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388 Transactions of S.M.P.E., Vol. XIII, No. 38, 1929
ing imparted to the film, but this was shown to be negligible for all practical limits of exhaustion; that is, the quantity of alum removed by a given quantity of film was not sufficient to affect the hardening properties of the bath. This was determined by quantitative estimation of the alum content of a bath before and after exhaustion.
The other products of exhaustion : namely, silver mono and disodium thiosulfate, developer oxidation products, and sodium aiid potassium bromide were found to have little or no effect on the hardening properties.
It is therefore apparent that the cause for the decrease in hardening and general usefulness of a hardening fixing bath when exhausted to the point at which aluminium sulfite precipitates is the unbalanced condition brought about by the neutralization of the excess acid by the carbonate of the developer carried into the fixing bath, and also by the added effect of the increased quantity of sulfite present. For all practical purposes, therefore, it would appear that the revival of a fixing bath should be accomplished by the addition of a given quantity of acid at certain intervals.
2. When Should a Fixing Bath he Revived? The frequency of revival or the stage of exhaustion at which the revival should take place depends upon the following factors:
(a) The original acidity of the bath, and
(b) The degree of uniformity of hardening required.
(a) As stated previously a large quantity of free acid in a fixing bath is desirable on the one hand because this increases the developer capacity ; but it is also undesirable because the propensity of the bath to blister and precipitate sulfur is increased while the hardening properties of the bath are impaired. If maximum hardening and sulfurization life are required, a minimum quantity of acid should be used but this will necessitate very frequent revivals with acid. If revival is not possible or undesirable, then the largest quantity of acid which will give the desired hardening and sulfurization life should be used, though experiments have shown that the best formula which it is possible to compound is inefficient unless revived, because the bath will precipitate aluminium sulfite long before the fixing powers of the hypo are exhausted.
It was considered that it might be possible to employ a high original acidity by suitable buffering of the acidity as suggested by F. A. Elliott (U. S. Patent 1,411,687), but it has been shown by