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Fixing Baths — Crahtree and Hart 389
practical experiments that, although certain buffer salts, such as sodium acetate, tend to maintain the hardening properties and sulfurization life, they do not materially decrease the propensity of the bath to blister and are therefore of correspondingly less practical value.
(b) From Fig. 8 it is seen that as a fixing bath becomes exhausted the hardening increases up to the point at which aluminium sulfite precipitates and then decreases so that the best point at which to revive with acid would be that just previous to the precipitation. It is dangerous, however, to approach the precipitation point too closely, because once the aluminium sulfite has precipitated, a relatively large quantity of acid must be added to bring it back into solution, the balance of the bath is destroyed, and it is liable to cause blisters. If the quantity of acid permissible for satisfactory hardening is added the precipitate disappears very slowly and it is necessary to revive at such a time when it is possible to allow the bath to stand for a day or two in order to dissolve the precipitate completely.
The effect of reviving a bath with varying quantities of acid at different stages before the aluminium sulfite precipitation point is shown in Figs. 9 and 10. These curves were obtained from results of tests made under practical working conditions in a motion picture finishing plant and are considered to represent average working conditions. The properties of the baths were tested only before and after revival, and therefore straight lines were drawn between the points representing the properties of the bath at the time of the tests.
The acidity, hardening, sulf urization life, and developer capaciey curves have all been assembled in one figure and the respective data are given for each stage of exhaustion determined by a definite number of feet of motion picture film fixed in the bath.
Test A (Fig. 9) represents the properties of a bath revived with too much acid at too infrequent intervals, while test B (Fig. 10) represents a more satisfactory method of revival where a smaller quantity of acid was added at more frequent intervals. The results indicate that there is a greater variation in the properties of the bath in test A than in the case of test B. In both cases the acidity was maintained within satisfactory limits while the relative hardening varied from 90° to 160° (M.P. °F.) in test A and from 120° to 155° (M.P. °F.) in test B. The sulfurization life varied in test