From: Richard Knoppow (dickburk@ix.netcom.com)
Date: 05/20/00-03:22:14 PM Z
At 03:14 PM 05/20/2000 -0400, you wrote:
>> If anyone wants the complete reply I will quote it to the list, I don't
>> think Mike would mind.
>
>It would be very useful, relevant, and applicable to this group. Appreciate
>it in advance!
>
>Dave Soemarko
I also received a request off list from Bob Kiss to post this so here
goes. I also recommend searching Deja for postings by Mike (Michael
Gudzinowicz). He's posted long pieces on the chemistry of fixing baths,
D-76 aging, etc.
The stain is about as stable as dirt or wood, since it is composed
of polymerized phenols. The polymers were purified from humus, and
were named "humic acids". They contained approximately ten monomer
units in the chain.
The reaction for hydroquinone in the absence of sulfite more or
less follows the following sequence (it's simpler than pyro and easier
to describe; pyro is similar).
An amino group (R-NH2) from the gelatin reacts with quinone (oxidized
hydroquinone) by a nucleophilic attack (non-bonding electrons of nitrogen
seek a relatively positive atom) on the carbon adjacent to the keto
group (=C=O). The carbon to carbon double bond in quinone rearranges
to a single bond; the single bonds to the keto groups become double
bonds, and the keto groups open up to form phenols. So the product
looks like hydroquinone with an amine bridge to gelatin attached
to one of the four carbons next to a phenol group.
C=C
/ \ C-C
O=C C=O // \
\ / -->> HO-C C-OH + quinone -->> see below
C=C \ //
+ C=C
R-NH2 /
R-NH
Next, another molecule of quinone reacts with one of the phenolic (-OH)
groups of the product (above) in much the same manner forming an ether
bridge (R-O-R), and causing a rearrangement to give another derivatized
hydroquinone on the end of the chain.
H
O
C-C C
// \ // \
HO-C C-O-C C
\ // | || + quinone repeatedly -->> polymer extension
C=C C C
/ \\ /
R-NH C
O
H
The reaction with quinone continues, extending the chain. Eventually,
the "last" monomer unit may become oxidized to form a quinone derivative,
and the quinone can react with an amine of gelatin forming a crosslink,
in much the same manner as the first reaction of the sequence.
There are all sorts of other reactions taking place analogous to those
involved in lith development which I once posted. Sulfite interferes with
the sequence in the same way as it does with lith development.
Some tanning developers use other agents such as amidol for the first
step. Then the adduct is oxidized with an agent such as ferricyanide, which
gives an oxidized monomer. The monomer can react with gelatin forming
a crosslink. In that case the bridge is just one oxidized amidol molecule,
and no stain is formed.
The stain is formed by the polymer, or actually by the "local" concentration
of a large number of double bonds which can delocalize electrons easily. A
single double bond will absorb in the far UV. When these become conjugated
or are linked by electron rich atoms, the they can absorb light of less
energy (or it takes less energy to push the electrons around - they are
"stabilized" when activated), so they will absorb in the visible region.
The "color" or intensity can be affected by the local environment in the
gelatin (pH, metals, etc.).
Anyway, that's a "rough" explanation without much of the jargon. The polymers
which are formed are very likely more stable than the gelatin. Similar
reactions in wood involving "tannins" give it its brown color.
---- Richard Knoppow Los Angeles,Ca. dickburk@ix.netcom.com
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