ACID FIXING BATH The " hypo '' (hyposulphite of soda) fixing bath made acid. Ordinary " hypo " fixing baths are neutral, not acid ; but acid fixing baths may be used for negatives and bromide and gas light prints, although not for prints on print out papers. Their advantages are that they immediately stop the action of the developer, prevent stains, and keep quite clear in use. It does not do simply to add any acid—say sul phuric or hydrochloric—to an ordinary " hypo " bath, inasmuch as this causes a yellow pre cipitation with the accompanying evolution of sulphuretted hydrogen, which militates against the permanency of the prints. Sulphurous acid, however, may be added to an ordinary hypo fixing bath in the proportion of 2 thins. to I pint. The best acid fixer is made by adding a little potassium metabisulphite to the ordinary solution of " hypo" ; the exact proportions are of no importance, -I- oz. to i pint being, however,
a good average. The following is a precise formula suitable for prints :— Sodium hyposulphite . 3 oz. i5o g.
Potass. metabisulphite. 25 „ Water . . . 20 „ 1,000 CCS.
This is suitable for negatives if the " bypo " is increased to 4 oz. A cheaper form of acid fixer is the following :— " Hypo " solution (I in 5) 25 oz. r,000 ccs.
To which add a mixture of :— Tartaric acid solution (i in 2) . . t oz. 3o ccs.
Sodium sulphite solu tion (i in 4) . . If „ 70 „ There is a danger of overworking acid baths and consequently of not fixing properly, as the clearness of the solution is apt to lead to the belief that it is still in a good working condition, although really it may be partly exhausted.