Re: APIS, hydroquinone hardening

----- Original Message -----
From: "Ryuji Suzuki" <rs@silvergrain.org>
To: <alt-photo-process-l@sask.usask.ca>
Sent: Thursday, July 14, 2005 4:41 PM
Subject: Re: APIS, hydroquinone hardening

> Sulfonation renders quinone a nonhardening and nondeveloping
> compound. (...)
> Quinone is extremely unstable in photographic
> solutions because it undergoes very rapid sulfonation
> reaction. Sulfonation renders quinone a nonhardening and nondeveloping
> compound.

Well, sorry for being unclear about that, my "hardening/bleaching" should
have read "hardening-bleaching", the way I referred to Quinone, was limited
on its use as a bleaching agent.

>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.

No doubt.

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

OK, you may give it a try all the same. I don't know what's exactly formed
by oxidizing Metol but it's surely less toxic than Quinone.
In the context of bleaching a silver halide emulsion, the oxidized Metol
definitely does harden a gelatin layer. By the way, that particular
rehalogenating bleach I am referring to, is based on an acid potassium
persulfate solution to which Metol (or in differnt formulas, Hydroquinone or
Amidol) has been added. Within a couple of minutes the Metol being oxidized,
a very potent bleach forms.

Martin

> 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 10:59:05 2005