REDUCTION O F SILVER HALIDES BY SODIUM S U L P H I T E AND S I T R I T E I S PRESEK’CE OF INDUCTORS AND A CHEAP DEVELOPER I N PHOTOGRAPHY BY RAM BEHARI LAL VERMA AND N. R . DHAR
In previous publications‘ from these laboratories it has been shown that sodium sulphite, which readily undergoes oxidation in air or oxygen initia.tes or accelerates the oxidation of sodium nitrite, potassium oxalate, ferrous sulphate, cuprous, nickelous, ferrous and other hydroxides. In photography, sodium sulphite has been largely used since its introduction by Berkeley, to prevent stains produced by the oxidation of developers in alkaline solutions. The exact function of the sulphite in development has not yet been cleared up.2 In this paper, the reactions between sodium sulphite and different sparingly soluble silver salts have been studied in presence of various organic and inorganic developers and an att,empt has been made to find out the real mechanism of the sodium sulphite added in photographic development. The reactions, however, differ from the usual induced reactions of sodium sulphite in the fact that the oxidation of sodium sulphite is not the primary change here but a secondary one. In fact, the developers used in photography, appear to induce the reduction of the sparingly soluble silver salts by sodium sulphite. The following reactions have been investigated : Reduction of ( I ) silver chloride, ( 2 ) silver bromide, (3) silver iodide, (4) silver thiocyanate by sodium sulphite in presence of (a) Hydroquinone. (b) Pyrogallol. (c) Metol. (d) Hydroxylamine hydrochloride. (e) Hydrazine sulphate. (f) Ferrous sulphate.
Experimental Procedure 20 cc. of a standard solution of silver nitrate (S,’99.5) were added t o 20 CC. of a standard solution of sodium chloride (S ’ 9 9 .j) taken in a bottle and silver chloride was precipitated. 20 cc. of a standard solution of the developer was then added and the volume was made up to I O O cc. The mixture
Dhar: J. Chem. SOC.,111, 694 (1927); Proc. .4kad. Wet. .4msterdam, 29, I 0 2 3 (1921); 2. anorg. Chern. 128 z o j ( 1 9 2 3 ) ; J. Phys. Chem., 28, 943 (1924);Mittra and Dhar: Trans. Faraday Soc., I?, 67b (1922);‘Z. anorg. Chem., 122, 146 (1922); J. Phys. Chem., 29,376; Palit and Dhar: 799 (1925); 30, 939 (1926);32, 1663 (1928); 34, 7 1 1 (1930). * Compare, “Photography as a Scientific Implement,” 1 5 0 (1923).
REDUCTIOK O F SILVER HALIDES BY SODIUM SULPHITE
1771
was shaken for 20 minutes and filtered. The experiments were carried on a t 30'. The precipitate remaining on the filter paper was washed several times with distilled water and nitric acid was added to it t o dissolve out the silver formed by reduction. The filter paper was washed and the filtrate titrated against a standard potassium thiocyanate solution using ferric sulphate as an indicator. From this the amount of silver reduced was calculated. In the case of silver bromide, sodium bromide was used and in the case of silver iodide, potassium iodide was used for the precipitation of the silver halides. Silver thiocyanate was precipitated with potassium thiocyanate. In all the cases the strength of sodium sulphite was hT,'24.3. Tables 1-111 show the experimental results:
TABLE I Hydroquinone Na2S03present CC.
0
hZ/200
-4gC1 present gm.
o
02881 7,
5
3,
IO
71
15 20
1)
Ag. obtained gm. 0.00144
0.0062 I 0,00909 0.01 170 0.01674
TABLE I1 Hydroquinone M/400 ?;a?SOI present cc. 0
5 IO 15 20
.4gCl present gm.
A g . obtained gm.
o 02881
0.0012 6
1, 2,
, 17
0.00450 0.00810 0.01008 0.01125
TABLE I11 Hydroquinone hf/800 S a ? S O i present
AgCl present
CC.
g*.
