British Kinematography (1948)

60 The Author : For a given material, and for a given gamma, you can develop in a wide range of Elon-hydroquinone developers without significantly altering the toe shape. Increasing the bromide, for example, has no significant effect ; changing from an Elon-hydroquinone developer to an all-hydroquinone developer has no significant effect ; pH will influence the rate at which a given gamma is obtained. But once you get to a given gamma, then the shape of the toe is the same with any Elon-hydroquinone developer. Mr. W. M. Harcourt : Have you found the peculiar phenomenon that you can mix a developer very carefully to a given Elon-hydroquinone balance — you immediately make a chemical analysis and find that the analysis gives different results from your mix ? The Author : There are two causes of that deviation. One comprises the errors in the analysis ; you must be very guarded about analytical results. Unless you take pains to control the sources of errors you must not expect an accuracy of better than 5%. Secondly, we found that the best way to mix a developer without loss of developing agent through oxidation, was to take a conical flask of distilled water, and bubble in nitrogen. It swept the oxygen out of the water, above which it formed an inert atmosphere. The chemicals were poured in a fine stream through the nitrogen layer and solution was assisted by the agitation induced by the bubbling gas. Mr. W. M. Harcourt : I assume that the thing to do, having made your initial mix and then made a chemical analysis, is to keep that chemical analysis from then on ? You would not try to balance the bath to what it should be? The Author : There is no real point in it, unless the discrepancies are very large, because you are faced with the probability of error in your analysis. Unless you are prepared to investigate that at length, you should see that your developer is made up with great care, and if the deviation is less than 5%, you should accept the bath as mixed. Only if you are getting a definite trend in the wrong direction should you try to counter it. Mr. C. F. Parkins : May not the question of chemical purity come into this matter ? The Author : The commercial brands of metol in general, and of hydroquinone, if they look white and clean, are usually very pure — over 99%. Sulphite may contain small amounts of carbonate, which can influence the pR of a D76 bath. The formula for D76 does not specify the pH ; unless you specify the pE. you have a bath which may be just as different from D76 as if you put in an extra grain of borax or took it away. Mr. C. F. Parkins : You raised the question of the pH of a D76 developer. Could you expound a little on the buffered borax developer ? The Author : The buffered type of developer, such as D76d, contains about 8 grams of borax and -8 grams of boric acid per litre, whereas D76 contains 2 grams of borax, which activates it sufficiently to do the job for which it is intended. But this quantity of borax has only slight buffering power. Buffering the developer is very expensive in boric acid and borax, because you usually find you have to put in a large amount, and if the aerial oxidation is high compared with the bromide oxidation, you may even find that you have to use a relatively acid replenisher to keep the pU. down. You can keep the pH down with strong acid added very cautiously. If the bath does show a steep rise in pH, it is best to check the pumps to make sure the glands are not J admitting air. Mr. R. Harding : You have not mentioned the effect of developer oxidation products in a developer. The Author : The effect is negligible on the sensitometric properties. When you have run a bath for a good while, you have present a considerable concentration of sulphonates, but their effect is less than 10% of that of unoxidised developing agents. Mr. R. Harding : They tend to give staining if you have a high concentration. The Author : If you run a strongly alkaline bath for a long time, reddishbrown humic acids are formed, which will stain. Mr. T. White : You did say, that with zero proportion of hydroquinone in the D76 bath, you would get a high emulsion speed. Would not that in turn tend to bring up the fog level ? The Author : Not much when working at gamma .65. Moreover, a small increase in the fog on a negative is not harmful, provided it is uniform.