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found in D76, at which the inertia speed reaches a maximum, and a considerable increase in Klon proportion beyond this leads to a decrease in speed.
At all levels of />H the Elon proportion is critical when lower than 10%, its influence being most marked on the rate of development. Fortunately, in the case of positive developers of the D16 type, where the Blon concentration is small, the position is relieved to a considerable extent by the fact that the relative consumption of Klon on silver bromide exhaustion is negligible, and so its concentration does not change much. However, care must be exercised in compounding the developer and replenisher so that the correct amount of Klon is used.
Evans, Hanson and Glasoe6 quote tolerances for the deviation of the concentrations of Klon and hydroquinone in a negative and a positive bath. Their figures are shown in the tables.
Negative Film in Negative Developer
Variable
Approx. value in Aged Developer
Per cent. Error Tolerable
Elon
Hydroquinone. KBr ... pH ... .
1.5 4.0 1.0
8.4
6 50 4-5 ^0.02 pR
unit
Positive Film in Positive Developer
Elon
1.0 10
Hydroquinone
3.0 10
KBr
1.0 10
pU
10.00 rL0.05^H
unit
Four things should be noted in connection with these figures. Firstly, the tolerances for Klon are valid only for the proportion shown and should not be taken as holding for any Klon proportion. Secondly, even if a variation of 50% in the hydroquinone concentration in the low pH negative developer does not affect the photographic results (although the present findings do not quite confirm this) such a change would have an influence on the relative consumption of Klon and hydroquinone and on the cost of running the bath. Thirdly, the pH tolerance for the negative bath is lower than that for the positive. Fourthly, the tolerance permissible in the analytical procedure should be about half these figures, so that a tendency of the bath to deviate can be noted and dealt with before the photographic performance is affected.
Reproducibility of Analytical Results
It might be helpful to those laboratory controllers who are newcomers to the field of chemical analysis to include here a word of warning about the danger of being misled by analytical results, especially those for Klon and hydroquinone. The results of a chemical analysis are accurate and reproducible only within certain limits. The limits are set, first, by the method nsed and, second, by the skill of the operator.
An error in the absolute accuracy of a method of analysis can be overcome by incorporating a numerical factor when working out the results, and this