Constituents of Fixer Solution

The modern fixer constituents are the fixing agent, acid and a hardener. The fixing agent is generally sodium thiosulphate but ammonium thiosulphate is used in some faster-working solutions. The acid is commonly acetic acid but it cannot be used alone because the thiosulphates are rapidly decomposed by direct addition of any but the weakest of acids. The proportions of the various constituents are not critically important and innumerable formulae have been published with minor variations. Kodak’s F-5 is typical one. Acetic acid is usually supplied in the glacial form which is converted to 28 per cent by diluting three parts with eight parts of water. The boric acid is believed to increase the hardening capacity of the bath and to minimize the formation of aluminum sulphite sludge. The fixing time for this type of solution is generally quoted as 10 minutes with a clearing time of 5 minutes. The Rapid fixers based on ammonium thiosulphate are available from most manufacturers but their formulae are not generally published. There was a suspicion when they were first introduced that they did not provide stable images and some also seemed to exhaust rapidly. The suspicion has slowly evaporated and appears. In fact it is not to have been soundly based. The arguments for and against ammonium thiosulphate depend ultimately on speed of reaction. The fixing time is reduced dramatically from 10 minutes to perhaps 90 seconds or less but there has to be some doubt about the efficiency of the hardening process. If speed is paramount and hardening is not vital then the ammonium thiosulphate solutions are invaluable. For many workers however, the longer fixing time is better suited to their working methods.