T H E ACTION OF HYDROGES SULPHIDE O K SOLUTIOKS OF KITRIC ACID BY H. B. DUNNICLIFF AND SARDAR XOHAVVAD
A. Yogel’ and S. A. F. Millon2 found that hydrogen sulphide has no action nitric acid free from nitrogen peroxide while R. Kempe? ascribed the action with nitric acid (s.g., 1.18) a t zs°C., giving sulphur, sulphuric acid, nitric oxide, nitrogen and ammonia, to traces of nitrogen peroxide produced by exposure of the nitric acid to air. I. W. F. Johnston4 and C. Leconte5 state that hydrogen sulphide reduces dilute nitric acid giving sulphur, sulphuric acid, ammonium sulphate and nitric oxide. P. T. .Austin6 observed that hydrogen sulphide burns in nitric acid vapour with a yellow flame giving clouds of nitrosulphuric acid. When hydrogen sulphide reacts with fuming nitric acid, J. Kesse17 states that an explosion results while A. W.Hofmann* reports that the reaction takes place with incandescence. Frequent reference will be made to the recent work of Bancroft, Milligan and Gilletteg on the reduction of nitric acid by metals and to the paper by L. S. Bagster’O on “The Reaction between Xitrous Acid and Hydrogen Sulphide.” Part I. The Soluble Products of the Action of Hydrogen Sulphide on Nitric Acid Solution In these experiments, hydrogen sulphide was prepared from ferrous sulphide and hydrochloric acid and purified by passing it first over dry iodine crystals and then through a dilute solution of sodium sulphide. The gas contains a little hydrogen, which was shown to have no action on solutions of nitric acid a t ordinary temperatures. Before passing hydrogen sulphide the air in the vessels was always displaced by carbon dioxide. It was observed that:Solutions containing less than 5% nitric acid are not attacked even I). if traces of nitrogen peroxide are addect; 2). Solutions containing 5-307~ nitric acid show no signs of reaction for some time but;.if nitrogen peroxide is added, a violent reaction with rise of temperature follows, and 3 ) When a 4 0 7 ~solution is used the reaction takes place after an interval of apparent passivity called in this paper the “induction period.” J. Phys. 82, 329 (1816). J. Pharrn. Chern. (3) 2, 179
(I~sI).
Ann., 102, 342 (1857). ‘Edin. J. Sci., 6, 6 j (18321. 5 .Ann. Chirn. Phys. (3) 21, 180 (1847). Am. Chem. J., 11, 172 (1889). Ber., 12, 230j (1879). Ber. 3, 6 j 8 (1870). J. Phys. Chern., 28, 492, 475, 754 (1924). l o J. Chem. SOC., 1928, 2631.
’
This has also betn notiecd in thc case of other reducing agcnts.’ Ferrous sulphatc, aodiurii siilphite, stannous chloride, and titanous chloridt react with pure nitric acid after long standing. The induction period becanie longpr when, before passing hydrogen sulphide, carbon dioxide was bubbled through the solution to remove any nitrogen peroxide resulting from the decomposition of nitric acid. Xitrogen peroxide is a well known catalyst in thp action of nitric acid on metals, both in the presence and absence of sulphuric acid.? I t also acts as a destabiliser to nitro-cellulose and nitroglycerine. Milkiness due to sulphur appeared on the surface and spread rapidly through the liquid. When, after stopping the flow of hydrogen sulphide, the vessel was shaken, the atmosphere above the solution turned brown owing to the presence of nitrogen peroxide.
+ HzS = HKOz + H?O + S HS03 + HTOz-eH,O + 2SOz
HSOB
(1) (2
j
The nitrous acid breaks down partly according to the reaction.3
3HSOz or zH’
+ 3(SOz)’*
+ HSOZ + H?O nS0 + ( S O 3 ) ’+ H20
2x0
(3)
(3a)
Xuch work has been done on this reaction A. Klemenc and F. Pollack have shown that the decomposition depends on the removal of nitric oxide the presence of which hinders the reaction. A. T‘. Saposchnikoff observed that nitrous acid breaks up with water and nitrogen trioxide which can be estracted by organic solvents. zHKO~
NzOZ
+ HzO
(4)
The nitrogen trioxide breaks down to nitric oxide and nitrogen peroxide
SzOs
No f
1 0 2
(5)
but equilibrium cannot be attained owing t,o the small solubility of nitric oxide and the hydration of the nitrogen peroxide. Xtrous acid decomposes spont’aneously giving nitric oxide and the rate of decomposition is increased by the presence of nitric acid or by the passage of even inert gases through the solution. F. Kaschig has shown that nitrous acid can oxidise nitrous oxide to nitric oxide and F. Hefti and JV. Schilt find that hydrogen sulphide with it’ yields thiosulphuric acid accompanied by sulphur and ammonia. Many have testified to the fact that, while the oxidieing action of nitric acid usually ceases !Then it is reduced to nitric oxide, a normal reduction product of nitrous acid is nitrous 0xide.j 3lilligan and Gillette: J. Phg-s. Chem., 28, 754 (1924); Joss: 30, I z j j (1926:. Gay-Lussac: d n n . Chim. Phys. ( 3 ) ,6 . 9 j i1842); Russell: J. Chem. Soc., 27, 8 (18741. Montemartini: h t t i Accad. Lincei. (4) 611, 263 (1890). hlellor: “Treatise on Inorganic and Theoretical Chemistry,” 8, 459 et seq. (19281. AIellor: loc. cit., p. 463.
H Y D R O G E S SULPHIDE .%XD S I T R I C ACID
I345
On passing a further supply of hydrogen sulphide, the brown colour disappeared as nitrogen peroxide reacts with hydrogen sulphide’ giving sulphur, water and nitric oxide.
