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368 Transactions of S.M.P.E., Vol XIII, No. 38, 1929
because it tends to reverse the action by virtue of its mass action.
The second type of preservative consists of the alkali salts of organic acids which are commonly referred to as ''buffer salts" and of which sodium acetate is a typical example. The effect of this type of substance is to buffer or reduce the hydrogen ion concentration of the acid employed below the limits at which sulfur is precipitated from hypo by acids.
(d) The hardening of gelatin may either be temporary cr permanent. Temporary hardening agents raise the melting point and prevent the swelling of gelatin only while the gelatin is in contact with the hardening solution. A concentrated solution of sodium sulfate is a typical temporarj^ hardener.'^ The hardening produced by such substances is reversible, that is, the gelatin will subsequently absorb water and swell. Such hardeners will not be discussed because their application is limited to tropical development when the temperature of the various photographic solutions is above 75° F.
Permanent hardening is characterized by a reduced absorption of water (swelling) by the gelatin during subsequent washing. Various materials may be used for permanently hardening gelatin such as formalin, quinone, tannin, organic developer oxidation products, and certain inorganic compounds. Formalin, quinone, and developer oxidation products harden gelatin only in alkaline or neutral solutions and their application is therefore limited to use in developers or to the hardening of completely washed film.
Of the inorganic compounds, the salts of iron, chromium and aluminium exert the most powerful hardening action on gelatin. Salts of aluminium are perhaps the most satisfactory hardeners because they are colorless, readily soluble in water, and do not form colored compounds with the common developing agents either in acid or alkaline solutions, while they give satisfactory hardening provided the wash water is not above 75° to 80° F. Aluminium chloride tends to hydrolyze when dissolved in water, forming a white precipitate of aluminium hydroxide but the double salts of aluminium sulfate with sodium or potassium sulfate called alums form a clear solution and are therefore to be preferred. Sodium and potassium alum are equally efficient but with the ammonium alum an evolution of ammonia takes place after the fixing bath becomes alkaline and this tends to produce dichroic fog.