Re: APIS, hydroquinone hardening

From: MARTINM <martinm@SoftHome.net>
Subject: Re: APIS, hydroquinone hardening
Date: Thu, 14 Jul 2005 12:20:07 +0200

> I believe what's actually taking place here, is the Hydroquinone
> (Quinol) is oxidized by the dichromate to form Parabenzoquinone (PBQ
> alias Quinone). PBQ is a well-known organic hardening/bleaching
> agent used for silver halide emulsions.

This is not really true. Quinone is extremely unstable in photographic
solutions because it undergoes very rapid sulfonation
reaction. Sulfonation renders quinone a nonhardening and nondeveloping
compound.

Another thing to note is that dichromate is a very potent oxidizing
agent and oxidation products of hydroquinone may be very different
from what we generally see when hydroquinone is oxidized in
photographic context (a lot more mild oxidation reaction).

As far as I know quinone is not used as a hardener in emulsion making,
coating or processing. It would be a bad choice for such purpose as
well.

On the other hand, hydroquinone reduces dichromate to make trivalent
chromium which has potent hardening effect, although the reaction rate
is slow in the range of concentrations generally used as a hardener
additive. In this case a large amount of trivalent chromium is
generated in the mixture so I am not surprised by a rapid hardening
reaction.

> By the way, you may get similar hardening action from oxidizing Metol
> instead of Hydroquinone.

I disagree.
Received on Thu Jul 14 08:42:01 2005