SOP
+ H,S = H20 + NO + S
(6) Sitric oxide reacts slowly with moist hydrogen sulphide giving some nitrous oxide and ammonium sulphide. When nitric oxide is bubbled through hydrogen sulphide solution, ammonia is formed (\ide p. 1352). In the first set of experiments with 40%~ nitric acid, four vessels B-E were connected in series with a reaction flask, A. The first, B, was empty and the others contained water, C, strong sulphuric acid, D, and acidified (H?SO1) ferrous sulphate, E, respectively. When the vessel, A, was shaken vigorously,
the excess of hydrogen sulphide and the brown gas passed into the empty flask, B. Here a further deposit of sulphur was observed and the gas which passed on was colourless and contained hydrogen sulphide. The atmosphere over the liquid in A became colourless and ferrous sulphate acidfied with sulphuric acid in E turned dark brown showing the presence of nitric oxide. The temperature in the flask, A, rose to about 70’ C. and the solution turned bluish green. Khen hydrogen sulphide was passed into A surrounded by melting ice, the solution became dark green on standing. This, when exposed to the atmosphere or gently warmed, slowly became paler and emitted brownish red fumes of nitrogen peroxide from the decomposition of nitrous acid or nitrogen trioxide to which the green colour of such solutions is usually attributed.* When hydrogen sulphide was passed for a longer time, the liquid in A became paler with the deposition of sulphur. Finally, the whole of the sulphur coagulated and separated and the solution became clear. At any time during the reaction, the solution in X gave tests for sulphuric acid, nitrous acid and “ammonium.” Hydroxylamine which is known to be Leconte: loc. cit. *Lunge and Porschneff: Z. anorg. Chem., 1, 209 (1894);H.B.and 51. Baker: J. Chem. Soc., 91, 1862 (1907); B.11.Jones: 105, 2310 (1914); Sanfourche: . h n . Chim. Phys., (IO), 1,
5 (1924).
1346
H. B . DUSNICLIFF AND SARDAR MOHAhlNAD
produced in the reduction of nitric acid by hydrogen sulphide under certain conditions was looked for repeatedly but was never detected. If it is an intermediate product HSOz
+
2s’’--t
NHzOH
+ zS(+
H20),
(7)
it is decomposed as soon as formed, (vide equation ( I I)). Feathery crystals, having the properties of nitrosylsulphonic acid or chamber crystals, KO.O(HSOa), appeared on the walls of the sulphuric acid flask, D. Quantitative Determination of the Products of Reaction in Solution: A constant stream of purified hydrogen sulphide gas was passed through 4374 nitric acid for different intervals of time. The products formed in solution are ammonia and nitrous, nitric, and sulphuric acids. The solution was free from hydrogen sulphide. Sulphur was not determined in these experiments. The methods, standardised by control experiments, were as follows : “Ammonium.” Sodium hydroxide solution was added in excess, steam blown through the solution and the ammonia collected in excess of standard sulphuric acid. Nitrous acid by Lunge’s potassium permanganate method. Total nitrogen including dissolced nitrogen gases by Lunge’s nitrometer method. Sulphuric acid as barium sulphate. Total acidity by titration against standard alkali. Each set of results in Table I records a separate experiment. They show that: I). The concentration of ammonia increases progressively but slowly and not in proportion to the nitric acid decomposed. If ammonia formed is not subsequently decomposed. its rate of formation is comparatively rapid a t first and slower in the later stages of the reaction. I n any case very little is found in the solution at any time. 2). The nitrous acid content shows a maximum a t the period of greatest velocity of reaction (between 1.j and 3 hours). The effect of the nitrogen is catalytic. I n his work on the reduction of nitric acid by metals, Veleyl found that the start of the reaction is at first delayed and that the amount of nitrous acid increases to a maximum. 3 ) . The total acidity decreases. The sulphuric acid formed has a marked influence on the rate of decomposition of the nitric acid and probably on the products formed: see also the results in Table 11, p. 1349. More detailed examination of Table I shows that (a) up to I$ hours, the nitric acid decomposed (11.9;grams) is very great compared with the sulphuric acid formed (4.471-grams). The equivalent ‘Phil. Trans., 182.1 (1891;.
HYDROGEN SULPHIDE ASD SITRIC ACID
I347
TABLE I Showing the nature and quantity of the reduction products in solution when hydrogen sulphide is passed through 42.93% nitric acid for different intervals of time Composition of the solution; grm/Ioo ( S O , )' (SO*)' 0 42.23
Time in hours 0.
40.47 39 .oo
0 . 2 5
0 . f j
31.1j 8.90
1.50
3 .oo 4.00
2.59 I .9i
j . 2 j
Time in hours
5.25
0.32
0.41
0,115
2.21
0.57
0,149
4.38
0.21
1.110
20.13
0.19
I .j89
24 49
0.15
I . 730
27.88 Colour of the solution of standing
Total acidity as hydrogen
0.67 0.66 0.61 0.49 0.46 0.45
4.00
(SO,)"
073
0.68
I.jO
0
0
3 .oo
SHa
0.15
0.25 0.75
C.C.
Colourless Pale Green Bluish Green Green Bluish Green Pale Green Colourless
quantity would be 9.31 grams. Reactions 1-3 above together with those given by Bagster'.
+ S" = 3HSO + SO, (+ 3H20) HKOz + S" = H S O + S (+ H 10) (HKO)? = HzO + ?;?O
3H?;On
(8) (9) (10)
would account for (i) the large amount of nitric acid decomposed compared with the quantity of sulphuric acid formed, (ii) the evolution of nitrogen peroxide in certain circumstances and (iii) the formation of nitric and nitrous oxides. P. C. RLy and A. C. Ganguliz record the decomposition of hyponitrous acid giving nitrogen 5H2Nz02 = 4H?O 2HNOs 4x2 (11)
+
and A. Hantzsch and L. Kaufmann3 that the same compound can yield ammonia
3H?x?O?= 2xH3 f 2N203 1
Loc C l t . J. Chem. Soc., 91, 1399, 1866 (1897). Ann., 68, 299 (1899).
