The developing solution completes the reduction of the silver salts started by light. The unexposed silver compounds, however, are left unchanged. The principal purpose of a fixing bath is to remove all this unreduced silver on negatives and prints and thereby stop any further reduction to silver metal by the developing agents carried over from the developing bath. Silver compounds, not acted on by light or developer, are readily soluble in a solution of Sodium Thiosulfate, commonly called “Hypo.” This chemical, therefore, is the most important ingredient in a fixing bath.
A plain fresh Hypo solution works well in cool weather for a time, but does not remain efficient very long. After a short time the reaction of the chemicals brought over from the developing bath into the Hypo solution, stains the prints and negatives fixed in it.
In general practice, all developing solutions are alkaline and may contain more than one kind of developer. These ingredients are brought over as impurities into the fixing bath with the negatives and prints, and some means must be provided to prevent them from contaminating the solution. If this precaution is not taken, they interfere with proper washing out of the unreduced silver and may cause stain and fog.
As a developing agent does not reduce silver salts except in an alkaline solution, and as the purpose of a fixing bath is to stop all development, an acid is added to neutralize the alkali. This is generally accomplished with Acetic Acid. Acetic Acid added to a Hypo solution, however, turns the bath milky. The Hypo decomposes into free sulfur and Sodium Sulfite. Fortunately this chemical action is reversible, since a solution of sulfur and Sodium combine to form Sodium Hyposulfite again upon boiling. This being the case, an excess of Sodium is maintained in the fixing bath to oppose the decomposition of the Hypo by the acid. In this way, it is possible to add sufficient acid to the fixing bath to neutralize all the alkali carried over into it, without decomposing the Hypo.
This excess of Sodium also prevents oxidation of the developing agents carried over into the fixing bath. It acts as a preservative by absorbing the oxygen dissolved in the solution from the air, before it reacts on the developers. If the did not perform this function, it would be necessary to employ something else, because an oxidized developer colors the solution, staining the prints and negatives.
All possible sources of trouble in the fixing bath are taken care of excepting excessive softening and swelling of the gelatin of the negatives and prints, caused by soaking in the solution of the developing and fixing baths. Some substance, therefore, should be used to harden the surface of the gelatin... (continued)