halide conversion (Re: Modifying bleaches)

From: MARTINM <martinm@SoftHome.net>
Subject: Re: Modifying bleaches
Date: Wed, 01 Dec 2004 08:34:31 +0100

> I understand some authors - e.g. Glafkidès, Frieser (?) - suggest
> there are a few stabilizers that don't lower speed.

I said I don't use "this stabilizer" (PMT) in bromide emulsions. I
use TAI (4-hydroxy-6-methyl-1,3,3a,7-tetraazaindene) for that. TAI
can be added in bromide emulsion in much greater quantity than PMT or
many other agent but do not seem to reduce speed. In many chloride and
chlorobromide emulsions PMT actually increases speed. 1H-benzotriazole
does not have this property. Tani did extensive study on this topic
and my experience is quite consistent with what he published.

TAI doesn't adsorb strongly onto AgBr (doesn't retard ripening) but it
does adsorb on AgCl (retards ripening). TAI is pretty effective in
fully desalted bromide emulsions in my experience. PMT adsorbs
strongly on all AgX and very strongly retards ripening.
1H-benzotriazole adsorbs depending on number of factors like pH, and it
stabilizes but the degree of retarding ripening seems less than PMT.

Note that in my original post I said "this halide conversion does not
work once stabilizing agents that work as a ripening retarder is
added" and did not make a statement that someone is assuming I said.
That's called "fallacy of straw man."

> Apropos, why do you have to add a stabilizer? Well, I may not know
> much about paper emulsions but speaking about ultra-fine grain
> emulsions, you don't necessarily have to incorporate stabilizers to
> prevent fog.

It all depends on how quickly you use emulsion and how stable you want
its performance to be. If you pursue good speed and contrast, and if
you want to keep them right at where you want them to be, it's almost
essential. You can refrigerate emulsion but you'll have to melt again
when coating and that would further the ripening/digestion. If you use
desalted (washed) emulsion without much digestion, this may not be an
issue but at the same time I don't see any practical benefit of
omitting stabilizer.

> By the way, like Ray, I also wondered about your method of
> converting AgCl into AgBr. For spectral sensitization this might
> have some interesting side effects - but for paper emulsions???

You can spectrally sensitize paper emulsion to get reasonable speed
from chloride paper so that you can enlarge onto them. A classic dye
erythrosin is pretty cheap (as a dye) and readily available, and this
is particularly effective for chloride/chlorobromide emulsions (works
on bromide also but less effectively).

But more of the reason I do conversion (despite increased number of
steps) is because I can get high contrast with unwashed printing
emulsion when the host crystals are doped with a divalent metal dopant
(not cadmium), and without using double jet. Because I don't have to
desalt this emulsion, I can prepare this in a matter of 30-40 minutes,
so I can "custom make" the emulsion for each printing session. For
other emulsions I tried sedimentation method by acylating gelatin,
sedimentation by inorganic salts, and noodle washing, but all take a
lot of time. I'm looking for ultrafiltration membrane used by dairy
industry. My gelatin has average MW of 60kD so membranes used to
concentrate milk should be usable.

> I assume the conversion process essentially depends on variables
> like: grain size, silver concentration, halide concentration of the
> bathing solution but also on diffusion speed, which in turn might be
> related to the kind of colloid and the degree of hardening. For the
> fine AgBr grains, I am dealing with, it only takes a few minutes to
> convert them into AgI (in say a solution of 2g KI/L).

That's probably because your emulsion doesn't have a stabilizer that
retards ripening (like TAI). Do you make that emulsion or are you
talking about commercial products?

--
Ryuji Suzuki
"People seldom do what they believe in.  They do what is convenient,
then repent." (Bob Dylan, Brownsville Girl, 1986)