Platinum Green by Richard Sullivan and Carl Weese

The following is a list of the compounds used in normal platinum and palladium printing with our judgments of the hazards they pose to personal health and to the environment.

Platinum and palladium salts

These metal salts pose very little danger to the environment, for several reasons. Platinum and palladium are not "heavy metals." They are indeed metals, and in fact they are heavy, but the term "heavy metal" is used to refer to metals such as arsenic, cadmium, selenium, lead, and mercury. These are insidious, nasty poisons that bind tightly with other elements and can accumulate in your body over a long period of time. "Noble" metals like platinum and palladium are so-called because they are almost always found in nature in their pure metallic state. The reason is that when they form salts by combining with other chemicals, they don't bond tightly. This weak bond breaks down quickly when the salts enter the natural environment. What are left are metallic platinum or palladium and chlorine salts. The palladium salt we use to make prints breaks down to palladium and sodium chloride (table salt) and in the case of platinum what remains is metallic platinum and potassium chloride, which is commonly used as a salt substitute in low sodium diets. Neither these salts nor the noble metals themselves pose any serious threat to people or their environment. The platinum and palladium metals are so non-reactive they could in fact be ingested and would pass through the body without harm.

Ferric Oxalate

Platinum printers use ferric oxalate, or more complex double ferric oxalate salts, as the light sensitive ingredients of the sensitizer coated onto a sheet of paper.
Ferric oxalate is a weakly bound chemical and if disposed of into the environment will quickly decay to ferrous oxalate and then to oxalic acid and ferric oxide (rust).

Developers

Platinum and palladium developers are salts of the weak organic acids. Some of them are toxic, but no more so than many natural environmental toxins. Organic acids are found commonly in the foods we eat. Citric acid gives lemons their sour taste, and oxalic acid in dark green leafy vegetables like spinach gives them their bitter flavor. Malic acid makes some apples more tart than others. Tartaric acid gives sour flavors to wine while acetic acid makes vinegar what it is. There's lactic acid in milk. Formic acid makes bee sting hurt and gives ants their family name of formicae.
If you take baking soda, and as we all did in school, and add it to one of these acids it will fizz and bubble. When the bubbling stops, the acid is neutralized and a weak organic acid salt is formed. For instance, use citric acid and baking soda and you get sodium citrate, which will do a nice job of developing platinum prints.
Out of this group the oxalates are the only salts that are poisonous, but even that does not mean they are harmful to the environment or severely hazardous to people. Ten pounds of spinach has a lethal dose of oxalic acid for a human but acres of the crop are perfectly safe on a farm. In small quantities the human body deals with the oxalates quite handily as the body produces natural
chelates (we'll get to those later) that render them harmless.
The most commonly used developers such as ammonium citrate, potassium oxalate, sodium acetate, and sodium citrate are quite safe.

The Chelating Agents

Platinum printers use a variety of different compounds to help dissolve the remnants of the ferric oxalate sensitizer left over alter the noble metals have been precipitated into the print paper during development. Since ferric oxalate is first cousin to rust, many of the compounds are the same as those used to remove rust stains from pools and spas.
The most commonly used compound is called EDTA tetra sodium. EDTA is used agriculturally as an aid to help plants absorb iron from the soil. EDTA is often combined with sodium sulfite in the clearing bath. The sulfite enhances the chelating action by physically "opening" the paper for better contact with the chemical. Neither of these chemicals is toxic.
Discarded EDTA clearing bath will be quite benign to the environment since the ferric contaminants
will have been reduced to rust.

Conclusion

Platinum and palladium printing is probably safer for the individual and the environment than either the usual black and white or color silver-halide based photography. It uses no chlorinated hydrocarbons and the organic compounds it does use are the reasonably safe weak
organic acid salts.