(12)
'338
H. B. D U S S I C L I F F A S D S.4HDAR 1IOHAXIMAD
If hydroxylamine is an intermediate product (equation i ) , it may be immediately decomposed giving nitrogen H2K202 z S H ~ ~= H 2 5 2 qH20 (13) (b) From 13 hours to 4 hours, the nitric acid decomposed is approximately equivalent to the sulphuric acid formed ( H S 0 3 : H2S04,iz= 461 :419). This suggests the reactions sunimarised in the equations :
+
+
+ + + +
S"+ 2HS03 = HZSOA 2x0 6HS02 3s'' = 3HCSO4 3x2 ~H?;03 132s = H2SZOj S H20 nitrosic acid
+ +
+
+
(14)
(15) (16)
= HZSO4 l i 2 0 HZO (17) which with those given in (a) would account for the existence of nitrous oxidc and much nitric oxide as well as nitrogen in the gaseous products at this stage (see Part 111). (e) When the concentration of nitric acid is very low, (under 3 5 ) , the sulphuric acid formed is in excess of that equivalent to the nitric acid decomposed. This is in agreement with the experiment5 described later in which it is shown that nitrogen is among the gaseous products and the inference is drawn that, since the power of oxidation of the nitric acid usually ceases when it is reduced to nitric oxide, oxidation by nitrous acid assisted by the increasing concentration of sulphuric acid is probably responsible for the nitrogen present (vide the reactions shown in paragraphs (a) and (b), p. 1347. Since solutions containing s% nitric acid and less are not attacked by hydrogen sulphide (v.s.) it is clear that, a t low concentrations, the nitric acid and its decomposition product, nitrous acid, have been rendered vulnerable to decomposition by the sulphuric acid present. This would suggest that HKOs and not (NO,)' ions are readily attacked. A. Kurtenacker' and Bagster2 conclude that, in the reduction of nitrous acid, it is the nitrous acid (HN02) and not the nitrite ions, (NOs)', which are the active agent. 4). Hence the sulphuric acid developed has a very marked influence on the course of the reaction and the products formed. Others have drawn attention to this phenomenon e.g. Divers and Shirnid~u,~ from a study of the influence of sulphuric acid on t'he reduction of nitric acid by zinc, conclude that sulphuric acid has a specific hydrogenising effect upon nitric acid in the production of hgdroxlamine but have not interpreted the mechanism of the reaction. Part 11. The Influence of Sulphuric Acid on the Reaction A solution was made up containing 42.95 percent of nitric acid and j 7 0 of sulphuric acid. Through separate quantities of I O O c.cs. each of this mixture, hydrogen sulphide was passed continuously for different intervals of time. The con>
llonstsheft, 41, 91 (192oj.
LOC.cit. J. Chem. Soc., 47, 61j (I88j).
HYDROGEX SULPHIDE A S D SITRIC ACID
I349
ditions of experiment were comparable in that considerable excess of hydrogen sulphide was used. This fact is important in view of the results presented in Part I11 of the paper where it is shown, Table X, that gaseous products are evolved up to 63 hours. This is the time required for “stasis” to be established when the hydrogen sulphide is admitted as required (vide p. 1356)and not passed in excess as in this set of experiments which are comparable with those detailed in Part I. The resulting solutions nere analysed as described in Part I. Again hydroxylamine was not found. The induction period was considerably increased but a trace of nitrogen peroxide started the reaction a t once. llilligan and Gillette observed that, when much sulphuric acid is added t o a mixture of nitric acid and ferrous sulphate, the period of induction is much increased and that traces of nitrouq acid acted as a positive catalyser. E. J. Joss showed that, when a I ;? solution of oxide-free nitric acid acts on copper, nitric oxide is evolved after I O minutes. After the addition of IC; of sulphuric acid, nitric oxide was given after 2 0 minutes and, nith 3 7 sulphuric acid, after 46 minutes. Higher concentrations decreased the time of inaction until, with I 17sulphuric acid,
TABLE I1 Shows how the reduction products in solution vary when hydrogen sulphide is passed through a 42.9jyc solution of nitric acid containing jyc of Sulphuric acid Strength of the nitric acid solution: 42.95grm. per IOO C.C. Sulphuric acid 4.898gm. of (SO,)” per IOO C . C . Composition of the solution gm,’Ioo (XOd’ (SO>)‘ SH3
C.C.
Time in hours
Total 0
i). ii). iii). iv). 17).
vi).
42.2j
0.2j
38.4j
0.75
1.50
36.02 28.63
2.50
3.50 j. o o
0.978
0.0jj
0.16; 0.096
-
4.90 7.13
2 ,23
3.92 i .63
22.9;
0.466 0.4Ij
0.2j9
8.82 j3 1j.30
23.25
0.275
0.259
1j.Ij
10.25
23,34
0.422
0.ITj
14.93
10.03
Time in hours
Total “acidity” in terms of hydrogen
12.
10.40
Total “acidity” in terms of hydrogen, less “acidity” added ! = o . ~ ograms).
0 .;8
0.68
0 . 2 j
0.;j
0.6;
0.;j
0 .i
0.63
I ,so
0.6; 0.64 0.64 0.64
0
i) . ii) . iii) . iv) . v) . vi) .
0.j63
(SO,!”
