POTASSIUM DICHROMATE The intermediate stage - ACS Publications

THE ACTION OF HYDROGEN SULFIDE ON CHROMATES POTASSIUM DICHROMATE M. A. HAMID Chemical Laboratory, Government College, Punjab University, Lahore, I n d i n H. B. DUNNICLIFF, G. S. KOTWANI,

AND

Downloaded by UNIV OF NEBRASKA-LINCOLN on August 28, 2015 | http://pubs.acs.org Publication Date: January 1, 1934 | doi: 10.1021/j150369a006

Received August 30, 1934

When a slow stream of hydrogen sulfide, purified by passing through scrubbers containing iodine and sodium sulfide, was passed into a 5 per cent solution of potassium dichromate of tested purity, the color of the solution first changed to a dark brown with the separation of a brown solid. As the current continued, the color changed to dirty brown and, passing through various shades, gave after four hours a whitish-green precipitate. When the color did not change perceptibly, the stream of hydrogen sulfide was stopped and the whitish green precipitate, which settled down, was filtered off. The filtrate was heated to coagulate colloidal chromium hydroxide and filtered. This filtrate was again subjected to the action of the gas but no more green precipitate separated, although there was a further deposit of sulfur. The green precipitate consisted of chromium hydroxide, thiosulfate, and sulfur, and the filtrate contained thiosulfate and polysulfide of potassium. The observation (4) that sulfate is formed in the early stages of the reaction was due (while testing for sulfur acids) to the development of sulfate from tetrathionate, which is now shown to be an intermediate by-product of the reaction. The precipitate formed by the action of hydrogen sulfide on chromic acid (3) has been shown to contain sulfate also. The mechanism of the reaction was studied by investigating the products a t certain definite stages. Stage I, subsequently called the “intermediate stage”: the gas was passed until the brown precipitate was just on the point of changing into green. Stage 11-the final stage-when hydrogen sulfide was bubbled through the solution until precipitation was complete.

The intermediate stage Hydrogen sulfide was passed into potassium dichromate solution (192OOC.) with continual stirring for about half an hour until the green precipi-

tate just began to form and the color of the reaction mixture was a dirty brown. The reaction mixture was allowed t o stand overnight in a stoppered flask 1217


1218

DUNNICLIFF, KOTWANI AND HAMID

and then filtered. An alkaline yellow filtrate was obtained, which contained potassium chromate and thiosulfate, but neither sulfide nor sulfate. Sulfate, in the presence of thiosulfate and chromate, was tested for by adding a 3 per cent solution of barium chloride to the alkaline filtrate in order to precipitate chromate and sulfate (if any). The washed precipitate was treated with dilute hydrochloric acid. The solid was completely dissolved, leaving no turbidity, showing the absence of sulfate, which is not formed if the concentration of hydroxyl ions is above a certain critical value. The brown solid was insoluble in cold and hot water but dissolved in hydrochloric acid with evolution of sulfur dioxide and chlorine, giving a green solution containing free sulfur. Potassium was absent from the intermediate precipitate. When treated with 0.3 N potassium hydroxide, a green solid remained which contained neither thiosulfate nor chromate. The alkali extract, however, contained both thiosulfate and chromate, showing that the coordinated complex was broken up by alkali treatment. The molecular ratio of the chromium sesquioxide (Cr2O3) to chromic acid (Cr03), calculated from chromate obtained by treatment of the brown solid with potassium hydroxide, was roughly constant. The mean of four determinations was Cr :0 1 . 9 3 , which corresponds approximately to chromium dioxide (CrO2) or chromium chromate (CrzO3.CrOs). Thus the brown solid is the same as that obtained by the action of hydrogen sulfide on chromic acid (3) and hence is different from the brown compound produced by the action of chromic acid on chromic sulfate. Chromate in the presence of thiosulfate was estimated as follows: The alkaline liquid was carefully neutralized with dilute acetic acid, and the chromate precipitated with barium acetate from a boiling solution. After heating for some time to make the precipitate granular, it was filtered and washed in a sintered glass crucible, dried a t 100-105"C., and weighed. The brown product was thoroughly washed, suspended in water, and a current of hydrogen sulfide passed until a green precipitate was obtained. This was filtered off, washed, and found to contain sulfate. The brown precipitate must be washed quite free from traces of both chromate and potassium, otherwise the reaction mixture develops alkalinity through the hydrolysis of KHS or K2S,, and no sulfate is formed. This shows that the oxidation of thiosulfate to sulfate does not take place in the presence of hydroxyl ions above a certain concentration. The properties of the brown substance were compared with those of chromium dioxide, obtained by the action of sodium thiosulfate on potassium dichromate (9). 2KzCrz01+ Na2S203 -+ K2Cr04+ KzSOl

