Maintaining Potassium Oxalate’s effectiveness for platinum/palladium prints.
For many years I was frustrated by variations in the density and contrast of platinum prints when nothing apparently had changed. The same negative printed with the same emulsion, paper, exposure time and developer would print out differently from one day to the next. A negative that yielded a good tonal range, deep blacks and crisp highlight, would be underexposed and muddy when I returned to make another print. This could be compensated for by increasing the emulsion’s contrast and exposure time but it made producing consistent prints difficult. The answer was found in the developer’s pH.
There are many developers used with platinum printing but potassium oxalate is the best in my experience. Most platinum/palladium developers are made of supersaturated combinations of two compounds with opposing pH. Potassium oxalate is a manufactured from potassium carbonate (11.5pH) and oxalic acid(1.3pH). This crystalline powder is mixed into the water until it will no longer dissolve. There are many good formulas available on the internet to make it yourself. Fresh potassium oxalate’s pH is just under 7.
Here is the variable that causes the inconsistency.
Platinum/palladium salts are sensitized to U.V. light with an iron and oxalate acid compound. As each print is processed, oxalic acid from the emulsion dissolves in the developer. Very quickly the pH will descend below 6 and after processing 30 or so prints, it will be closer to 5. As the developer becomes more acidic the precious metals become less responsive, particularly the platinum. If the pH is not kept at 6 or above the metal isn’t fully developed and contrast and exposure will vary and diminish.
The remedy is adding potassium carbonate to raise the pH. With a plastic spoon, sprinkle small amounts potassium carbonate into the developer and stir (1/2 a gram to 1L of P.O. at a time). Check the pH with litmus paper and repeat until the solution is up to 6.5. If the developer has become very acidic there will be some effervescing. I recommend doing this every twelve prints. Keep oxalic acid handy as it is easy to go over 7, making the solution alkaline and useless. If this happens, the color will change from the usual unappetizing green to an equally unpleasant red.
Some sediment may form. This can be filtered out but I’ve never seen any noticeable effect on a print.