Re: Tanning theory of dichromated colloids

"...Incidentally, primary alcohols, alkenes and alkynes can also be
oxidized to carboxylic acid with strong oxidizing agents like
permanganate or chromium (VI)."

That has been interesting me for quite a while. Do you know what kind of
condition (pH/temperature/time) is required to initiate that reaction?

"So one possible test might be to oxidize gum with suitable oxidizing agents
and see if chrome alum can harden it better. But the amount of oxidizing
agent must be suitably chosen, because you don't want gum molecules to break
down at once before they get hardened."

What would be an appropriate oxidizing agent? Thinking of some special
bleaches occasionally used for holographic silver-halide materials and
involving Cr bleaching, copper sulfate or potassium persulfate come to my
mind...

Martin

----- Original Message -----
From: "Ryuji Suzuki" <rs@silvergrain.org>
To: <alt-photo-process-l@sask.usask.ca>
Sent: Sunday, May 09, 2004 5:12 AM
Subject: Re: Tanning theory of dichromated colloids

> From: Ryuji Suzuki <rs@silvergrain.org>
> Subject: Re: Tanning theory of dichromated colloids (was gelatin
> Date: Sat, 08 May 2004 11:10:02 -0400 (EDT)
>
> > > You say, so dismissively, "consider the building blocks." Well, I've
> > > considered them, and I don't get it. In the one case carboxyl
> > > groups, in the other, what?
> >
> > Are you saying there is no carboxyl group available for reaction in gum?
>
> Ok, someone pointed out to me that you already mentioned this. So here
> it goes.
>
> From: Katharine Thayer <kthayer@pacifier.com>
> Subject: Re: Glutaraldehyde: a different kind of cautionary tale
> Date: Thu, 29 Apr 2004 18:47:27 +0000
>
> > I meant to say that while it's quite unlikely IMO that amine groups
> > are involved in gum crosslinking, it's also somewhat unlikely that
> > carboxyl groups are highly involved either, since only one of the
> > four sugars that make up the gum (glucuronic acid) contains a
> > carboxyl group, and it's not one of the more prevalent sugars (18%
> > of total sugar content) in the gum.
>
> This is actually a fraction of the carboxyl groups you get. In
> dichromated colloid process, chromium in dichromate (+6 oxidation
> state) must be reduced by the colloid molecules to +3 oxidation state
> before engaging in crosslinking reactions. Mannivannan et al (1993)
> studied this with PVA, and there are other dichromated gellatin
> studies as well. Dichromated colloid material also has significant
> rate of dark reaction involving dichromate as well. So, sugars in gum
> should be viewd with this in mind.
>
> Gum arabic, according to my food chemistry reference, typically
> consists of 70% polysaccharide chains with little nitrogenous
> material, 30% polysaccharide chains attached to protein
> molecules. Both of the polysaccharide chains are highly branched,
> branch-on-branch structure. Like in pectin, end of the chains tend to
> be uronic acids where terminal carbon is likely to occur in oxidized
> form (that is, carboxylic acid). That's what the D-glucuronic acid is.
>
> Other non-terminal sugars (galactose, arabinose, rhamnose) are all
> aldoses, sugars containing aldehyde group. These sugars and
> ketone-containing sugars (ketoses) are called reducing sugars because
> the aldehyde (and ketone if in alkaline condition) can be oxidized to
> make carboxylic acid. Dichromate is definitely a powerful enough
> oxidizing agents. What this means is that virtually any sugar in gum
> can provide carboxyl group.
>
> Note however that crosslinking at very small fraction of these
> molecules may suffice to render the gum insoluble in cold water. Note
> also that I'm merely suggesting that hardening of gum by chromium
> bridge between carboxyl groups is very possible, and this does not
> preclude other possible reactions co-occuring.
>
> Incidentally, primary alcohols, alkenes and alkynes can also be
> oxidized to carboxylic acid with strong oxidizing agents like
> permanganate or chromium (VI).
>
> So one possible test might be to oxidize gum with suitable oxidizing
> agents and see if chrome alum can harden it better. But the amount of
> oxidizing agent must be suitably chosen, because you don't want gum
> molecules to break down at once before they get hardened.
>
> Another test might be to "deactivate" or esterify the carboxyl groups
> by acidic methyl compounds. I don't remember what was used for the
> analogous gelatin study, but probably dimethyl sulfate. This stuff
> makes good ester with carboxyl groups in protein and amino acid as
> well, but the stuff is pretty nasty because you don't have acute
> irritation, pain, or smell during exposure but the toxicity may be
> severe. Mixture of methanol and mineral acid would also work, but it
> also would have the same problem. Methyl ester is weak and it
> hydrolyzes at neutral or alkaline pH, so the hardening step with
> chrome alum should be done in pH of 5 to 5.5 range. If carboxyl
> groups are involved in hardening, this treatment would significantly
> weaken the hardening effect.
>
>
> From: Katharine Thayer <kthayer@pacifier.com>
> Subject: Re: Glutaraldehyde: a different kind of cautionary tale
> Date: Mon, 03 May 2004 09:26:08 +0000
>
> > Since there are very few amino groups, (and of those little and none
> > is lysine and hydroxylysine which are said to be the two amino acids
> > that link in glutaraldehyde hardening of gelatin)-- if
> > glutaraldehyde works for gum, it must work by an entirely different
> > mechanism, or...... something.
>
> I'm not sure about what happens here. It might be hemiacetal
> formation. As you say it's hard to believe it's due to epsilon-amino
> groups as in gelatin. But if you want to test it, it's fairly
> straightforward. You can "phthalate" gum by mixing phthalate anhydride
> and NaOH and cook for a couple of minutes at 40C. After this, amino
> groups are inactivated. If the crosslinking is due to amino groups,
> the gum should harden with chromium (III) to significantly less extent
> after this treatment.
>
> In both chromium and aldehyde cases, the degrees of crosslinking
> measured at several different pH would add another piece of
> information as well.
>
> Another thing to note is that, in general, coated and dried material
> responds to smaller quantity of hardener better. Thick colloid
> dispersion (solution) is next best. Dilute ones are poor. In
> dichromated colloid process, hardening takes place in dried
> layer. When you do hardening on a dish before coating (with gum or
> gelatin), especially with aldehydes, the result can't be compared with
> that from dried material.
>
> --
> Ryuji Suzuki
> "You have to realize that junk is not the problem in and of itself.
> Junk is the symptom, not the problem."
> (Bob Dylan 1971; source: No Direction Home by Robert Shelton)
Received on Sun May 9 12:53:40 2004