[pure-silver] Re: Kodak Wash Aid Patent
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Mon, 16 Apr 2007 15:41:55 -0700
----- Original Message -----
From: "Eric Nelson" <emanmb@xxxxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Monday, April 16, 2007 11:51 AM
Subject: [pure-silver] Re: Kodak Wash Aid Patent
So Richard, given the formulas discussed previously on
this list, would you suggest switching from using
sodium citrate to citric acid or a combination
thereof?
Eric
Citric acid is mentioned in the patent and is listed in
the MSDS. I don't know the concentration of it in the
commercial product. Both the Citric Acid and the EDTA Na5
are listed in the MSDS as being present in 1% to %5 amounts
but that does not mean they are in equal amounts. That is
only a guess. Other than the patent the formula for the
commercial products remains proprietary. If I have ever
posted a formula with Sodium citrate it was in error.
The target pH for the wash aid is 7.0, or neutral. The
reason given for this in the Kodak research paper is that at
this pH the mordanting effect of white alum hardener is
broken but the hardening remains. Also, this value is a bit
above (more alkaline) than the isoelectric point of the
gelatin so electrical charges are right to repel rather than
attract the thiosulfate and thiosulfate-silver-complex ions.
Sulfite has the effect of displacing the thiosulfate ions
but this appears to be independant of pH.
The patent states that the sequestering agent or agents
are there to prevent precipitation the sulfite by calcium or
magnesium salts in water or carried over alum hardner.
Citric acid is a good sequestering agent as is the EDTA.
Probably both are used in the commercial products because
one may be more effective for certain salts than the other.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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