According to the German Standard Method for water analysis, also called "platinum color number". Visual comparison of the coloration of water samples with solutions of platinum and cobalt chloride of different concentrations. The APHA and ASTM standards designate the respective scale with Hazen units, where 1 mg/l Pt equals one Hazen.
The stock solution is prepared as follows: Dissolve 2.49 g K2PtCl6 and 2.02 g CoCl2·6H2O in 200 ml of concentrated pure HCl (d=1.19) and dilute to 1 liter with distilled water. This solution represents 1000 Hazen.