0
5 IO 1.5 20
0.02281
,, ,f I,
,,
Ag. obtained
w.
0.00099 0.00342 0.00657 0.00729 0.00872
From the foregoing results we see that the amount of AgCl reduced decreases as the concentration of the solution of the developer decreases but increases in all the cases as the amount of S a l s 0 3 increases. Exactly similar results have been obtained with other developers and various silver salts.
Iii2
RAM BEHARI LAL VERMA AKD N. R. DHAR
The above results show that silver chloride is reduced by the developer solution even in the absence of sodium sulphite. Similar results are obtained with other silver salts also. The greatest reduction is observed with metol and the least with ferrous sulphate. The results in Table IT will show that in the absence of sodium sulphite, the reducing power of the developer is in the following decreasing order: Metol >Pyrogallol >Hydroquinone >Hydrazine sulphate >Hydroxylamine hydrochloride >Ferrous sulphate.
TABLE IT‘ M/20o
RIetol Pyrogallol Hydroquinone Hydrazine sulphate Hydroxylamine hydrochloride Ferrous sulphate
BgCl
Amount of Sg. reduced from AgBr AgI
AgCNS
o oooj4
0.0004j
0.0020j
-
0.0004j
o.ooo27
0.00144 0.00063
0.00351 0.00063
0.0004j o.00013
o.00022
0.00216
o.oo~j9
0.0002j
0,00054
-
-
o.00052
0.00009
0.000
I8
-
As has been mentioned before, if no developer is used, sodium sulphite does not reduce the silver salts and hence the silver salts are the actors. From the results it will be seen that even without the sodium sulphite the developers reduce the silver salts (though to a slight extent), and hence are called the inductors. Sodium sulphite in its turn is the acceptor. It is remarkable that in the presence or in the absence of sodium sulphite, the reduction of silver salts with the organic developers is greater than with the inorganic developers of the same concentration. If the reduction by sodium sulphite be considered in presence of the inductors the inducing powers of these inductors come out in the same order in which they reduce the silver salts in the absence of sodium sulphite :Metol >Pyrogallol >Hydroquinone >Hydrazine sulphate >Hydroxylamine hydrochloride > Ferrous sulphate. The results in Table 5’ will illustrate this point. The results quoted in Table V are those in which 2 0 cc. of sodium sulphite were used. Another thing to be noted from the results is that the greatest reduction is in the case of chlorides and the least in the case of iodides i.e. the order is AgCl >AgBr >AgCKS >AgI. The reason for this may be that silver iodide is the least soluble and silver chloride is the most soluble of the silver salts investigated. These results show that the reduction of the silver salts by sodium sulphite can take place only in the presence of substances which themselves can reduce the silver salts. I t appears that these inductors in being oxidised
REDCCTION O F SILVER HALIDES BY SODICM SULPHITE
TABLE
1’
Inductor
Salt
XgCl.
M/200
Metol Pyrogallol Hydroquinone Hydrazine sulphate Hydroxylamine hydrochloride Ferrous sulphate
0 . 0 2 I33
M/40o
1773
m/800
0.02052
0,01935 0.0161I
0.01782 0,00792
0.016j4
0.OIIZj
0.00872
0,01377
0.00918
o.ooj29
0.0098I 0.006;5
0,00549
0.002jo
0.00486
0 .002fO
I1 AgBr.
Metol 0.01872 Pyrogallol 0.0144 Hydroquinone 0.00999 Hydrazine sulphate 0.00369 Hydroxylamine hydrochloride 0.00369 Ferrous sulphate -
0.01j82
0.01269
0.00846 o.oojo2 0,00342 0.00306
0.0oqj7 0.00306 0.00288
0.00261
0.001 j3
0.0008j
0.000; 2
0,00063
0 . ooojq
0.0002 ;o
0.00022 5
0 . 0 0 0 27 0
0 , 0002 2
0,01449
0 , 0 1 2j I
0.005i6 0.00198 0,00036 0.00036
0.00261
0,00909 0.00198 __
0.00450
I11 AgI.