+ NazS03+ 3CrOz

(1)

ACTION OF HYDROGEN SULFIDE ON CHROMATES

1219


The solid was washed free from chromate, sulfite, and sulfate. It gave chlorine with hydrochloric acid, and chromate and chromium hydroxide with potassium hydroxide. The dark brown chromium dioxide was suspended in water and hydrogen sulfide passed until a green precipitate was obtained. This contained coordinated sulfate, chromium hydroxide, and sulfur. Thiosulfate was absent because the chromium dioxide contained no coprecipitated chromium hydroxide and sulfur (3). The chromium dioxide in the precipitate from potassium dichromate was reduced in an alkaline medium and hence thiosulfate but no sulfate was formed. TABLE 1 Examination of the products at the intermediate stage Values are shown in mg. for 1 g. of potassium dichromate

I EXPERIMENT

I

FILTRATE

PRECIPITATE

-

TIME FOR WKICl ALLOWED TO STAND

0

5

ZQ

8-

." .

&Q

0

0

C

*iY

%E

a6 EL4

TOTAL 0

KzCraO7

2

LCCOUNTEI

$

FOR

TOTAL TRIOBULFATE

5

- -(a).

.....

(b) . . . . . .

mg .

mg.

mg.

mg.

mg,

mg .

mg.

313

58

79

303

586

11

978

69

309 319 326 326 319 318

598 616 631 631 618 615

13 12 13 15 14 14

989 983 986 984 987 995

69 69 65 74 77 82

56 54 52 59 63 68 -* Calculated from chromate.

. . . . .. (d) . . . . . . (e). . . .. . ( f ) . .. . . . . (g). . . . . . (c)

Filtered immediately 1 hour 1 hour 4 hours 4 hours 12 hours 12 hours

mg.

316 295 292 300 304 312

75 72 62 68 65 67 -

-

The following determinations were carried out a t the intermediate stage (table 1): (1) The brown solid was extracted with potassium hydroxide (0.3 N ) and the chromate and thiosulfate determined in solution. (2) The residual green solid was dissolved in hydrochloric acid and the chromium precipitated as hydroxide, which was weighed as Crz03. (3) Chromate and thiosulfate were also estimated in the filtrate, which does not contain any sulfide. The lack of uniformity in the results is not surprising, as it is impossible to arrest the reaction at exactly the same point in each case. The values represent intermediate stages of a continuous reaction. Experiments b and e, d and e, and f and g were carried out under exactly similar conditions and show that the reaction proceeds systematically. Since the filtrate is alkaline and potassium hydroxide extracts chromate

1220

DUNNICLIFF, KOTWANI AND HAMfD


from chromium dioxide, one would expect that no sulfide of potassium is formed in the filtrate until all the chromium dioxide is removed from the solid phase. Careful investigation showed that, so long as chromate is present in the reaction mixture, potassium sulfide does not form. This was established by examining the action of hydrogen sulfide on potassium chromate, stopping the reaction a t different stages, and testing for chromate and sulfide. As soon as chromate disappears from the liquid phase, sulfide makes its appearance,

The final stage Hydrogen sulfide was passed until the precipitation was complete. On filtering, a whitish-green precipitate and a strongly alkaline, golden-yellow filtrate were obtained. TABLE 2 The final green precipitate

__ EXPT NO.

ONCENTRALTION OF AMOUNT

KnCrzOl

*AKEN

CrrOa

SOLUTION

__

[odometric method

TOTAL SULFUR

Bas04

method

FREE BULPUR ( B Y DIFFERENCE)

p e t cent

cc.

grams

grams

grams

grams

grams

grams

1 2

2 2

50 50

0.51448 0.5128

0.0212 0.0214

0,0208 0.0200

0,0059 0.0061

0.0524 0,0522

0,0475 0.0461

3 4

3

50 50

0.7708b 0.7712

0,0322 0.0321

0,0316 0.0312

0.0094 0.0089

0.0781 0,0766

0.0687 0,0677

5 6 7

5 5 5

50 50

1,28920 1.2906 1.2902

0,0532 0.0529 0.0531

0.0522 0.0514 0.0524

0.0150 0.0142 0.0149

0.1260 0.1245 0,1264

0.1110 0.1103 0.1115

-

3

50

Theory: a = 0.517 g.; b = 0.776 g.; c = 1.292 g.

