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Acid Fixing

bath, quantity and solution

ACID FIXING BATH.—The question as to whether the acid fixing bath should be acid. alkaline or neutral is still an undecided one. Eder was the first to advise the addition of sulphite of soda to the hyposulphite solution ; others preferred carbonate of soda and bisulphite. Dr. Higinson, after a series of experiments, concluded that the pure hyposulphite bath was the best, as it acts more promptly, with more certainty, is the least costly, and simplest to make.

The acid bath has, however, the property of removing all yellow discoloration and of giving a clear, transparent negative, and of stopping the action of the alkaline developer left in the film. Long immersion in the bath tends to weaken the image, however, and for this reason some workers prefer to fix first in an ordinary bath and finally in an acid one. Whenever acid is added direct to the hypo bath, sulphur is separated, the solution is turned milky, and the negatives rendered opalescent.

Professor Lainer discovered that if a solution of sodium sulphite (with citric or tartaric acid) be added to the hypo bath, a slight discharge of sulphurous acid takes place, after which the solution remains clear.

The following formula was afterward recommended : ACIDIrlETER (synonym, acidometer).—An instrument for measuring the strengths of acids.

A ACIDIMETRY.—The process of determining the quantity of real acid in a sample of hydrated acid. This is accomplished by volumetric or by weight analysis. The former method is carried out by ascertaining the measured quantity of a standard alkaline solution required to saturate a given quantity of the acid. By weight analysis a known weight is decomposed with an excess of acid, carbonate of sodium or potassium, and estimate by weight the quantity of carbonic anhydride evolved. When this is done the quantity of acid can easily be ascertained.