Platinum Toner.

Platinum toning gives enhanced black images. This method uses Platinous Chloride ( PtCl2 ) in the presence of an alkali chloride which gives a chloroplatinite e.g. PtCl2.2KCl. The platinum is displaced by the silver within the photographic image from within these compounds. This is then precipitated as a metal whilst the silver is converted to a insoluble halide: PtCl2 + 2Ag = Pt + 2AgCl.

The reaction has to take place in an acid solution. The preferred platinum salt is potassium chloroplatinite. The toning process leaves approx. 25% of the image silver unaffected.