THE ACTION O F HYDROGEN SULPHIDE OK CHROMATE?. PART L 3 Y H. B. DUNNICLIFF AXD C. L. SON1
“,Vhile studying the action of hydrogen sulphide on potassium chromate, :>otassium dichromate, and chromic acid in aqueous soiution, it was found tkat with potassium chromate the products of the reaction are potassium thiosulphate and pentasulphide, sulphur and chromium hydroxide. When, however, an aqueous solution of chromic acid is treated with excess of hydrogen sulphide, the filtrate is neutrai and the green precipitate which is formed contains the sulphate group. This coordination compound is dowdy 5ydroiysed by water giving chromic hydroxide and sulphuric acid. It is decomposed by alkalies giving chromium hydroxide and a soluble sulphate and by hydrochloric acid giving a chromic salt and sulphuric acid. i n neither case have thionic acids been detected. Further, when potassium dichromate is treated with hydrogen sulphide, a brown precipitate first forms which contains the sulphate group and the filtrate from the incomplete action also contains sulphate. Obviously therefore sulphates can be formed by the action of chromic acid and a dichromate on hydrogen sulphide and the details of these reactions will be given in another communication. Since sulphates are not decomposed by hydrogen sulphide, it appears that the presence of hydroxyl ions presents the formation of sulphate in the action of hydrogen sulphide on potassium chromate. The interpretation of this reaction is the subject of the following report.
Experimental Potassium chromate was recrystallised until free from carbonate and sulphate and analysed by standard methods. Found : Cr = 26.;37C. K = 40.22Yc,C r 0 4 = j9.77c. (K2Cr0,. requires Cr = 26.77%; K = 40.28% and CrOa = 59.74%). h 2 % solution of potassium chromate was used. I t was found necessary to pass sulphuretted hfdrogen, purified by passing successively over iodine and through water, for about 16 hours in order to complete the reaction. The following sequence of colour changes took place :Yellow-dirty yellow-light green-green-dirty green. On standing, there was a green precipitate formed and the supernatant liquid was golden yellow. The reaction is accompanied by marked rise of temperature and the final solution is strongly alkaline. The precipitate was filtered off but, on evaporating the filtrate, a further quantity of precipitate separated. The filtrate from the second precipitate could be concentrated without any deposition of sulphur. The two precipitates consisted of chromium hydroxide mixed with sulphur. KOsulphur acid, possibly present as a basic salt, could be detected. When the precipitate was digested with dilute hydrochloric acid, the chro-
82
H. B . DUNNICLIFF AND C . L. SON1
mium hydroxide went into solution and the sulphur remained and was easily filtered off. The chromium was precipitated as chromium hydroxide and weighed as Cr203. Found Cr = 26.75-26.82%. K2Cr04requires Cr = 2 6 . 7 7 % . The golden yellow, strongly alkaline, filtrate was treated with white lead to remove hydrogen sulphide, and filtered. The solution then contained a considerable amount of potassium carbonate and, on adding barium nitrate to it, a copious precipitate was obtained. When thoroughly washed this precipitate was shown to he free from sulphate and sulphite. The latter was not likely to occur in view of the large excess of sulphur present in the strongly alkaline solution. The filtrate was, carefully neutralised and tested for sulphur acids in the manner discussed by Dunnicliff and Nijhawan.’ Thiosulphate was present hut none of the thionic acids was detected. The reaction was carried out under different conditions of temperature and concentration, but the end products were always the same : chromium hydroxide contaminated with sulphur was precipitated and, in solution, sulphide, polysulphide and thiosulphate of potassium and colloidal sulphur.
H. Bottger2 states that if sodium pentasulphide is boiled with lead hydroxide, lead sulphide and sodium thiosulphate are formed, and gives this as evidence for stating that the alleged pentasulphide behaves like a mixture of sulphur and sodium monosulphide, lead sulphide and sodium hydroxide being first produced, and sodium hydroxide subsequently reacting with sulphur to give thiosulphate. Hence to avoid the use of white lead for the removal of sulphuretted hydrogen, the excess of hydrogen sulphide was removed by heating the clear filtrate under reduced pressure. In order to obtain a higher concentration of filtrate, 10% solutionsof potassium chromate were used and, to accelerate the completion of the otherwise very slow reaction, the reaction vessel was maintained a t about 80-85OC on a water bath. The liquid was filtered and the filtrate was evaporated under reduced pressure until no further precipitate of chromium hydroxide and sulphur occurred. The liquid was filtered into a distillation flask and evaporation under reduced pressure continued until crystals appeared in the liquor on cooling. Provided these crystals were kept out of air contact they could be redissolved, re-evaporated and recrystallised without the separation of any sulphur. The yellow crystals were dried on a porous plate and were completely soluble. A turbid liquid resulted if they were shaken with water owing to access of oxygen. With dilute acids, a copious evolution of hydrogen sulphide took place and much sulphur separated. All efforts to separate the polysulphide by means of alcohol were unsuccessful. This confirms the observations of W. P. Bloxham3 who states that, “though potassium pentasulphide is soluble in alcohol, it cannot be separated from the thiosulphate by means
* J. Chem. Soc., 128, I ( 1 9 2 6 ) ;Kiirtanacker and Wollok: Z. anorg. allgem. Chem., 161, ZOI;Kurtenarker and Goldbach: 166, 177 (1927). Ann. Suppl., 223, 352 (1884). J. Chem. SOC.,77, 753 (1900).
