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Transactions of S.M.P.E., Vol. XIII, No. 38, 1929
a straight line from the i'resh to the exhausted bath. However, it was concluded that revival with acid in the proportion of 1 pint (500 c.c.) per 120 gallons of bath after processing 7,500 feet of film was sufficient to prevent the bath from becoming impaired with regard to the propensity of the bath to precipitate aluminium sulfite and that the acid necessary to restore the properties of the bath and maintain satisfactory developer life until the next revival was not sufficient to impair the hardening properties and sulfurization life of the bath. It was also concluded that practical procedure in a motion picture laboratory would not warrant more frequent revival because of the time and delay accompanying such procedures.
3. Methods of Determining the Revival Point, (a) As outlined above, the revival point or stage of exhaustion at which the bath should be revived is largely arbitrary, depending upon the degree of uniformity of hardening desired; that is, for more uniform hardening the bath should be revived at frequent intervals although in practice it is usually satisfactory if revival takes place just previous to the point at which aluminium sulfite is precipitated.
At the outset it was thought that it might be possible to add to the bath an acid indicator which would change color just previous to the point at which precipitation of aluminium sulfite occurs, although the color of the indicator either in the fresh or exhausted bath would have to be complementary to the safelights used in order to permit of detection of the color change in the darkroom. No perfectly satisfactory indicators for the purpose are at present known.
(b). It is possible to determine the quantity of sodium sulfite and sodium carbonate added to the fixing bath during the process of fixation by titrating the fixing bath with a known alkali using suitable indicators. From this information the quantity of acid necessary to bring the bath back to its initial stable condition can be calculated. However, in order to determine this it would not only be necessary to consult the curves for the compounding of a fixing bath, but it would also be necessary to determine the effect of the exhaustion products from the other curves indicated above. This would complicate the procedure to such an extent that it would not be applicable to average darkroom procedures.
(c). Since none of the above methods is readily adaptable for darkroom use the only alternative was to determine the period of revival by means of a quantity factor. Every darkroom worker