Formed in reaction
.so 3.50 5 00 2
3
0.57
0,54
0.54 0.54
I 3 50
H. B. DUNNICLIFF AND SARDAR MOHAMYAD
copper was attached more readily than with nitric acid alone. These results are explained on the basis d the increased solubility of nitric oxide by the presence of sulphuric acid. The results of entirely separate experiments are recorded in Table 11. The methods of experimentation and analysis were the same as those reported in Part I. The results may be summarised as follows:I ) . The nitrous acid present is small in quantity and, after rising to a maximum value earlier than in the first, experiment, falls to a constant concentration which is higher than in the first set. Since, in the first set of experiments, the ammonia is formed in 2). quantity diminishing (assuming no decomposition) with increase in concentration of sulphuric acid, it is not surprising that, in these experiments, the amount of ammonia formed is less. Thus it appears that, if no ammonia is decomposed, the presence of sulphuric acid inhibits the formation of ammonia in the reduction of nitric acid by hydrogen sulphide. Hence the question as to whether ammonia is formed and subsequently decomposed by the nitrous acid giving nitrogen assumes importance. 3 ) . When the concentration of nitric acid has fallen to 23% and the total sulphuric acid content is 1 5 7 ~the ) reduction comes to a stand-still. This is a most unexpected result and its interpretation will be dealt with in detail below. 4). At no stage of the reaction is the amount of sulphuric acid formed equivalent to the nitric acid decomposed, the sulphuric acid formed being considerably less than that demanded. This would forecast a greater volume of gaseous products, i.e. oxides of nitrogen for a given rise in concentration of sulphuric acid. Experiments demonstrated that this is actually the case. To review these observations in more detail:I ) . The higher concentration of nitrous acid may be due to the establishment of new stability relations i n v o k n g the formation of small quantities of nitrosyl sulphonic acid
HXO1+ HgS04
e XO.HSO1+
HzO
(18)
which may be considered as nitrous acid stabilised by solution in sulphuric acid.' Dilution decomposes this acid with evolution of nitric oxide and formation of nitrous acid. 2). To ascertain whether ammonium salts are decomposed by the reaction, known amounts of ammonium sulphate or nitrate were added to the solution before passing hydrogen sulphide. The results are recorded in Table 111. It was also shown that nitrogen is an important decomposition product (Table IX).
Milligan and Gillette: J. Phys. Chem., 28, 744 (1924).
HYDROGEN SULPHIDE ASD SITRIC ACID
13.51
TABLE I11 The decomposition of ammonium sulphate and ammonium nitrate in nitric acid by hydrogen sulphide: Time for the passage of HIS in hours
1.5 3 .o 3 .O
Ammonium salt used
(KH4)?S01 (KH4)eSOd SH~SOI
Quantity of Ammonium .Immonium Ammonia lost ammonium contained recovered in the after,the salt used salt in reaction in IOO C.C. 100 C.C. in grm. gram. gram. gram.
2.86
0.74
11 , 3 7
2.93
0.37 0.94
4.71
1.00
0.27
0.36 0.99 0.74
7Loss
48.7 30.4 74.0
Acworthl and Acworth and Armstrong2 have shown that when ammonium nitrate is added to nitric acid used for the solution of copper, the principal gases evolved are nitrogen and nitrous oxide, and with zinc, mercury and iron, nitrogen is the principal gaseous product. E. J. Joss explains these results on the assumption that ammonium nitrite produced by the action of copper on nitric acid in the presence of ammonium nitrate decomposes under ordinary conditions into nitrogen and water.3 Because ammonium nitrite can decompose into nitrogen and water, it is often assumed that the nitrogen produced in the reduction of nitric acid results from the interaction of nitrous acid and ammonium salts. Though this is possibly true in some cases, Bancroft shows that the generalisation is not justified because cases are known in which nitrogen is evolved when there is no reason to suppose that ammonia is formed at all. It was essential, therefore, to identify the gaseous products of the reaction with which this paper deals. This has been described in Part 111 and it is shown that nitrogen is not a product of the reduction of 4 0 7 ~nitric acid by hydrogen sulphide in the early stages. Hence the conclusion is drawn that the ammonia first formed is due to a side reaction and not to the main process of reduction of the nitric acid. There are three possibilities, all of which may be realised to some extent : a). The ammonia is formed in solution by the reduction of nitrogen peroxide, nitrous acid or nitroxyl sulphuric acid HSO,
+ 3s’’ = KHa + 3s +(zH?O)
(19)
Hyponitrous acid can give ammonia (equation 1 2 ) . c ! . Ammonia may be produced by the action of hydrogen sulphide on nitric oxide.’ b!.
HZS
+ S O --t ISH;)?S+
9 2 0
(20)
Scrubbed hydrogen sulphide was passed into water in a flask filled with scrubbed carbon dioxide gas to prevent thc backnard diffusion of oxygen (air). Sitric oxide as passed into the hydrogen sulphide solution and interact,ion J. Chem.
28. 6 2 8 (1S7j:. 32. 67 116ijr. J A r n d t : Z. physik. Chem.. 39. 64 f19or’~ 1
,&IC.,
* J. Chem. &IC..
