catechol interchangable with hydroquinone?

hi everyone,

i've read in a few places that hydroquinone is almost the same as catechol. are they interchangable in formulas? yes, hydroquinone stains differently and doesn't tan but could you use the tried and tested synergy ratios of metol - hydroquinone in the metol - catechol pairing? and visa versa? any insights welcomed.

Well it only differs in the positioning of one hydroxyl on the benzene ring. Same molecular weight.

Hydroquinone, 1,4-Dihydroxybenzene
Pyrocatechin, 1,2-Dihydroxybenzene

In practice they have been interchangeable, Kodak used Pyrocatechin in HC110 instead of Hydroquinone for a time, and Agfa used it in Graphic arts developers in Germany while the same (or very similar) Agfa Ansco formulae in the US used Hydroquinone.

In practice Pyrocatechin & Phenidone have slightly more super additivity than Phenidone & Hydroquinone but Pyrocatechin is more toxic than Hydroquinone and has a strong aromatic smell so wasn't used much in commercial developers, it also oxidises faster and that can lead to uneven staining. It forms Meritol a combination product with PPD and this was the basis of many Johnsons fine grain & super fine grain developers.

Resorcinol, 1,3-Dihydroxybenzene, is another similar developing agent, only really used in research.

Ian

In addition to what Ian says, catechol can be used as the sole developing agent in a developer and produce a normal contrast image while hydroquine cannot be so used. During the 1920's and 1930's catechol developers were very popular for 35 mm work.

While catechol can be substituted for hydroquinone the reverse is not usually true. Since catechol is more expensive than hydroquinone it is used only in certain special developers.

As an interesting sidelight, the developing agent in caffeinol and similar developers is caffeic acid which is a substituted catechol.

From the literature, I get the impression that catechol stains the image more than hydroquinone. Otherwise, they seem pretty similar. That does not mean they are identical in action, however. Catechol is often used as the sole developing agent in developers for moderate contrast, while hydroquinone is generally used as the sole developing agent only when high contrast is wanted. That indicates some difference in activity. Resorcinol (1,3-dihydroxybenzene), unlike its cousins, is not an effective developing agent. This is similar to most other aromatic developing agents where the ortho and para configurations are active but the meta compounds are inactive.

The major advantage of Pyrocatechin over Hydroquinone is its tanning action. It's why Windisch recommended it as a surface developer in Die Neu Foto Schule, the formula has since been given his name but in fact he doesn't claim that in the book (luckily also published as "The New Photo School" in English).

Pyrocatechin is also warmer working which is synonymous with finer grain than Hydroquinone so in developers with PPD, p-Aminopheol (and derivatives), Metol & Phenidone the combination of tanning & fine grain gives developers with excellent all round properties giving good fine grain, tonality and excellent sharpness/

It was used in many highly favoured "Cult" European fine & super fine grain developers, Agfa Atomal (Orwo/Calbe A49), May & Baker Promicrol, various Johnsons developers, Definol, Unitol, MCM100 etc.

Ian