ACTION OF HYDROGES SULPHIDE ON CBROMATES
83
of this solvent because in the presence of potassium pentasulphide some thiosulphate dissolves.” Attempts were made to find the composition of the mixture of polysulphide and thiosulphate which separates in the crystalline form though, in view of the work of Kuster and Heberlein’, its appeared probable that the nature of the product would depend upon experimental conditions such as temperature, concentration and time. W. P. Bloxham states that “hydrogen sulphide rapidly decomposes polysulphides, if passed through a cold solution, sulphur being copiously deposited and potassium hydrosulphide formed. If, however, hydrogen sulphide is passed through a hot solution of a polysulphide, no deposition of sulphur is observed, but the depth of colour is increased. It is suggested that some potassium hydrosulphide is formed on passage of hydrogen sulphide through a hot solution of K&, and that the liberated sulphur, a t the moment of its separation, is taken up by unaltered K4S9, forming the higher compound K4Slo”. The following are the properties of the crystals obtained from filtrate, which is golden yellow when concentrated and bright yellow when dilute. If the crystals are left in contact with the mother liquor for some time the polysulphide is oxidised to thiosulphate and free sulphur appears. The crystals are hygroscopic and cannot be dried completely. They decompose if complete dehydration is attempted and also on preservation, hydrogen sulphide being evolved. This reaction finally ceases and very old crystals contain no sulphide but much free sulphur. Fresh crystals are very soluble in water giving a clear solution which decomposes potassium chromate giving chromic hydroxide and which is decomposed by salts and dilute acids giving free sulphur. When heated on a platinum loop the crystals burn. DrescheP states that this indicates the presence of a pentasulphide. I n view of the impossibility of drying the product, the following method of analysis was adopted. The crystals were filtered from the mother liquid as far as possible out of air contact and then quickly pressed on a porous plate or between filter paper and the product used for analysis. (a) Total Sulphur was determined by treatment with sodium peroxide and estimation of sulphate in the melt. (b) Total Potassium. A solution of a weighed quantity of the substance was divided into two parts. To one, strong hydrochloric acid was added and the solution boiled in order to coagulate all the sulphur which separated out. The filtrate from this was evaporated to dryness in a platinum dish and the amount of potassium chloride in the dish determined by titration against standard silver nitrate. The corresponding amount of potassium was calculated. (c) Thiosulphate. To the other half of the solution, acetic acid was added. The solution was kept just acid and gently warmed. After filtering off the
* Z. anorg. allegem. Chem., 43,j3 (1905). 2
J. prakt. Chem.,
(2) 4,zo
(1871).
H. B. DUNNICLIFF AND C . L. SOSI
84
sulphur, and removing hydrogen sulphide, the clear filtrate was titrated against standard iodine solution to determine the amount of potassium thiosulphate present. This method was shown to be valid by control experiments. These estimations make it possible to calculate the formula of the polysulphide of potassium, KzS,, present in the crystals (vide Table I, column I). Hydrogen sulphide was passed through six different samples at 8j°C and the filtrate obtained from them was evaporated, crystallised, and analysed separately. The results so obtained are given in Table I. The variations in the results indicate the instability of the crystals. Each result represents the average of two or three determinations.
TABLE I Results of Analysis of Different Samples Analysed immediately after crystallisation Concentration of KgCr04 solution used
a. Total potassium present b. Total sulphur present c. Potassium thiosulphate KzSz03, jHz0 d. Potassium present as thiosulphate (Calculatedfromc) e. Sulphur present as thiosulphate (Calculated from c) f. Potassium present as sulphide = ( a 4 g. Sulphur present as sulphide = (b-e) h. Calculated formula KzS,
2%
5%
10%
15%
Analysed Analysed after 24 after 4 hours days 10%
approx. approx.
approx. approx. approx.
29.40
22.31
23.30
32.02
10%
approx.