1352
H. B . DUSSICLIFF A S D SARDAR YOHAMMAD
took place. Fumes of nitrogen peroxide were not observed inside the flask nor any reaction in the gas phase even when nitric oxide was in excess and the gases emitted from the flask gave brown fumes when they mixed with the air.1 The solution contained much sulphur and, after removing hydrogen sulphide, it was found to give tests for ammonia readily. The liquid also contained sulphuric acid but nitric and nitrous acid were not detected even by the most delicate tests. This reaction is under further investigation. See aLo Bagster: loc. cit. Hydrogen sulphide interacts with nitrous acid giving much nitric oxide either in the presence of excess of nitric acid or of hydrogen sulphide2 while nitric oxide with hydrogen sulphide and oxygen (Bagster, Table I) yields much ammonia and also nitrous oxide, nitrogen, and hydroxylamine. I t is possible therefore that, in the later stages of the reaction under investigation, ammonia is formed by the action of hydrogen sulphide on nitric oxide and immediately decomposed giving nitrogen. .-is has been pointed out, however, it is not necessary to assume the intermediate formation of considerable quantities of ammonia and the authors are not of the opinion that this is the source of the nitrogen p r e ~ e n t . ~ 3). The remarkable condition of chemical "stasis" which comes about when the concentration of the nitric acid has fallen to Z ~ and ~ that C of the sulphuric acid has reached 1 57'may possibly be due to the combination of these two acids in solution a t these specified percentages TThich correspond very roughly with equivalent quantities of the two acids, the nitric acid being slightly in excess. Salts of nitrato-sulphuric acid4 HSO,, H:!S04 or (HO)2 = X0.0S02.0H e.g. KN03, KHSO, or HK03, II?SO4 and 3H4S03, corresponding with the constitution (HO) (SHdO)
\
(SH?O)--?;
\
- 0 - SO1
have been obtained. It seems not improbable that a compound
a H S 0 3 , H2S04or (HO)?= S O
-0 \\
SO?
/
(HO):! =S O - 0
'Leconte: loc. cit.; Lunge: Ber., 14, 2196 (1881'8. Bagster: loc. cit., 2683, Table 11. Sardar Mohammad a n d H. D. Suri, working in this laboratory have shown t h a t , when nitric oxide is bubbled through aqueous solutions of hydrogen sulphide in the absence of air, action takes place with the formation of a m m o n i k compounds one of which is the tetrathionate. Hydroxylamine nitrate and nitrite are absent. The gaseous products are nitrogen and nitrous oxide. Quantitative estimations show t h a t nitrogen is the principal product comprising in the early stages 9 5 5 of the nitrogennitrous oxide mixture while. in the latter stages of the reaction, the proportion of nitrous oxide increases, the maximum recorded being z j c > of the mixture. When nitric oside is bubbled through a solution of hydrogen sulphide in 5 s sulphuric acid, only traces of ammonia are formed in the solution and the small quantity of gas formed is practically all nitrogen. Jacquelain: Ann. C h i m Phys., ' 2 , 70, 310 (1839); Friedhein a n d lIozlin: Z. anorg. C'hem., 6, 297 (1894:.
HYDROGES SCLPHIDE AND SITRIC ACID
I3 5.3
corresponding with the above proportions of nitric and sulphuric acids may be formed in solution and that it may be passive to hydrogen sulphide. Physicochemical methods would determine whether this compound exists in solution or not. This investigation will be undertaken later. In order to confirm these results, excess of hydrogen sulphide was bubbled through a solution containing 23yc nitric acid and I jyc sulphuric acid. No action resulted even after the passage of the gas for four hours at laboratory temperature ( c . 2 ~ ~ )If. the action was started by the addition of traces of oxides of nitrogen, it subsided after a short time. It should be noted that i t would have been possible to pass through a stage in set I, Table I , in which the relative concentrations of the two acids were in equivalent proportions. This would have been in the I . j-3 hours’ period when it has been observed that the rate of decomposition was most rapid. By calculation t,his would have been when the nitric acid had fallen to about 19% and the sulphuric acid was about 14%. The fact that, in this set of experiments, the reaction did not cease is probably due to the lowness of the total coneenfration of t,he acids resulting in the dissociation of the postulated compound. These are speculations and must be examined by scientific method but the possibility of their truth is contributed to by t’he fact t’hat Friedheim and Mozlin obtained the salt K2S04.HK03referred to above froin a solution containing one molecule of sulphuric acid and two molecules of nitric acid. Part 111. The Gaseous Products of the Reaction The gases from the reaction flask viere free from nitrogen peroxide and had to be examined for nitrous oxide, nitric oxide and nitrogen. Carbon dioxide and traces of hydrogen sulphide might be present. The former was remorrd by the potash in G (Fig. I ) and the latter by means of solid cupric phosphate.‘ See also Lunge’s “Technical Gas -Analysis,” p. 249. Sitric oxide was estimated by absorbing it in a mixture of fire volumes of saturated potassium bichromate solution wit,h one volume of concentrated sulphuric acid.2 The reagent is stable at ordinary tcniperatures and does not evolve oxygen when agitated with indifferent gases. Sitric oxide is quantitatively oxidised to nitric acid. zS0
=
KnCr?Oi
+ 4HnS04
=
+ Cr,lS0,)3 + 3H,O + 2HS03
K2SO4
(21)
Sitrogen and nitrous oxide were estimated in a specially designed apparat,us (Fig. I ) , which Jyas a modified form of that used by Milligan for the same purpose. The cocks, TI, T2,TS,and T,of a S-way capillary tube, W, were connected with ( I ) , a Lunge’s nitrometer, H, i z ) , the tube, G, containing caustic potash solution and used later to receive t,he gases formed in actual experiments ( 3 ) , a supply of pure hydrogen, AI, prepared by the electrolysis of saturated baryta solution and ( A j , the combustion pipette, Q; respectivel>-.
*
Harding and Johnson Ind. Eng. C,hem., 5 , 836 i19131. Von Iinorre: Chem. Ind.. 25, j34 (1902).
