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47
is usually done by the author of a particular method. However, inaccuracy due to poor reproducibility cannot be dealt with in this way, and the likely error from this source must always be remembered when considering the results. For example, if a developer containing 5.00 grams per litre of hydroquinone is analysed using a method having a reproducibility of 5%, two consecutive results could be 4.87 and 5.13 grams per litre, although the concentration in the bath had not changed.
There are a number of methods of analysis for Elon and hydroquinone. Ibl general, some accuracy has to be sacrificed in order to achieve speed of working. Evans, Hanson, and Glasoe6 claim an accuracy for Elon and hydroquinone of better than 5%. Baumbach7 claims 2% for his iodimetric method, and Shaner and Sparks8 indicate an accuracy of just better than 5% for hydroquinone and Elon as determined by Baumbach's more recent method9 in which Elon is determined by acid titration. The potentiometric
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Fig. 7. Relationship between developing agent, consumption, and initial concentration.
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method of Stott10 is prone to give large errors, especially for Elon, which can be 14% in error. In the present author's modification of Stott's method11, the errors are reduced to 1% for hydioquinone and 2% for Elon.
The most rapid methods of analysis are those in which the concentration of one component is found by subtracting the titre of the other component from the total titre of the two. This is not a good method when the Elon concentration is found by subtracting the hydroquinone value from the sum for Elon and hydroquinone, as in the Atkinson and Shaner method12. In D16, for example, the equivalent concentration of Elon is less than 5% of that of the hydroquinone. The error in the hydroquinone determination is of the same order of magnitude. Baumbach's later method is better in this respect because the Elon is determined directly and it is the hydroquinone that is estimated by difference.
Sulphite
The concentration of sulphite is not critical and may vary by as much as 10%. A considerable reduction in the quantity used in the D76 type developer leads to a loss of solvent action and a loss in inertia speed.
The iodimetric determination of sulphite is perfectly straightforward and is sufficiently accurate for control purposes,