Developing agents pyro developers

Sodium bisulfite is often preferred in pyro developers and in Phenidone concentrates (Appendix 3 Pharmacopoeia Phenidone). Sodium bisulfite3 is often used in formulas that are divided into two solutions, as its weak acidity helps to inhibit the oxidation of the concentrated developing agent. When carbonate, contained in the B solution, is added to make a working solution, the bisulfite is immediately broken down into sulfite and bicarbonate, producing a useful buffering effect. 

Also, the higher the pH, the faster a developer will oxidize. To prevent oxidation, developers often are stored in separate A and B solutions that are mixed just prior to use. Developers using caustic alkali should always be formulated in this manner. Developers containing easily oxidized developing agents, such as pyro, should also be stored this way, regardless of which accelerator is used. 

The Trouble with ABC The first problem is that when pyro is used as the sole developing agent, especially with low concentrations of sulfite, it tends to rapidly exhaust. As a result film developed... 

As a developing agent, catechol tans the surface gelatin of the negative (see Tanning Developers, below). It is also easy to use catechol to compound formulas that create a desirable image stain density when low quantities of preservative (e.g., sodium sulfite) are used (see Image Stain, below). Both of these properties it shares in common with pyro. 

There are other developing agents, chief among them hydroquinone, that will produce a tanned image when used in a formula similar to D-175 (Formulas Extreme Compensating Developers D-175 Tanning Developer). Even so, pyro/cat are the two most commonly used for this purpose. 

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