I3 54
H. B. DUNNICLIFF AND SARDAR MOHAMMAD
The reservoir, B, and oxygen tube, S , were closed by guard tubes containing solid caustic potash. Any desired volume of hydrogen could be transferred into the Lunge’s nitrometer. The nitrometer was cooled by means of a water jacket, K, containing supply, Kz, and outlet tubes, K1, and a thermometer, KI. The syringe bulb, L, was used to bubble air through the water to keep it stirred. The combustion pipette, Q, made from an inverted separating funnel, was surrounded by a copper spiral, T, perforated with small holes directed towards the pipette. Air, from an electric blower and chilled by passing through a spiral cooled by melting ice, was driven on to the glass walls and tap of the pipette to protect them from breakage during a combustion. Through the rubber stopper of the pipette was passed a glass tube from the mercury reservoir and two copper rods, SI and S:! (3.5 mm. diameter), having a t their ends vertical sockets PLATINUM EXT€N S IO W into which were set platinum rods ( I mm. diam.) ending in hooks beG L A S S TUBE tween which the platinum heating SET IN BUNG spiral was stretched. Platinum extensions were used so that the copper RUBBER RUNG rods were never exposed to the gases during combustion. The rods were connected through a sn;itch and a rheostat to a battery. Fig. z makes PPER RODS these points clear. SIand SZare the copper rods, TI and Tz, the platinum rods and F, the platinum spiral. Mercury is shown as black and the apparatus is shown set for the heating FIG.2 of the spiral. The copper rods are covCombustion Pipette ered and electrical connection between the two terminals is prevented by the setting in the rubber stopper, two glass tubes, T and TI. It will be seen that the gases can be swept from the pipette and also that, when in the position shown, the spiral can be heated without fear of a short circuit. Before performing any experiment, air was completely expelled from the capillary tubes and pipette by filling them with mercury, using the nitrometer as a pump. A measured volume of hydrogen was purified by drawing it several times into the combustion pipette (and heating the spiral) by raising and lowering the reservoir of the nitrometer. Khen the volume of the cooled gas was constant a t atmospheric temperature and pressure, it was finally transferred to the combustion pipette. Pure nitrous oxide prepared by T. Meyer’s method‘ and was then introduced into the nitrometer and measured. The J. Chem. SOC.,141, 175 (1875).
HYDROGES SULPHIDE AND SITRIC ACID
1355
screw clamp, H,, on the pressure tubing was closed and the mercury reservoir of the nitrometer raised above the level of the three-way cock. The taps of the pipette and the nitrometer were opened and then the screw clamp just opened. The mercury in the nitrometer should rise so slowly as to be just perceptible. At the same time the spiral was made red hot and the cooling device started. I n this way, the whole of the gas was slowly brought under combustion. The gases were then passed backwards and forwards slowly over the heated spiral until there was no further contraction when measured several times in the nitrometer by bringing the mercury reservoir to a fixed position. The gases were ultimately measured a t atmospheric pressure and the contraction noted. This should be equal to the volume of nitrous oxide introduced. Actual values are given in Table IV showing a high degree of accuracy (about 0.3%).
TABLE Is’ Volume of nitrous oxide
Volume after combustion
c.cs.
c.cs.
34.30
26.2
343
26.2
43.6 40.2
35.2
43.5
35.3
30 ’
40.
30.1
Volume of hydrogen c.cs.
i) . ii) . iii) .
I
Contraction
Error r-
0
-0,3 -0.3
Estimation of the Gaseous Products o j the Reaction. The apparatus is shown in Fig. 3. Two Kipps were used, one for making carbon dioxide by the action of hydrochloric acid on marble chips and the other for preparing hydrogen sulphide by treating blocks made of calcium sulphide and plaster of Paris with hydrochloric acid. The pressure necessary to force the carbon dioxide through the series of liquids was attained by making it possible to raise the acid reservoir of the Kipp to a considerable height by extending it with a long rubber tube. Carbon dioxide was scrubbed by sodium bicarbonate solution and hydrogen sulphide by dilute sodium sulphide. The tower, D, contained copper phosphate. The reaction flask, C, was connected by means of rubber tubing inside which paraffin wax had been run to prevent the oxides of nitrogen from attacking the rubber. The reaction flask could be shaken easily. To allow hydrogen sulphide to react on the nitric acid, the cock, F’, was closed and the hydrogen sulphide cock opened,
H. B . DUNNICLIFF AKD SARDAR MOHAXYAD
I356
while the reaction flask was shaken. When the action ceased, the cock, F’, was opened and the products of reaction swept forward. I n this way the correct amount of hydrogen sulphide for the reaction was admitted as required and its reaction completed before a further supply was taken in. Thus, excess of hydrogen sulphide was avoided. The absence of the free gas was shown by the fact that the cupric phosphate in the scrubber did not become Ez, Ea contained potassium bibrown or black. The absorbing vessels, El, chromate solution mixed with sulphuric acid. Cock F, (Fig. I) could be connected either with the potassium hydroxide tube, G, or the air. The lower part of the tube, G, was connected by a rubber bung with the lower portion of the Schiff’s nitrometer containing mercury. This arrangement prevented the liquid in G from being sucked back into the absorption vessels. Before commencing the experiment, air was driven out of the apparatus by a current of carbon dioxide. The scrubbing gas (COz) was let out through the cock, F, into the air. When, on trial, carbon dioxide was completely absorbed by the caustic potash in G, the experiment was commenced. It was observed that there was very little or no blackening of the cupric phosphate. Unabsorbed gases were collected in G over caustic potash solution. When sufficient gas had collected, it was transferred to the nitrometer. After the experiment, the residual gases were driven into the vessel, G, by a current of carbon dioxide gas. The residual gas was examined, as described above, for pure nitrous oxide and nitrogen. The standard bichromate solution was back titrated and the amount of nitric oxide absorbed was calculated. The results with the residual gas obtained by the passage of hydrogen sulphide up to 30 minutes showed that the gas is practically pure nitrous oxide (see footnote, p. 13j8).
