Dan,
With regard to Hypo Clearing Agents (HCA) I quote this 1997 posting by Richard Knoppow, followed by a comment of my own.
"Kodak Hypo Clearing Agent and other so called washing aids are the
result of the discovery that sea water was more effective for washing out
hypo than fresh water. This was first noted around the turn of the century
but extensive research was not until WW-II when the problem of washing
photographic materials on ship board or other locations where fresh water
was in short supply became critical.
It would appear that wash aids work mostly by an ion exchange mechanism.
I've never seen a clear explanation of this so cant provide a citation.
Kodak Wash Aid is approximately: Sodium Sulfite, dessicated, 100 gm; Sodium
Bisulfite, 10 gm; Water 1.0 liter. Dilute 1 part stock to 4 parts water
for use. The commercial product will contain the usual chelating agents in
it to deal with hard water.
Kodak HE-1 Hypo Eliminator is works on a different principle. It
converts the residual hypo to Sodium Sulfate which is more soluble. Kodak
has recommended against using hypo eliminator for several years. It can
damage emulsion and may be too effective washing out the hypo. It was
discovered around 1960 that very small residues of hypo actually stabilize
the silver!
I am not sure what advantage Ammonium salt would have over Sodium but
Crabtree et al of Kodak mentions the use of Ammonium Carbonate as a washing
accelerator in an early paper. It was thought at that time that the main
function of the salt was in changing the pH of the gelatin rather than the
more complex one of ion exchange.
----
Richard Knoppow
Los Angeles, Ca
dickburk@ix.netcom.com
Dead Link Removed
In summary: For fixers that already contain sodium sulfite (and many do), an additional HCA is redundant. If your fixer does not contain sodium sulfite, add about 15 grams per liter (2 teaspoons) to the working strength fixer bath and forget a separate HCA. Tom Hoskinson, 5/31/04