AIetol 0.0009 Pyrogallol 0.0009 Hydrazine sulphate 0,0003I 5 Hydroxylamine hydrochloride o.00045
Is’ AgCSS. llletol Pyrogallol Hydroquinone Hydrazine sulphate Hydroxylamine hydrochloride
0,0011;
5
0.0002;
0.0002~
start the oxidation of sodium sulphite by the silver salts and when once the oxidation of sodium sulphite is started, it goes on reducing the silver salts wElen its concentration is increased. For instance metol reduces the silver salts thus: hletol silver chloride = silver oxidation product of metol. This primary reaction starts the oxidation of sodium sulphite according to the following equation: S a 2 S 0 3 ahgCl H?O = Sa2SOI nhg 2 HCl, the reduction by sodium sulphite being a secondary change. Similarly for the other reactions primary and secondary reactions occur. In all these cases, the primary change, i.e. the oxidation of the easily oxidisible substances, pyrogallol, metol etc. takes place and this primary change induces the secondary or the induced reaction i.e. the oxidation of sodium sulphite by the silver salts. The view that hydroquinone can act as an inductor and sodium sulphite as the acceptor has also been advanced by Bancroft.
+
+
+
+
J. Phys. Chem., 33, 1188 (1929).
+
+
RAM BEHARI LAL VERMA A T D N. R . DHAR
I774
K e have observed that sodium nitrite can also behave as an acceptor in a similar way in the reduction of the silver salts in the presence of inductors such as metol, hydroquinone and pyrogallol. Sodium nitrite alone does not reduce the silver salts. The experimental results are given in Table VI.
TABLE VI K/200 N a K 0 2 added
AgCl present gm.
cc.
I. 11
Hydroquinone 0.02881 ii
5
i,
IO
ii
15
li
20
11. 31,:zoo Hydroquinone. .oz8S1 il
20
0 20
Fm.
IOO
0
0
AS. redured
111. 111 400 Hydroquinone. o 02881 o 02881
0.00189 0.00324 0.00387 0.00432 0.00486 0.00144 0.00369
0.00108 0.002;o
The results with hletol are given in Table VII.
TABLE VI1 NaNOl added
AgCl present
w.
cc.
hI 0
5 IO
I O O Metol 0.02881
0,002 52
ii
0,00360
((
0.00423
li
0,00468
ii
0.0ojL)o
15
20
Ag. reduced gm.
M j z o o hletol 0
20
0.02881 li
0.002
16
0,00423
hI/400 Jletol 0 20
0.02881
0 . 0 0 17 I
ii
0.00324
This result very well bears out the observation of Dharl that one chemical change will either promote or induce another chemical change of the same nature. The reduction of the silver salts by metol, etc. brings about their reduction by sodium nitrite. '€'roc. Akad. !?-et. Amsterdam, 29, ~ o r (1921). j
REDUCTION O F SILVER HALIDES BY SODIUM SULPHITE
‘iij
The reduction may be represented in the following way: metol = Ag oxidation product of metol (primary change) AgCl z AgCl Ka?iOz H 2 0 = nAg T\’aSOa n HC1 (secondary change) It is interesting to note from the results that of all the organic developers the best inductor is metol and this is followed by pyrogallol and hydroquinone. This is probably due to the side chain present in metol. Of the inorganic developers hydrazine sulphate is more reducing than hydroxylamine hydrochloride or ferrous sulphate. V e have carried on some experiments to prove the practical use of the reduction of silver salts by sodium sulphite. We have tried to show experimentally that sodium sulphite plays an important part in the development in photography. I t has been observed that the efficiency of the developer is well maintained even when the amount of metol is considerably decreased and the amount of sodium sulphite is largely increased. Photographic plates were used and the developing solutions containing different amounts of sodium sulphite were utilised. The developing solution used in practice has the following composition:
+ +
+
I.
I
+
+
+
gm. metol
4 gms. hydroquinone
2 5 gms. PvrazSOa
11.
zj
gms. Ka2COs
1
in
500
