Concentrations and Working Dilutions
Not all of our chemicals are used as supplied: some are conceived as concentrated solutions, to be diluted at the time of use in different proportions according to the result one intends to achieve. This guide illustrates the principle, with examples drawn from our own manuals.
Whosoever approaches photographic chemistry for the first time might suppose that every preparation is to be employed in one single, fixed proportion. Such is not the case. A good part of our special-application preparations are conceived as concentrated solutions, of which only a small quantity is drawn off and diluted with water at the time of use; and even the developers of most common employment may, in certain cases, be diluted to obtain an effect different from that of the bath used at full strength. We set forth below the principle, with concrete examples drawn from the manuals of our own products.
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The Notation of Dilutions.
Dilution proportions are commonly expressed with the notation "A+B", where A represents the parts of concentrate and B the parts of water added. Thus, a dilution of "1+1" signifies one part of concentrate to one part of water (the concentrate being brought to twice its own volume); a dilution of "1+9" signifies one part of concentrate to nine of water (the concentrate being brought to ten times its own volume).
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Example: PYRO-HD
This preparation for special applications is stored in two amber glass bottles, A and B, in strongly concentrated form. The working solution is obtained by pouring 5 ml of A and 5 ml of B into a jug containing 200 ml of water, then completing the volume to a total of 300 ml. This amounts, in other words, to a dilution on the order of 1+58 for each component relative to the final volume: a small quantity of concentrate generating a considerably larger working volume, to be employed moreover in the shortest possible time from preparation, the diluted solution being far less stable than the concentrate.
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Example: IP
The same principle governs the preparation of the IP developer, though the proportions differ: 10 ml of A and 10 ml of B, likewise brought to a total volume of 300 ml. The greater quantity of concentrate drawn, compared to PYRO-HD, reflects the different energy required of this preparation to fulfil its specific function of tolerance to push processing.
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Dilution as a Tool for Contrast: the Case of Zone N-1
At times dilution serves not to prepare the working solution from the concentrate, but to modify the behaviour of a developer already ready for use. Such is the case with MICROPHEN and DK-50, where, to obtain a reduction in contrast (zone N-1), it is prescribed to dilute the normal developer in a 1+1 ratio, employing it however as a single solution (one-shot), with no possibility of storage once diluted. The dilution, in this case, does not economise the product, but deliberately alters its photographic rendering: a more dilute bath acts with greater gradualness upon the shadow areas, restoring an image of more extended tonal range.
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A General Rule.
Whenever a dilution is carried out, whether dictated by the need to prepare the working solution from a concentrate, or intended to modify its photographic behaviour, the resulting solution is always to be considered less stable than the concentrated product or the full-strength bath, and is therefore to be employed shortly after its own preparation.