acid has been known for a long time. Wohler (38) was probably the first to attempt to prepare pure thiocyanic acid and to recognize its instability. Some of its decomposition products were studied in the laboratory of Wohler’s friend, Liebig (19). Since then the literature on thiocyanic acid and related compounds has become very voluminous, but excellent papers have been written by Volckel (S6), Klason ( I C ) , and Stokes and Cain (36). Wohler (38) found that thiocyanic acid is a colorless liquid at ordinary temperatures but decomposes on standing, with the formation of a yellow substance which is now recognized as isoperthionic acid, HzCzNzS3,and given the following structure:
Hrl.-c=xH
I
According to statements in the literature (l4,36) the following substances are also likely to result from the decomposition of thiocyanic acid: hydrocyanic acid, formic acid, carbon dioxide, carbonyl sulfide, carbon disulfide, hydrogen sulfide, and ammonium salts. The action of mineral acid on a thiocyanate is also likely to cause the formation of HSCSNHz,
/CS-NHz S \CS-NH?
S-CS-NH,
’
and
$--CS--NH2
The behavior of thiocyanic acid toward oxidizing agents has been the subject of numerous papers and many quantitative methods of analysis have been based on the formation of thiocyanates and subsequent oxidation of the thiocyanic acid. Thus, potassium permanganate in acid solutions oxides thiocyanic acid to sulfuric and hydrocyanic acids. This reaction appears to have been discovered by Erlenmeyer ( 5 )but other chemists have studied it (1, 4 , 6 , 7 , 9 ,12,21,86,25, 26, 27,S1, 37) The reaction can be expressed by the equation 5HCXS
+ 6Mn04- + 8H --+ +
5HCN
+ 5S04-- + 6MnTT+ 4H20a
A similar oxidation of thiocyanic acid is the basis of many iodometric methods. Here also the oxidation of thiocyanic acid results in the formation of sulfuric acid from its sulfur and either hydrocyanic acid or a product such as cyanogen iodide from its nitrogen. Thus with potassium iodate in the presence of hydrochloric acid, Jamieson, Levyy,and Wells (11) found the reaction to be 2SCN-
+ 3108- + 4H+ + 3C1-+ 2SO4--
+ 31C1 + 2HCN + HzO?
but Rupp and Schied (29) under different conditions accomplished the oxidation with iodine according to the equation SCN-
+
4x2
+ 8HCOa- + SO*-- + 71- + 4HzO + ICN + 8COn
Similar oxidations have been carried out with bromine, potassium bromate, hypobromite, hypochlorite, and chromic acid ( 2 , S, 6, 8,13, 15, 16, 17,23, 24, 28, SO, SS, S4,35). This by no means exhausts the literature on the oxidation of thiocyanate in methods used in quantitative analysis. During the last thirty years, for example, a t least twenty-five papers have been published on the thiocyanate content of saliva, urine, blood, and other biological fluids. Some were published in journals which are not easily accessible and abstractors often forgot to mention how the analyses were made, but enough were examined to show that in practically every case the sulfur atom of the thiocyanate anion is oxidized to sulfate and in most cases the nitrogen is left as hydrocyanic acid; in some cases the determination is based upon the argentometric titration of the hydrocyanic acid formed. Under other conditions the oxidation of thiocyanic acid does not take place quantitatively in any one direction and the literature describes products called Schyefelcyan, Persulfocyan, Pseudoschwefelcyan, Kanarin, uberschwefelblausaure, Persulfocyansiiure, Dithiocyansaure, Dithiocarbaminsulfid, and Melanin which have been obtained directly or indirectly by the oxidation of thiocyanic acid. In some cases,
INDUSTRIAL AND EXGINEERING CHEMISTRI-
396
the same substance has been described under different names, and the study of these papers is complicated by the fact that in many papers mitten in the nineteenth century, the atomic weights and equivalents were different from those recognized today. As long ago as 1846, the complaint was made that the literature on the decomposition products of thiocyanic acid was confusing and later workers have not been able t o duplicate some of the results (18). During the last six months, several sophomore students in this laboratory have experimented with small quantities of thiocyanic acid and selenious acid in the presence of considerable hydrochloric acid. As in the experiments described by Ljung (to),the solutions were boiled for a short time after the addition of the thiocyanate. The deposited selenium was weighed in nearly every case and futile attempts were made to establish definite relations between the quantities of selenium deposited and of some decomposition product formed. From these experiments some interesting conclusions can be drawn. 1. When potassium thiocyanate is added to boiling 6 N hydrochloric acid containing no selenious acid, decomposition of the thiocyanic acid, formed by the action of hydrochloric acid on the alkali salt, takes place promptly and the principal products appear to be XH4+, hydrogen sulfide, and carbon dioxide. The decomposition can be expressed fairly well by the equation HCNS
+ 2H20 + H +
+ “ I
+ COz + HIS
