[pure-silver] Re: Self-Made HypoClear any good?
----- Original Message ----- From: "john stockdale" <j.sto@xxxxxxxxxxx> 
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Sunday, November 26, 2006 7:16 PM
Subject: [pure-silver] Re: Self-Made HypoClear any good?



At 05:24 AM  27/11/2006, Richard wrote:

<snip>
The sequestering agents are present to prevent deposition of aluminum from the hardener in hardening fixing baths and minerals from the wash water. They are probably not necessary if the wash aid is used for a small quantity of film or paper but I have not experimented with this. 
<end quote>
Would these sequestering agents prevent the calcium compounds causing the problems that Ralph has experienced? And/or would the bisulphite reduce the problem as a result of the pH being lower? I've been using just sulphite and metabisulphite in the quantities recommended by Richard with no problems, but the water here has a fairly small mineral content.
Yes, the sequestering agents should stop the deposit of minerals from the water. I don't know what effect the pH has on these but the principle reason for including it, according to the Kodak paper, is two fold: one, to prevent loss of hardening as would happen if a simple alkaline bath were used, and 2, to minimise swelling and maintain the shortest diffusion path for the ions leaving the emulsion. The wash aid works in three ways:1, the Sulfite is an ion exchange agent which is taken up preferentially by the emulsion thus displacing the thiosufate ions and silver-thiosulfate complex ions. 2, White Alum hardener when in acid environment, causes a sort of mordanting which binds the above mentioned ions pretty tightly to the emulsion. At about neutral pH this mordanting no longer is effective but the hardening not destroyed as it would be at higher pH. 3, The pH of the emulsion is important because when acid the electrical charges in the atoms tend to bind thiosulfate ions and their complexes by electrical attraction. When neutral or alkaline the charges are such as to repel these ions. Gelatin has the peculiar property of taking on the pH of the last bath its had, however, it has a preferred pH called the Isoelectric point. At this pH its charge is neutral and the swelling is minimum. Most photographic gelatin has an Isoelectric point slightly on the acid side of neutral. Thus when made neutral the electric charges are right for repelling thiosulfate ions but its still very close to the minimum swelling point. So, while a simple 2% soluton of Sodium Sulfite will work pretty effectively as a wash aid Kodak had good reasons for buffering it. 

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Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx 
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