[pure-silver] Re: Chemical substitution A CORRECTION!
----- Original Message ----- From: "Kurt J. Griffin" <kjg@xxxxxxx> 
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Saturday, April 28, 2007 9:05 PM
Subject: [pure-silver] Re: Chemical substitution A CORRECTION!
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
The target pH of Kodak wash aid is 7.0. The formula given in the patent is: 
Sodium Sulfite                20 grams
Sodium Bisulfite               5 grams
EDTA Na4                      0.5 grams
Water to make                  1 liter
Working solution.
The MSDS for the commercial product shows that it also contains Sodium Citrate in an amount similar to the EDTA Na4 but, of course, does not give the actual amount of any of the ingredients, only ranges. 


The patent is USP 2,860,978


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


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