The green solid was insoluble in water. With hydrochloric acid it gave sulfur dioxide and a green solution containing suspended sulfur. Thiosulfate was present, but neither sulfate nor polythionates. All the thiosulfate was extracted by treatment with potassium hydroxide and estimated in the filtered solution. The residue contained chromium hydroxide and free sulfur. The statement (4)that sulfate is produced in the reaction was especially investigated (see p. 1224) and found to be incorrect. Sulfate is an important product in the action of hydrogen sulfide on chromic acid (3) and the fact that the final stage in the present case does not contain sulfate sets a limit to the oxidizing power of potassium dichromate. As already stated (p. 1218) the development of a considerable concentration of hydroxyl ions prevents the formation of sulfate in this case. The strongly alkaline, final filtrate contained only thiosulfate and polysulfide of potassium.



1221

Chromium hydroxide, thiosulfate, and free sulfur were estimated in the precipitate as described by Dunnicliff and Kotwani (3), in an attempt to define its composition (table 2), Kurtenacker and Wollak’s method (8) for the removal of sulfide in the presence of thiosulfate was also used, but it was observed that in the absence of sulfite the cadmium carbonate method gives accurate results. Even when special precautions are taken (l),the free sulfur is invariably higher than theory, owing to dissolved oxygen in water, air contact, and, to some degree, the action of light. The values for thiosulfate (column 5 ) obtained by titration agreed closely with those calculated from barium sulfate obtained by oxidation with bromine water. The latter value was not higher than the former in any instance, as would have been the case if a thionate had been present. The absence of a polythionate in the final stage was thus confirmed, and the only sulfur compound in the precipitate was thiosulfate. The existence of a trithionate was thus ruled out since, if formed, it should persist in the final stage (7), being the only thionate stable in the presence of hydrogen sulfide,

Coordinated and ionic thiosuljate While trying to oxidize the sulfur and thiosulfate in the precipitate to sulfate with bromine water, it was observed that, even after refluxing for two hours, the remaining solid still contained thiosulfate. This slow attack of the thiosulfate by bromine suggested a method for distinguishing between ionic and coordinated thiosulfate groups in the complex molecule. Portions of the precipitate were shaken with bromine water of various concentrations in the cold and filtered. The ratio A/B, between A, the thiosulfate estimated directly in the residue thus obtained, and B, that calculated from sulfate in the filtrate, shows very wide variations (table 3). The inference is that bromine attacks both the ionic and coordinated thiosulfate. It is probable that, as the ionic thiosulfate is removed, the equilibrium between coordinated and ionic thiosulfate is disturbed and more ionic thiosulfate is formed. Possibly bromine is catalytic in this sense. A series of experiments was tried using iodine. (1) The washed precipitates from a number of samples of 50 cc. of 5 per cent dichromate solution were treated with excess of different strengths of iodine solution and allowed to stand. The iodine disappeared entirely after times which increased with the strength of the solution. This was due to the adsorption of iodine by the solid particles. Hence methods involving excess of iodine solution were rejected. (2) After first making acid with dilute acetic acid, direct titrations with iodine of various strengths (0.1 N to 0.01 N ) against a fine suspension were then performed (titer d cc.) in the hope

1222


that, when the ionic thiosulfate had all been attacked, a momentary end point niight be obtained with starch used as an external indicator. After filtration, the precipitate was treated with potassium hydroxide and the acidified extract titrated against iodine (titer b cc.). It was found that N/40 to N/GO iodine gave a / b = 2 (approximately) (see table 4). TABLE 3 Ionic and coordinated thiosulfate in the precipitate (bromine method)


TIME OF CONTACT

Bas04 FROM FILTRATE

A

(s&) CALCULATED FROM

Bas04

B

(szos) I N

T E E PPT.

