Two of the most commonly used developers in the Platinum/Palladium processes are Potassium Oxalate and Ammonium Citrate. Potassium Oxalate is preferred for several reasons. It produces warmer tones, it does not become exhausted but can be replenished as needed and used indefinitely, and it can be used with the dichromate method of contrast control.
Photographers' Formulary 07-0028 Specs
Oxalic acid, dipotassium salt Ethanedioic acid, dipotassium salt
Transparent, colorless crystals
Platinum printing Platinotype process To remove stains in photography and in analytical chemistry
K2C2O4 . H2O
Standard mixture for potassium oxalate is 454 grams (1 pound) to 48 ozs. Water. Basically the hotter the developer the warmer the print. Developer should be around 150°F. Caution: Potassium oxalate is very poisonous substance, use with care.