The chemical changes produced in a Hyposulphite Bath by addition of acid, may be explained thus:—The acid first displaces the feeble Hyposulphurous acid from its combination with Soda.
| Acetic Acid | + | Hyposulphite Soda. | |
| = | Acetate Soda | + | Hyposulphurous Acid. |
Then the Hyposulphurous Acid, not being a stable substance when isolated, begins spontaneously to decompose, and splits up into Sulphurous Acid—which remains dissolved in the liquid, communicating the characteristic odour of burning Sulphur—and Sulphur, which separates in a finely divided state and forms a milky deposit.[21]
[21] From the Vocabulary, Part III., it will be seen that commercial Chloride of Gold usually contains free Hydrochloric Acid; hence a considerable deposit of Sulphur takes place on adding it to the Hyposulphite solution, and the liquid must not be used immediately.
Observe therefore that free acids of all kinds must be excluded from the fixing Bath, or, if inadvertently added, the liquid must be set aside for some hours until the Hyposulphurous Acid has decomposed, and, the Sulphur having settled to the bottom, the Bath has regained its original neutral condition.[22]
[22] The chemical reader will understand the decomposition of free Hyposulphurous Acid by the following equation:—S2O2 = SO2 and S.
b. Decomposition of the Bath by constant use.—It has long been known that a solution of Hyposulphite of Soda undergoes a peculiar change in properties when much used in fixing. When first prepared it leaves the image of a red tone, the characteristic colour of the reduced Silver Salt, but soon acquires the property of darkening this red colour by a subsequent communication of Sulphur. Hence a simple fixing Bath becomes at last an active toning bath, without any addition of Gold.
This change of properties will be found more fully explained in the abstract of the Author's researches given in the next Section ([p. 156]). At present we remark only that it is due principally to a reaction between Nitrate of Silver and Hyposulphite of Soda, attended with decomposition of Hyposulphite of Silver ([p. 130]); and hence, if the prints are washed in water before immersion in the Bath, the solution will be less quickly liable to change.
Many operators state that the toning Bath having at first been prepared with Chloride of Gold, no further addition of this substance will be required. This no doubt is correct, but in such case the proofs will at last be toned by Sulphur more than by Gold, and will not possess the same stability; the Bath will also, after long use, be found to acquire a distinct acid reaction to test-paper, the acidity being due to a peculiar principle generated by decomposing Hyposulphite of Silver, and which is shown to have an injurious action upon the print ([p. 158]). To avoid this the solution should be kept neutral to test-paper by means of a drop of Ammonia, if required; and when it begins to be exhausted, and does not tone (quickly) a print from which the free Nitrate of Silver has been removed by washing, a fresh quantity of Chloride of Gold should be added.
c. Tetrathionate in the Hyposulphite Bath.—The Author has shown that the Tetrathionates, which are analogous to the Hyposulphites, have an active sulphuretting action upon Positive prints (see the papers in the next Section). Very fine colours can be obtained in this way; but toning by Sulphur having been proved to be wrong in principle, the formulæ given in the first two editions of this Work have been omitted.[23]