grams

grams

grams

1 1 1

0.0242 0.0366 0.0624

0.0058 0.0088 0.0149

0.0026 0.0051 0.0072

2.24 1.17 2.07

2 2

0.0532 0.0746

0.0127 0.0179

0.0040 0.0058

3.17 3.09

4 4 4

0.0382 0.0522 0.0388

0.0092 0.0125' 0.0093

0.0024 0.0042 0.0038

3.83 2.97 2.46

12 12

0.0524 0.0692

0.0125 0.0166

0.0026 0,0031

4.81 5.35

hours

TABLE 4 Titration of suspensions w i t h iodine solutions a

b

a/b

~-

a

b

~~

cc.

cc.

2.30 3.80 2.30 2.45 2.60

1.05 1.70 1.20 1.15 1.10

2.19 2.23 1.92 2.15 2.36

cc.

cc.

1.95 2.10 2.60 2.50 3.30

0.90 1.15 1.20 1.25 1.55

alb

a

b

cc.

cc.

2.85 3.20 2.45 1.80 2.30

1.30 1.55 1.05 0.85 1.05

-__-

2.17 1.83 2.17 2.00 2.13

2.19 2.07 2.33 2.11 2.19

Mean of 15 determinations = 2.14

Considering the small titration volume and the presence of solid, which in any case adsorbs a little iodine, the agreement is significant and the ratio of the ionic to coordinated thiosulfate is probably of the order 2: 1.

Composition of the final Jiltrate Although the final filtrate is alkaline, it does not extract the whole of the thiosulfate from the precipitate as it does in the case of potassium chromate (4). Table 2 shows that the percentage of thiosulfate in the precipi-


1223


tate varies directly with the concentration of the solution used, and the constant values for experiments 1 and 2 , 3 and 4, and 5 , 6 , 7 show that an equilibrium is set up between the thiosulfate (total) content of the precipitate and the concentration of thiosulfate in the solution. After the removal of colloidal chromium hydroxide under reduced pressure the final alkaline filtrate contained only polysulfide and thiosulfate of potassium. The color varied between yellow and orange, with the amount of hydrogen sulfide passed and the temperature of the reaction. The whole of the potassium was in the filtrate. Potassium from 50 cc. of TABLE 5 Thiosulfate in filtrate (6per cent solution of potassium dichromate)

(by titration) X lo4 (actual).. .... .. .. . . . 3321 3256 3241 3186 3146 3283 3132 3374 3125 3286

g&Oa

----------

12.97 12.58 12.53 12.50 Calculated per 100 g. of 13.28 K2Cr20T. . . . . . . . . . . . . . . 13.02 12.74 13.13 13.49 13.14

.

TABLE 6 Values i n grams per 100 grams of the polysulfide (8)

(b)

(d)

(0)

POTABSIUM NO.

TOTAL POTABBIUM

TOTAL BULFUR

-~ I I1 I11 IV

-

K$:$~, 5Hz0

(e)

(f)

(9)

PoTA881uM

POTABBIUM BULFUR CALCULATED PRESENT AB PRESENT AB p ~ ~ o ~ & p f~ B ~ o ~ ~ L ~ B p o ~ poLysuL~ ~ ~ -FORMULA FATE CALFATE CALKzSy FIDE CULATED CULATED (a) (d) (b) (e’ FROM ( 0 ) FROM (0)

-

-

-

-

grams

grams

grams

grams

grams

grams

grams

28.45 27.72 28.43 21.75

31.63 29.60 32.42 23.75

78.46 84.35 76.52 61.21

21.85 23.50 21.31 17.05

17.93 19.30 17.49 14.00

6.60 4.22 7.12 4.70

.13.70 10.30 14.93 9.75

5 per cent potassium dichromate: found 0.6605, 0.6614, 0.6612, and 0.6616 g.; theoretical value = 0.6633 g. Thiosulfate in the filtrate was estimated after removing the polysulfide by means of cadmium carbonate. The filtrate was acidified with dilute acetic acid and titrated against iodine. These values (table 5 ) varied slightly. Probably this was due to the conversion of some of the polysulfide into thiosulfate by contact with air (10). The determination had, therefore, always to be made in fresh filtrate. When the reaction was allowed to proceed at the laboratory temperature (19-20°C.), the formula of the polysulfide worked out to KzS3 (mean of three determinations = K.J&.l).


