The reaction of Hyposulphite of Soda with Nitrate of Silver.—In order to understand more fully how decomposition of Hyposulphite of Silver may affect the process of fixing, the peculiar properties of this salt should be studied. With this view Nitrate of Silver and Hyposulphite of Soda may be mixed in equivalent proportions, viz. about twenty-one grains of the former salt to sixteen grains of the latter, first dissolving each in separate vessels in half an ounce of distilled water. These solutions are to be added to each other and well agitated; immediately a dense deposit forms, which is Hyposulphite of Silver.
At this point a curious series of changes commences. The precipitate, at first white and curdy, soon alters in colour: it becomes canary-yellow, then of a rich orange-yellow, afterwards liver-colour, and finally black. The rationale of these changes is explained to a certain extent by studying the composition of the Hyposulphite of Silver. The formula for this substance is as follows:—
AgO S2O2.
But AgO S2O2 plainly equals AgS, or Sulphuret of Silver, and SO3, or Sulphuric Acid. The acid reaction assumed by the supernatant liquid is due therefore to Sulphuric Acid, and the black substance formed is Sulphuret of Silver. The yellow and orange-yellow compounds are earlier stages of the decomposition, but their exact nature is uncertain.
The instability of Hyposulphite of Silver is principally seen when it is in an isolated state: the presence of an excess of Hyposulphite of Soda renders it more permanent, by forming a double salt, as already described.
In fixing Photographic prints, this brown deposit of Sulphuret of Silver is very liable to form in the Bath and upon the picture; particularly so when the temperature is high. To obviate it, observe the following directions:—It is especially in the reaction between Nitrate of Silver and Hyposulphite of Soda that the blackening is seen; the Chloride and other insoluble Salts of Silver being dissolved, even to saturation, without any decomposition of the Hyposulphite formed. Hence if the print be washed in water to remove the soluble Nitrate, a very much weaker fixing Bath than usual may be employed. But if the proofs are taken at once from the printing frame and immersed in a dilute Bath of Hyposulphite (one part of the salt to six or eight of water), a shade of brown may often be observed to pass over the surface of the print, and a large deposit of Sulphuret of Silver soon forms as the result of the decomposition. On the other hand, with a strong Hyposulphite Bath there is little or no discoloration, and the black deposit is absent.
The print must also be left for a sufficient time in the fixing bath, or some appearance of brown patches,[18] visible by transmitted light, may occur. Each atom of Nitrate of Silver requires three atoms of Hyposulphite of Soda to form the sweet and soluble double salt, and hence, if the action be not continued sufficiently long, another compound will be formed almost tasteless and insoluble ([p. 44]). Even immersion in a new Bath of Hyposulphite of Soda does not fix the print when once the yellow stage of decomposition has been established. This yellow salt is insoluble in Hyposulphite of Soda, and consequently remains in the paper.
[18] The writer has noticed that when sensitive paper is kept for some time before being used for printing, these yellow patches of imperfect fixation are very liable to occur. The Nitrate of Silver appears gradually to enter into combination with the organic matter of the size of the paper, and cannot then be so easily extracted by the fixing bath.
In fixing prints by Ammonia the Author has found that the same rule may be applied as in the case of Hyposulphite of Soda, viz. that if the process be not properly performed, the white parts of the print will appear spotted when held up to the light, from a portion of insoluble Silver Salt remaining in the paper. Prints imperfectly fixed by Ammonia are also usually brown and discoloured upon the surface of the paper.
More exact directions as to the strength of the fixing bath and the time occupied in the process, will be given in the Second Part of the Work; at present it may be noticed only that Albuminized paper, from the horny nature of its surface-coating, requires a longer treatment with the Hyposulphite than the plain paper.