British Kinematography (1948)

38 bath is poor the pH rises, and this rise is usually more than enough to maintain or even increase the rate of development in spite of a fall in the developing agent concentration. This process continues until all the hydroquinone is consumed, and then the drop in Elon concentration becomes more rapid and the bath collapses. Therefore, if one wants to operate a bath continuously, it is highly desirable to gain some knowledge of the trend of the chemical changes that take place. For this and for other reasons, there has been a marked tendency in recent years to introduce chemical analysis as a routine practice in motion picture laboratories. Application of Chemical Analysis This paper is primarily addressed to the controllers who use, or who contemplate using, analytical methods ; and one of its objects is to draw attention to the fact that the carrying out of chemical analyses on the baths is not always followed by an improvement in the quality and consistency of the product. There is a great virtue in " leaving well alone," and some laboratories who have established consistent procedures after many years of patient trial and error methods are quite likely to find that chemical analysis can lead to no improvement. Some laboratories prefer to instal a fresh negative bath, to use it until it shows signs of deteriorating, and then to discard it, rather than to attempt to maintain it for a long period. In this case, the most useful purpose to which analysis can be put, apart from being used as a check on the accuracy of the mixer, is to enable the pH. to be adjusted, because negative baths are rather sensitive to pH changes. The most important application of chemical analysis is, however, in connection with baths that are to be continuously maintained and which are difficult to control photographically. Most laboratories attempt to maintain a positive bath for a long period and some to maintain a negative bath. The controller who uses analytical methods requires also to know the effect on the performance of the bath of a given change in the concentration of one, or more, of its constituents in order that the analytical data may be used to best effect. Without this knowledge, the best that he can hope to do is to maintain the composition of the bath constant according to the formula that experience has shown to give satisfactory results. However, it is unlikely that the controller will know whether the bath formula established at that laboratory by trial and error could be modified without disadvantage to a more economical mix, because few laboratories can spare machine time to carry out such experiments. Elon-Hydroquinone Ratio Not much help in this direction can be obtained by reference to the literature. For example, the work of Frotschner1 and Shiberstoff2 led these authors to conclude, that only slight modifications in performance could be achieved by changing the Elon/hydroquinone ratio within wide limits On the other hand, practical experience seems to indicate that increasingthe hydroquinone concentration in a positive bath helps to increase the gamma obtained in given time of development. A systematic study was therefore undertaken of the Elon-hydroquinone developer to find the influence on the performance of the developer of changes in the Elon/hydroquinone ratio, and in the concentrations of sulphite and bromide, and in the pH. By performance is meant the behaviour of the bath with respect to sensitometry, oxidation by silver bromide, and oxidation by atmospheric oxygen. The work on aerial oxidation is not yet