Storage of Chemicals and Verification of their Reactivity
Every photographic chemical, however well preserved, is subject with the passing of time to a natural decay of its reactivity. This guide sets forth the correct manner of storing developers and fixers, together with some simple methods for verifying their residual reactivity before use.
It is no rare occurrence that the operator, finding at the back of a cupboard a bottle of developer prepared months before, should wonder whether that chemistry be still fit for use. A legitimate question, for every photographic solution, however scrupulously preserved, is subject with the passing of time to a natural decay of its own reactivity. We set forth below, firstly the correct manner of storing prepared chemicals, and thereafter some simple methods — suited to amateur practice, requiring no laboratory instrumentation — for verifying their condition before use.
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On Storage
The following indications hold for all photographic chemicals, be they developing, regeneration, or toning baths:
1. Prepare all chemicals with demineralized or boiled water.
2. Store the chemicals in dark glass bottles, or bottles wrapped in aluminum foil.
3. Employ bottles fitted with spring-loaded caps and gaskets, or laboratory caps with O-rings.
4. Limit as much as possible the residual air volume within the bottle; alternatively, spray some butane for two or three seconds before closing the cap.
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Verification of the Developer's Reactivity.
Three are the methods we recommend, in order of practicality:
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Observation of colour.
A fresh developer presents itself colourless or slightly amber. With oxidation, the colour darkens progressively. When the solution assumes a tint resembling that of tea (a reddish brown), it is a sign that oxidation is already advanced: we recommend in such case that the chemistry no longer be employed, or at the least that its efficacy be verified by one of the following methods before proceeding.
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The clip test.
Cut a small strip of already-exposed film (the leader protruding from the roll serves the purpose perfectly) and develop it separately, in the same bath, for the canonical time prescribed. Compare then the density obtained with that of a reference negative developed with fresh chemistry: should the strip prove noticeably lighter, the developer has lost reactivity.
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The pH.
Every developer possesses its own characteristic pH value, there being no general figure valid for all formulations. We recommend, therefore, measuring with a common pH strip the value of the freshly prepared chemistry, and noting it down; a subsequent significant drop in this value, measured with the same strip, will indicate chemical exhaustion in progress.
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Verification of the Fixer's Reactivity.
The classic method, of assured reliability, is the clearing test:
1. Immerse a strip of virgin film (unexposed, undeveloped) in the fixing bath.
2. Time the interval required for the strip to become perfectly transparent, free of the milky film characteristic of undeveloped silver halide. For our fresh fixer, this time stands at 4 minutes.
3. The fixing time to be applied to the film shall equal twice the clearing time thus measured.
4. Should the clearing time reach or exceed 8 minutes (twice the reference value), the bath is to be considered exhausted and must be replaced.
The same holds for the fixer's pH: this being an acidic environment, a value approaching neutrality, relative to that noted at preparation, indicates a fixer nearing exhaustion.
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A Final Consideration
None of these methods claims the precision of a laboratory analysis, yet all, practised with a modicum of consistency, allow the amateur operator to avoid the most common error: blindly trusting the capacity declared upon the label alone, without regard for the actual conditions of storage and use to which the chemistry has been subjected.