1224-


In addition to the method of estimating the polysulfide sulfur given below, that of Kurtenacker and Bittner (6) was found to give satisfactory results, but the modified procedure of Szeberenyl (12) was found to be more convenient, as it avoids the necessity for tedious corrections. In order to get the most polysulfide and to accelerate the otherwise slow reaction, hydrogen sulfide was passed into a 10 per cent solution of potassium dichromate at 80435°C. for nearly ten hours. The chromium hydroxide which separated when the filtrate was concentrated under reduced pressure was filtered off, and the polysulfide allowed to crystallize out under reduced pressure at about 50°C. The crystals were pressed between folds of filter paper, keeping them, as far as possible, out of air contact. Potassium, “sulfide,” and thiosulfate were determined in a weighed amount of the dried polysulfide (4). Table 6 shows that the molecular ratio K2Sz03:K2Sx is not constant. Possibly, under ideal conditions, potassium thiosulfate and pentasulfide are formed in equimolecular proportions. With the exception of the potassium hydrosulfide, the analyses in tables 2 and 4 can be represented approximately by the following equation :

+ 3H2S + H2O) + Cr2(S203)3 + 11OCr(OH)3 + 20(K2S203 + KHS) + 26K2S3 + 245 + 49HzO

56(K2Crz07

(2)

Note on the observation (4) that sulfate is formed in the action of hydrogen sulfide on potassium dichromate If the intermediate filtrate is acidified and then treated with barium chloride solution a white precipitate of barium sulfate is produced, but the final stage contains no sulfate. Since sulfate is not found in the final products and it is not decomposed by hydrogen sulfide, it is obvious that it was developed in testing, and the only source of such sulfate would be by the action of hydrochloric acid on a polythionate. The reaction was studied in order to determine the exact stage at which the formation of sulfate on acidification is suppressed, i.e., when polythionate ceases to be a by-product, and the following procedure was adopted: A saturated solution of hydrogen sulfide was mixed in small but increasing quantities (10 cc. a t a time) with 20 cc. of 5 per cent potassium dichromate and the products analyzed next day. The reaction was divided into three stages: (a) When up to 40 cc. of saturated hydrogen sulfide solution was added, A brown solid and a golden-yellow, slightly acidic filtrate were obtained. (b) On the addition of from 50 to 130 cc. of hydrogen sulfide solution, the precipitate was green and chromate was present in the filtrate. (e) When more than 130 cc. of hydrogen sulfide was added, the polysulfide made its appearance and the composition of the final

1226


precipitate (expt. 16, table 6 ) was almost the same as given in table 2, p. 1220. Tables 7a, 7b, and 7c give results in which sulfate was precipitated by previous acidification and addition of barium chloriae. The scheme for the analysis of all other radicals is as given previously. The results show that (a) the amount of sulfate formed from 1 g. of potassium dichromate remains practically constant up t o experiment 8; (b) so long as the solution is acidic, thiosulfate is not formed; (c) a t a certain stage the sulfate entirely disappears, the disappearance being first


TABLE 7a

Precipitates Values are shown in mg. for 1 g. of potassium dichromate

-

(1)

(2)

(5)

EXPT. NO.

COLOR OF PPT. AND

CrzOs I N PPT.

REACTION

KzCrz07

--

-1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Brown, acidic Brown, acidic Brown, neutral Brown, alkaline Dirt y-brown, alkaline Green, alkaline Green, alkaline Green, alkaline Green, alkaline Green, alkaline Green, alkaline Green, alkaline Green, alkaline Green, alkaline Green, alkaline Green, alkaline

(6) :ORRESONDING

(7)

AC:OUNTID FOR I N

FROM

KPCr207 TEIONATI

PPT.

--

cc.

mg.

mg.

mg.

mg.

30 40 50 60 70

62 64 62 64 22

120 158 302 305 342

233 306 584 593 663

14 14 11 12

352 359 351 363 478 49 1 513 516 514 512 514

681 695 679 699 925 950 993 999 995 990

80 90 100 110 120 130 140 150 160 170 180

(9)

KzCrzO7

(804)

-

4

w.

ma.

