The usual preparation of Krogmann's salt involves the evaporation of a 5:1 molar ratio mixture of the salts K[Pt(CN)] and K[Pt(CN)Br] in water to give copper-colored needles of K[Pt(CN)]Br·2.6 HO.
::5K[Pt(CN)] + K[Pt(CN)Br] + 15.6 HO → 6K[Pt(CN)]Br·2.6 HO
Because excess Pt or Pt complex crystallizes out with the product when the reactant ratio is changed, the product is therefore well defined, although non-stoichiometric.