I n opinion formed before reading the literature, the reaction is not as simple as this equation would indicate. Wohler and Liebig knew this a century or so ago. The author and his associates have never been able t o get approximately one molecule of ammonium salt, one of carbon dioxide, and one of hydrogen sulfide from the decomposition of one molecule of potassium thiocyanate, although in every case they have found these three products. They have been unable to get tests for any appreciable quantity of hydrocyanic acid either by the odor of the distillate or by special tests for hydrocyanic acid, including the argentometric test which has been used for determining the small quantity of thiocyanic acid in saliva on the basis of the hydrocyanic acid formed by treatment with an oxidizer. 2. The reaction between thiocyanic acid and selenious acid varies, as stated by Ljung, a t different concentrations of hydrochloric acid and a t different temperatures. When the thiocyanate and selenious acid solutions are mixed, the first thing noticed is a yellow color, which may be due to the formation of isoperthionic acid (19,38) or to the yellow complex studied by Ivanov ( I O ) . At the boiling temperature, selenium soon begins to precipitate and the reaction is capable of detecting the presence of very small quantities of selenium, but cannot be recommended for the determination of this element. The equation ZH2SeOa
+ HCNS + 2Se + COz + SO4-- + H + + NH4+
shows how the reaction can take place. I n two experiments Robert N. Bonnett obtained 2 Se to 1 SO4--, but in other experiments he obtained quite different results. S. V. Arnold obtained 1.43 Se t o 1 SO4--, and Max Cohen 1.57 Se to 1 801--. Bonnett observed that when the concentration of the hydrochloric acid was changed from 6 N to 8 N only 65 per cent as much SO,-- was formed and some of his precipitates were proved by both physical and chemical examination to contain sulfur. Corroborative experiments were carried out by others, especially W. P. Lamb and R. D. Haworth, Jr. Working under different conditions, Ivanov ( I O ) found that sulfur was always precipitated with the selenium. It is possible to conceive the CNS- anion as containing carbon with a valence of +4, nitrogen with a valence of -3,
VOL. 10,NO. 7
and sulfur with a valence of - 2 . The experiments of the author and his associates show that the principal products of the decomposition of thiocyanic acid are carbon dioxide, in which carbon has a valence of +4, h-H+in which nitrogen has a valence of -3, and hydrogen sulfide in which sulfur has a valence of - 2 . In other words, the spontaneous decomposition of thiocyanic acid from a boiling solution which is 6 N in hydrochloric acid results for the most part in a reaction which is neither an oxidation nor a reduction, because the sum of the valences of carbon, nitrogen, and sulfur is the same in the products as in the original compound. I n the presence of selenious acid, the principal product of the oxidation appears to be sulfuric acid in which the sulfur atom has the valence +6, so that the oxidation is really due to the sulfur atom and a part of the sulfur is sometimes oxidized only to the neutral state. Hydrogen sulfide, which is a product of the spontaneous decomposition of thiocyanic acid, reacts with selenious acid as follows: HzSeOJ
+ 2HzS +25 + Se + 3H20
For quantitative purposes, the reduction of selenious acid by potassium thiocyanate and hydrochloric acid is not satisfactory because it is practically impossible to make the reduction take place in accordance with a single definite equation. The fact that the ammonium ion, but rarely if ever the hydrocyanic acid molecule, is formed when selenious acid is reduced by alkali thiocyanate is of particular interest, but is not surprising because selenium or selenious acid has found extensive use as a catalyst in the Kjeldahl digestion, when it is desired t o convert the nitrogen of an organic compound into ammonium salt.
Literature Cited At, Ber., 32, 3258 (1889). Baumann, Sprinson, and Metrger, J. Biol. Chem., 105, 269-77 (1934). Bicskei, 2. anorg. allgem. Chem., 160, 271-2 (1927). Chelle, Bull. 8oc. pharm. Bordeaux, 1919, 140; J. pharm. chim., 20, 156-7 (1919). Erlenmeyer, Z. Chem., 1859,202. Golse, Bull. SOC, pharm. Bordeaux, 67, 221-5 (1929) : Bull. soc. chim., 47, 655 (1930). Grossman and Holter, Chem.-Ztg., 33, 348 (1909). Hartner, Mkkrochemie, 16, 141-52 (1934). Illarionov, 2. anal. Chem., 87, 26-32 (1931). Ivanov, Chem.-Ztg., 32, 468 (1908). Jamieson, Levy, and Wells, J. Am. Chem. SOC.,30, 760 (1908). Jumeau, Bull. SOC. chim., [3] 9, 346 (1893). Kahane and Coupechoux, Bull. 8oc. chim., [ 5 ] 3, 1588-95 (1936). Klason, J. prakt. Chem., [2] 36,57-64 (1887); 38,383 (1888). Knupffer, 2. phys. Chem., 26, 262 (1898). Konig, J. prakt. Chem., 84, 558-60 (1912). Korenmann and Anbrokh, Zavodskaya Lab., 5, 749 (1936). Laurent and Gebhard, Compt. rend., 22,46 (1846). Liebig, Ann., 10, 1-46 (1834); 50, 339 (1844). Ljung, H. A., IWD. ENCI.CHEM., Anal. Ed., 9, 328 (1937). Masino, Chem.-Ztg., 33, 1173 (1909). Meurice, Ann. chim. anal. chim. appZ., 2, 272-3 (1920). Orella, Anales farm. biopubm. (Buenos Aires), 6, 41-51 (1935). Page1 and Ames, J. Am. Chem. SOC.,52, 2698-2702 (1930). Parr, Ibid., 22, 685 (1900). PeOn de Saint Gilles, Ann. chim. phys., [3] 55,390-1 (1859). Reinitzer and Pollet, 2. anal. Chem., 81, 286-308 (1930). Rupp, Arch. Pharm., 243,458-67 (1905). Rupp and Schied, Ber., 35, 2191-5 (1902). Schulek, Z. anorg. allgem. Chem., 144, 257-62 (1925). Stamm, Angew. Chem., 47, 791-5 (1934). Stokes and Cain, J. Am. Chem. SOC.,29, 443-7 (1907). Swicker, 2. anal. Chem., 77, 278-80 (1929). Thiel, Ber., 35, 2766 (1902). Treadwell and Mayr, 2. anorg. allgem. Chem., 92,127-34 (1915). Volckel, Ann., 43, 74-106 (1842). Volhard, Ibid., 190, l(1878). Wohler, Friedrich, Gilbert’s Annalen, 49, 271 (1824). RECEIVED April 16, 1938,