13 12 13

295 370 646 657 685

12 16 12 11 14 16 16 16 16 18 18

681 695 679 699 925 950 993 999 995 990 995

apparent in the precipitate; and (d) so long as chromate is present in the filtrate or precipitate, a test for sulfate is given on acidification, but as soon as chromate disappears (expt. 12) the test for sulfate is negative. The explanation is that sulfate is not present as such but is formed owing to the oxidation (by the chromic acid produced on acidification) of some thiosulfate or a thionate. Since, however, the values for thiosulfate show the same regular increase (expts. 11 and 12) without any abrupt change, the formation of sulfate is clearly due to the decomposition of a thionate.

1226


TABLE 7b Filtrates* (1)

(2)


PIXPT. NO,

C

CORRPSPONDING ~ TO COLLOIDAL

K ~ c ~ ~ o ~Cr(0H)s

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

(4)

(3)

KzCrzOi ~

~

(6)

KrCrlOr

(sod)PROM ~

~

THIONATE

mg.

mg.

mg.

625 578 306 299 292 293 290 294 266 49 15

54 22 21 18

59 61 62 61 59 61 69 73 19 19

~

ACCOUNTED N FOR [ N T E E FILTRATE

.

mg.

mg

679 600 327 317 292 293 290 294 266 49 15

58 59 60 55 61 68 70 83 94 98 110 117 121 121

8

* The filtrates correspond t o the precipitates in table 7a. TABLE 7c Total potassium dichromate accounted for and total sulfate and thiosulfate formed i n precipitate and filtrate BIXPT. NO.

KzCrzO7 (TOTAL OB COLUMN 9 IN TABLE 7a AND COLWMN 6 I N ,

7b)

TABLE

.

1 2 3 4 5 6

7 8 9 10 11 12 13 14 15 16

(sod)

SULFATE (COLUMN 7 IN TABLE AND COLUMN 4 I N TABLPI 7b)

.

mg

mg

974 970 973 974 977 974 975 973 965 974 965 993 999 995 990 995

73 75 73 73 63 61 69 73 19 19 8

7a

TAIOSULFATE (9208) 8 I N TABLE 7a AND COLUMN 5 I N

(COLUMN

TABLE

7b)

mo.

71 71 73 77 77 80 81 97 110 114 126 133 139 139

1227



That sulfate is not a product of the reaction under consideration was shown as follows: The intermediate filtrate (and the alkali extract from . the precipitate) were treated separately with barium chloride without acidifying. The precipitates were filtered. On treatment with hydrochloric acid the barium chromate dissolved out and no cloudiness or precipitate remained in solution. This was confirmed by determining sulfate by the old and new methods (see table 8).

Direct proof of the existence of a thionate The alkaline filtrate from the intermediate stage was treated with barium chloride to remove chromate, and the filtrate made up to a known volume. An aliquot portion, after acidification with dilute acetic acid, was titrated against iodine. This gave the titer for thiosulfate existing normally in the filtrate. TABLE 8 VOLUME OF HIS USED

NO.

_

1 2 3 4

WEICIHT OF

KzCi-107

_

Bas04 ON

Bas04 WITHOUT

ACIDIFYINQ

ACIDIFYING

~

cc.

grams

grams

grama

70 80 90 100

1.00 1.00 1.oo

0.0875 0.0768 0,0764 0.0787

0.0012

1.oo

0.0009 0.0014 0.0011

Another portion of t,he filtrate was treated with sodium sulfide; the white precipitate which formed was filtered off and the sulfide removed with cadmium carbonate. The filtrate from this when titrated with iodine gave a titer almost three times as large as that obtained before. This is conclusive evidence of the existence of a thionate. The existence of trithionate is ruled out since, if formed, it should persist in the final stages (7) (p. 1221). The polythionates which are possible under the existing conditions are the di- and tetra-thionates. Quantitative results show that a tetrathionate, which is easily converted into thiosulfate by an alkaline sulfide (11, 2), is present.

+ Na2S

K2S406

-+

KzS203

+ Na2Sz03+ S

(4)

This would account for the fact that no potassium sulfide is present so long as tetrathionate exists. It also explains the regular increase in the thiosulfate content and the ultimate elimination of tetrathionate. M. J. Fordos and A. Gellis ( 5 ) state that mild oxidizing agents like ferric chloride react with sodium thiosulfate to give tetrathionate.