TABLE V Combustion wit’h the residual gas Volume
of Hz
.
i) ii) . iii) ,
Volumes of the gas
Volume after Contraction the combustion
42 .o
30.2
42 4
40.0
29.3 36.2
40.7
45.8
46.3
29.8 29.6 36.7
Residual gas -0.5
+0.3 +o. j
The absence of nitrogen precludes the possibility of any reaction betwcen ammonium salts and nitrous acid in the early stages. Some nitrogen would have been formed. Hence it is probable that, a t first, ammonia is only formed a s a result of side reactions and is not an important product of the main reaction. I n a carefully conducted experiment, all the soluble and gaseous products were determined. The results obtained after 40 minutes exposure to the gas as just described on p. 1356, (not bubbling unknown excess of the hydrogen sulphide) are shown in Table VI :
HYDROGES SCLPHIDE ASD SITRIC ACID
1357
TART,E S’I Initial strength of nitric acid = 42.95%, H I 0 3 containing 9.54 grams of nitrogen: Analysis of products of reaction H S 0 3 = 3 9 . 8 2 7 ~containing 8.85 grams nitrogen. 1) ” 0 I4 ” HxO3 = 0 46% 1 2 % ” HQSOI = 2 ,, 0.13 ” XH3 = o 1 6 7 ~ ” ,, o 38 ” s o = 0 8170 1) S ? O = 0 21SC !1 0.13 s = 0 47% )’
’)
$’
9.63
Error = +o 09 gram of nitrogen, equivalent to 0.4 gram of nitric acid. The results of a second experiment (Table S’III) in which the solution was examined after I hour 2 0 minutes but from which the gases were only collected during the last hour of the experiment are also given.
TABLE S’II Initial strength of nitric acid = 4 2 . 9 j z HSO, containing 9.54 grams of nitrogen. “03 = 33.50% H2S04 4.79% HSOZ = 0 . 1 8 7 ~ SH3 = 0.~7yc SO = I , 2 4 7 0 containing 0.j j gram nitrogen. 1, s*o = O , Z 8 Y c 0.18 gram nitrogen. s = 0.817~ 1
Again no nitrogen was found and the ratio of nitric oxide to nitrous oxide formed, in terms of nitrogen, remained the same, viz. 3:1. I n Table S’III are given the results of a further series of determinations in which the gaseous products were allowed to escape up to a certain time (col. i) and then the gaseous products collected between the times given in columns i and ii. Columns iii and iv show the proportions (per cent, by volume) in which nitrous oxide and nitrogen were found in t,he gaseous mixtures collected in the intervals between these times (cols. i & ii).
TABLE VI11 Collection of gases Time started Time finished in hours in hours 0
0.50 2.25
3.50 j .oo
0 .j
o I .66 2,75
4.00 j .jo
Sitrous oxide c100.0
Sitrogen “5 0
0.0
100.0
0.0
79.7
20.3
jj.1
44.9
30.0
70.0
13 58
H. B. DUNNICLIFF AND SARDAR MOHAMMAD
The amount of nitrous oxide decreases continuously.1 Nitrogen makes its appearance and finally predominates though nitrous oxide is never completely absent. The appearance of nitrogen starts about the time when the rate of chemical action is a maximum and when the sulphuric acid formed is only a little less than equivalent to the amount of nitric acid decomposed. This is an interesting observation as it coincides with the fact that the more I early the nitric (or nitrous) acid is exerting its maximum power of oxidation the greater the proportion of nitrogen in the gaseous products, The increase in the concentration of sulphuric acid may be responsible in part for the instability of nitrous acid formed as an intermediate product or the explanation may be that the presence of sulphuric acid facilitates the formation of ammonia from nitric oxide by the action of hydrogen sulphide (vide Bagster: loc. cit). The ammonia would then be decomposed by the nitrous acid present and give nitrogen (vide supra p. 1356) though, as shown previously, it is not essential to suppose the formation of ammonia because nitrogen is one of the products of the reaction and one must not come too hastily to the conclusion that ammonia is the intermediate product. After a short exposure of the solution of nitric acid containing ammonium sulphate or nitrate to a current of hydrogen sulphide, much nitrogen was evolved together with nitric oxide. Table IX shows the analysis of the gas from two of these experiments.
TABLE IX Tolume of hydrogen
i) . ii) .
44.6 41.7
Volume of the gas
31 . o 27.9
Volume left after combustion
Contraction
N*
74.7 68.7
0.9
3 0 .I
0.9
27
.o
This suggests that nitrous acid is an early and important product of the reaction. Nitrous acid is almost certainly the procursor of hyponitrous acid, which, in acid solution, decomposes into water and nitrous oxide. This experiment also shows that nitrous acid-the parent of hyponitrous acidis destroyed by ammonium salts. I n this way, the side reaction prevents the appearance of the normal products of the reaction. These results also show that, in the early stages the action of hydrogen sulphide on nitric acid, ammonium salts are not formed and suusequently decomposed. Hydrogen sulphide was passed into 43% nitric acid cont,aining jcG of sulphuric acid as described on p. 13j6. The reaction proceeds rapidly a t first but slows down towards the finish. The gaseous products were collected between the intervals of time shown in Table X, columns ii and iii, and nit'rous oxide and nitrogen were obtained in t,he proportions sliown in columns iv and v. A small amount of nitrous oxide is lost by absorption in the caustic potash solution G. This does not affect the general conclusions drawn.
HYDROGEN SULPHIDE A S D NITRIC ACID
Sitric acid Sulphuric acid
Concentrations Time in hours
so.
Sitrous oxide
= 42.95% = j ooyo Sitrogen Ratio SZO SZ by volume
CC Pure nitrous oxide never obtained.