20.68 24.50
39.05 27.44 30.98 44.00 27.90 29.51 61.18 j6.34 45.43 67.39 42.38
17.05
15.69 12.65 18.77 11.80 19,j3
13.98 12.87 10.39 Ij,40
12.35 2j.07
70.12
7.61
9.66
13.25
9.69 16.03
8.88
4.97
14.57 20.59 30.60 18.21 13.43
K2S4.95 K2S4.67
KZSS.WI K2S6.24 KBSS.W KZS6.61
Discussion of Results In sample 6, the formula K2S6.6 is obtained for the polysulphide. When crystals of this sample, apparently quite clear, were dissolved in water the liquid became cloudy from the separation of sulphur. From this it is conjectured that some of the sulphur was present in solid solution. It was probably developed by the action of the air on part of the polysulphide present since H. Rose1 showed that all polysulphides take up oxygen from the atmosphere forming thiosulphate and sulphur. 'Pogg. Ann., 17, 327 (1829); 5 5 , 353 (1842).
ACTION O F HYDROGEN SULPHIDE ON CHROMATES
85
+
+
2KzSs 302 = 2K2S203 6 s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (I) This would lead to high values for the thiosulphate and for y in the formula KzS, (Table I). Pentasulphides react with water to give thiosulphate and hydrogen sulphide' KzSj 3H20 = K2Sz03 3 H z S . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (2) This would result in an increase in the relative amount of thiosulphate a t the expense of the polysulphide present without affecting the composition of the resulting sulphide. The crystals obtained smell strongly of hydrogen sulphide and the variation in the thiosulphate content in the final moisture when the formula of the polysulphide is KZSSmay possibly be accounted for in this way. Sample 5 was closed up in a small bottle and may not have suffered oxidation as shown in the above equation (I). The 6th sample was kept in a desiccator which happened to be opened several times during the four days it was kept there. It was not possible to determine the proportions in which thiosulphate and polysulphide are formed. Bloxham (loc. cit.).
+
+
The Mechanism of the Reaction:It has been shown2 that the sulphate ion consists of four oxygen atoms grouped tetrahedrally round a central sulphur atom (S6). If a coordination linkage is represented by the sign, - and a covalent linkage thus, -+ the formula of sulphuric acid becomes:
[
0i(
J.
2 0 ' '
s \r
. . . . .2H*
or
or, Oi(
,OH
S ' O H
[I > O+H’ Cr”
H-0’
\O-+H’
which immediately resolves itself into HO,
/OH
Cr“
HO’
‘OH
a rearrangement involving the reversion of electrons from a valency orbit to one of the inner orbits. Tetravalent chromium or chromium compounds showing a coordination number = 4 have so far not been observed and, even if they existed they would, on decomposition in the presence of a reducing agent, probably yield chromous compounds. If chromous chloride is dissolved in ammonia, the complex [Cr”(NH&]” . . . .2C1‘ is formed. This compound resolves itself spontaneously into the Weinland and Fridrich: J. Chem. SOC.901, 37 (1906). See also Wagner and Preiss, 2. anorg. allgem. Chem., 128,265 (1928). Sidgwick: “Electronic Theory of Valency”, p. 286 (1927).
ACTION O F HYDROGEN SULPHIDE ON CHROMATES
87
trivalent compound [Criii(NH3)6]”’, . . .3Cl’ with evolution of hydrogen. Similarly the compound Cr” (OH) (v.s.) apparently rearranges itself into a coordination compound of the type :-
[ 1::]
. . .OH’.
Criii
which reacts with the ionisation products of hydrogen sulphide, HZS $ H‘
+ HS’ $ zH‘ + S”,
the latter stage being considerable in alkaline media.‘ The hydroxy and hydrogen ions form water and the electrons associated with the S” or SH’ ions are transferred to the complex with the formation of colloidal sulphur or polysulphide and, also in the colloidal form, the compound: k r 3H20] (OW3 , a gray-green substance* also formed by the complete hydrolysis of CrC13. 6H20. This loses its coordinated molecules of water and eventually coagulates giving chromium hydroxide3 C r ( 0 H ) Conventionally the reaction may be represented by the equation : zH2CrO4 3H2S = z C ~ ( O H ) ~z H 2 0 3s. A confirmation of this equation by a determination of the sulphur acids and free sulphur is not practicable on account of the complicated action between sulphur and potassium hydroxide.
+
+
+
Department of Chemistry, Gocmmenf College, Puvjab Uniccrsify, Lahore, India. Bassett: “Theory of Quantitative Analysis,” pp. 40, 160, et seq.
* Bjerrum: Z. physik. Chem., 73, 724 (1910).
a Bjerrum: loc. cit., where evidence for the single formula is given; see also Weiser: J. Phys. Chem., 24,277 (1920);26,401(1922).
