Electrodeposition of Platinum from Chloroplatinic Acid

SYNOPSIS

The electrodeposition of platinum from aqueous solutions containing chloroplatinic acid (15 to 25 g/l) and hydrochloric acid (10–390 g/l HCl) has been investigated at rates of deposition from about 0·1 to 1·1 × 10⁻³ in./hr and at temperatures from 45° to 75°C in a successful attempt to define the conditions for the production of ductile plates. The quality of the plates was assessed by microexamination, by their hardness and by a simple rolling test. The process was operated with platinum anodes, which behaved as soluble anodes in all electrolytes which were acid enough to yield ductile deposits. The plating cell was operated both with and without a diaphragm; the non-diaphragm cell was preferred because it was simpler to operate. The amount of stirring was critical; best results were obtained without any stirring except that provided by the thermal convection currents when the bath was at 60–70°C. Plating conditions could not be defined in terms of current density because the plating rate was affected by other important variables. For the production of ductile deposits (hardness 155–250 KHN) there appeared to be a minimum rate of deposition which varied with the acidity of the bath, namely from 0·7 ×10⁻³ in./hr at 225 g/l HCl to 0·35 × 10⁻³ in./hr at 290 g/l HCl. Below these minima the deposits were harder (250–350 KHN) and were either cracked as plated or they cracked on rolling; as the rates of deposition were increased above these minima the deposits remained ductile up to the rate (about 1·0×10⁻³ in./hr regardless of acidity) at which they became rough, spongy or semi-burnt. No fully ductile deposits were obtained at acidities below about 220 g/1 HCl. The electrolyte was found to have a smoothing effect at appropriate rates of deposition; smooth deposits up to 0·013 in. thick were obtained. Heating the electrodeposits above 1100·F caused blistering which was attributed to the presence of occluded platinum compounds. The hydrolysis pH f chloroplatinic acid, which was measured for the first time, was found to be about pH 2 2. An explanation is proposed for the unusual relationship between rate of deposition, acidity of bath and quality of deposit. On account of its unusually high acid content, the plating solution is only suitable for the deposition of platinum on noble metals.