[pure-silver] Re: Chemical substitution A CORRECTION!

----- Original Message ----- From: "BOB KISS" <bobkiss@xxxxxxxxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Saturday, April 28, 2007 6:44 PM
Subject: [pure-silver] Re: Chemical substitution A CORRECTION!



 DEAR RICHARD,
 Just one favor...when you come out from under the bed, PLEASE tell
 me how much Sodium Citrate I should add per liter along with the other
 reagents to make stock solution of Hypo Clearing Agent.  I want to correct
 my formula.
 CHEERS!
 BOB
I don't know. The only _authentic_ formula I have is the one in the patent which does not include Sodium Citrate only EDTA. The amount of citrate by weight listed in the Kodak MSDS is the same as the EDTA but what is listed is a 5:1 _range_ not a specific amount. I suspect the exact amount is not critical and I would try it at the same strength as the EDTA namely 2.5 grams/liter for the concentrate or 0.5 grams/liter if you mix working solution directly.
I have since discovered a bunch of old correspondence stored on a Zip disc including some from Doug Nishimura at IPI on permanance. I am going through this stuff to see what is relevant. For instance, he contradicts a statement made somewhere in the Kodak paper on wash aids, namely that the diffusion rate of gelatin increases with swelling where the Kodak paper speaks of minimising the swelling to make the diffusion path as short as possible. This is simply beyond my chemical knowledge. Doug Nishimura is a Phd chemist directly involved with photographic chemistry so I give considerable weight to anything he says. 

---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx


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I just picked through most of this thread; apologies for the late entry. Citrate is a reasonable buffer, depending on the pH of interest. Unless the pH of wash aid is extreme in one direction or the other, the citrate/citric acid will be a mixture of various protonated/deprotonated forms. pKa appear to be 3.1, 4.8, and 6.4 for the 3 carboxylic acid groups in citric acid.
The best bet is to start with citric acid and then adjust the final mix to the correct pH with NaOH. Doing it backwards with NaCitrate and bringing pH down with HCl will *almost* accomplish the same thing, but will generate some extra NaCl along the way. In a more complicated mix, this can be minimized by starting with the various free acid / free base forms of each chemical, then adjusting the pH as needed. 

Hope this helps,

Kurt Griffin
Tucson, AZ
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