Supersaturated Potassium Oxalate?

[tgtaylor]

Many Pt/Pl developer formulas call for a supersaturated solution of Potassium Oxalate - i.e., 350 grams per liter. The solubility of PO is 30 grams/100 grams of water (=300 grams/L). What benefit is there in supersaturating the developer when the excess will probably precipitate out of solution anyway? As a side note the B&S label says to dissolve 250 grams into 1 Liter.

Thomas

[koraks]

None. The only I can think of is that the solution will be more or less of the same concentration as the oxalate is slowly used up; a sort of auto-replenishment. But given the tolerances in development, I doubt this has much practical significance. The same with saturated dichromate solutions in various alt processes.

[Paul Cunningham]

The question would be supersaturated at what temperature? Room temperature? Many use potassium oxalate significantly above room temperature. That said, I mix at 250g/l, warm to 90F, and develop for 120 seconds.

[tgtaylor]

I maybe wrong but the greater the temperature of the water the greater the amount of the compound dissolved. B&S says to dissolve 250 grams in 1L of water at 150F while others say to dissolve 350grams in 1L @ 120F.


Thomas

[koraks]

Solubility of PO is listed as 360g/l at 20C, so 350 at 120F should dissolve with relative ease. Upon cooling down, a little bit may crystallize out of solution, which can be reverted by reheating the solution. Of course all this doesn't answer your question, what the purpose is of a supersaturated solution. I can't think of any.