The impedance of a natural galena (PbS) crystal polarized in solutions with a borate buffer was studied under conditions similar to those used in flotation plants for sulphide minerals. Measurements were first carried out with solutions containing only the buffer in order to study the electrode oxidation. A behaviour quite unusual in electrochemistry was observed : dependence on time but practically not on potential. With the aid or literature or other new results and in the light of a recent treatment of a model predicting film growth, the impedance data were explained by the formation of a growing film consisting in Pb(OH)2 + S. It was also shown that when less than one monolayer of xanthate is adsorbed, it partly inhibits the oxidation. These results bear new insights into the flotation of sulphide minerals in the presence of low xanthate concentrations. Flotation indeed results from the formation of a mixed potential but the anodic reaction is not the oxidation of xanthate but that of the mineral and the mixed potential determines the amount of xanthate adsorbed. The connected cathodic reaction, the oxygen reduction, is studied in part II.