Start
Finish
I
0.05
0 ,I O
2
0.00
0.50
ij.6
3
0.50
I . j O
1.75 .is
2.25 3 .oo 3.75
The gas was practically all nitric oxide. 58.5 41 .j 3 : 2
5
5 6 7
8
2
3.25
.oo 6 .oo j
I359
5.50
6.50
38.8 29.1 23 .o
24.4
61.2 70.9
3
: I
:3 3 :7 2
11.0 2 : 7 Segligible: only 4-5 c.cs. in all. A very little nitric oxide is evolved. m-
Free HSOI still undecomposed
39,1% 36.55
29,1% 27.6% 24.15
2 3 . iYc
Too much importance should not be attached to the time-composition rela. tionship, cols. ii and iii and col. vii, as the 'method of working and the temperature variations make the relationship only of a general nature. When these results are compared with those in Table VIII, it will be seen ( I ) that the presence of sulphuric acid affects the action by eliminating a preliminary stage in which nitrous and nitric oxides are among the products of reaction but nitrogen is absent and ( 2 ) that, although pure nitrous oxide cannot bc obtained, until chemical stasis sets in, a condition is never realised in which it is formed at all.
Part IV. Summary and Discussion I n order to facilitate the reading of the paper, much discussion has been introduced into the body of the report. To avoid duplication only the most general conclusions are summarised in this section. I). Solutions containing j% nitric acid and less are not attacked by hydrogen sulphide even if nitrous fumes are added. Solutions of higher concentrations are attacked after a more or less 2). long interval of time. This "induction" period is removed if nitrous fumes are introduced or slight decomposition of nitric acid is induced b,y insolation. The addition of sulphuric acid increases the induction period. d 43Tc (c.6.8N) solution of nitric acid was used. 3 1 . The products of reaction are sulphuric acid, nitrous acid, ammonia; sulphur, nitric oxide, nitrous oxide and nitrogen. 4). A possible explanation of the evolution of nitrogen in the later stages is that ammonia is formed and immediately decomposed. If this is so, ammonia is not formed in the early stages of the reaction in any quantity though there is evidence to show that it might be formed later. Apparently the presence of sulphuric acid exerts considerable influence on the formation of nitrogen. There are however explanations for the existence of nitrogen
1360
H. B. D U S S I C L I F F A S D SARDAR M0HhMJl.W
other than through the agency of aninionia as an intermediate compound, and it is probable that any ammonia fornied is the result of side reactions or minor secondary reactions. There is a concentration of ammonium salt below which there is no interaction with nitrous acid. j ) . The presence of sulphuric acid affects the progress and ultimate products of the reaction. If to the nitric acid, sulphuric acid is added before passing hydrogen sulphide, the reaction stops when the concentration of the nitric acid has fallen t o 237c and the total sulphuric acid concentration is 155%. These are roughly equivalent quantities but isotonic solut'ions of lower concentrations do not eshibit this stoppage in the progress of the reduction. It is probable that the nitric acid and sulphuric acid enter into a chemical combination which is inert to the action of hydrogen sulphide, but which at lower concentrations become decomposed or dissociated and attackable by hydrogen sulphide (p, 1 3 5 2 ) . Suggestions regarding the mechanism of the formation of sulphuric acid are given on p. 1348. 6) From a study of absorpt'ion spectra and conductivity measurements A . Hantzsch' suggests that nitric acid consists of an equilibrium mixture of (i) O2 IiOH (pseudo-nitric acid), (ii) true acid present as hydroxonium salt Ii03(H.0H2) and (iii) acid forming salts like nitronium nitrate (XO3)2 [(OHJ] N. The hydroxonium salt may be
OH I i
/\
0
0
I 1
H
H
v 0
compared with the accepted views on sulphuric acid and leads one t'o the analogous but unstable hydrogen sulphide derivative which breaks down thus:
OH h-
4%
0
0
OH
I n strongly acid solutions this nitrous acid is probably the source of the nitric oxide and also of the nitrogen peroxide when it occurs. E. J. Joss2
*
Ber. 58,941 (192j). J. Phys. Chem., 40, 1254 (1924).
1361
HYDROGEN SULPHIDE AND S I T R I C ACID
concluded that the depolariser in the reduction of nitric acid is nitrosic acid1 ( H 2 S 2 0 5;)the formation of which in the action under review may take place in t x o ways:
OH 0
1)
\/ s
HO
SOH
//
+
o
=
0
0
0
OH
\/ s //
+
0
OH 0
\/ S \
Jr HzS+
OH
\/ x /
(24)
? e
0
0
(23)
0
OH
0
\/ s \
or
N
//
2)
0
v\ hT S O H
H
H
\/ s 0
OH
OH
I n other words nitric acid is activated owing to the formation of nitrosic acid, the nitrous acid being the catalyst. Kitrous acid also acts as an oxidising agent. From nitrous acid, by the action of a reducing agent, hngeli’s acid2 or nitro-hydroxylamic acid is produced thus:
OH
HO
\
/
S
\ ; 0
0
H
H
I L
HO
OH
I I X - K + \ /
H20
+
S
0
\/ S
1
Oddo: Gazs., 45 I, 413 (1915).
* Angeli: Gazz., 26 11, 24j (1897,; .Ingeio and Angelico: 33 11, 245
(1903).
1362
H. B. DUSNICLIFF .4ND SARDAR MOHAMMAD
This is further reduced to H2N202,hyponitrous acid, which, in acid solution, decomposes giving nitrous oxide and water
HO
OH
I
I
+ HBS+
X-S
\/
7'
N = N
+ HBO+ S .--+ S20 + H 2 0
(27)
0 Hyponitrous acid can also decompose giving nitrogen (equation 13), or possibly ammonia (equation 1 2 ) one of the other potential sources of which is the action of hydrogen sulphide on nitric oxide (p. 1351). Department of Che?nzstry, Goceriiment College, Piuijub
l-?iai
ersatij,
Lahore. Iiidia. M a y 21, 1929.