+

2NazSz03 2FeC13 -+ NazS40s THE JOURNAL OF PHYSICAL CHEMISTRY, VOL. XXXIX, NO,

+ 2NaC1 + 2FeC12 9

1228


Thus the tetrathionate is formed due to the oxidation of thiosulfate by

. the very weak chromic acid present in the initial stages. This reaction accounts for the fact that no test for tetrathionate is obtained when chromate is absent. Direct estimation of tetrathionate was found to be cumbersome, and therefore the values for sulfate found by the acidification method have been given in tables 7a, 7b, and 7c.


SUMMARY AND DISCUSSION

The brown solid formed in the intermediate stages of the reduction of 5 per cent potassium dichromate by hydrogen sulfide consists of chromium dioxide and hydroxide, a coordinated chromium thiosulfate, chromium tetrathionate, and free sulfur, while the filtrate contains unattacked potassium dichromate together with potassium thiosulfate and tetrathionate. The amount of tetrathionate decreases with the amount of chromate present, until eventually both disappear simultaneously. So long as chromate remains sulfide is not present, and the dichromate accounted for as thiosulfate is less than theory by the amount of tetrathionate formed. Accepting views previously advanced (3,4) for the development of thiosulfate in these reactions, it has been shown that thiosulfate is the source of the tetrathionate formed in a side reaction, owing to the mild oxidation of part of the thiosulfate by chromate. The tetrathionate is ultimately reduced to thiosulfate by the alkaline sulfide. Sulfate is not formed in these reductions if the concentration of hydroxyl ions is above a certain critical value. The final products are (a) a precipitate containing chromium hydroxide, sulfur, and a complex chromium thiosulfate in which the ratio of ionic t o coordinated thiosulfate is approximately 2 :1, and (b) potassium thiosulfate and polysulfide in solution. The polysulfide formed depends on the temperature of the reaction, being KzS3a t laboratory temperatures and the pentasulfide at temperatures approaching 90°C. REFERENCES (1) (2) (3) (4) (5) (6) (7)

BAGGTER, L. S.: J. Chem. SOC. 123, 2631 (1923). BASSETT AND DURRANT: J. Chem. SOC.1927, 1418. DUNNICLIFF AND KOTWANI: J. Phys. Chem. 36, 3214 (1931). DUNNICLIFF AND SONI:J. Phys. Chem. 33,81 (1929). FORDOS, M. J., AND GELIS,A . : Compt. rend. 16:920 (1842). KURTENACKER AND BITTNER: Z. anorg. Chem. 142, 115 (1925). KURTENACKER AND GOLDBACH: Z. anorg. Chem. 166, 177 (1927).

ACTION O F HYDROGEN SULFIDE ON CHROMATES

(8) (9) (10) (11)

1229

KURTENACKER AND WOLLAK:Z. anorg. Chem. 161, 201 (1927). POPP:Ann. 166, 90 (1870). ROSE,H.: Pogg. Ann. 17, 327 (1829); 66, 353 (1842). SMITH,W., AND TAKAMOSTSU: Compt. rend. 37, 592 (1880); Chem. News 41,


290 (1880). (12) SZEBERENYL: Z. anal. Chem. 78,36 (1929).

Editor's Note: Owing to unforeseen circumstances, the publication of this paper has been unavoidably delayed. A private communication from Professor DunniclifT states that further investigation has proved that, if the reduction of sodium, potassium, or ammonium dichromates is carried out rapidly a t 90-95"C., some sulfate appears in the product, but no sulfate is formed after the reaction mixture becomes alkaline. The formation of sulfate is totally suppressed in the alkaline media which result from the reduction of the chromates of sodium, potassium, and ammonium. A study of the reduction of the insoluble chromates by hydrogen sulfide has also yielded valuable results. Among the reduction products of silver, mercurous, and thallous chromates a t medium or low temperatures, sulfite has been found, while the chromates of lead, barium, strontium, etc. give no sulfite in any circumstances. Evidence is adduced t o show that sulfite is the precursor of both thiosulfate and sulfate in the general reaction. Details of this work will be published shortly.


POTASSIUM DICHROMATE The intermediate stage - ACS Publications

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