The pulp potential Eh was also measured against Ag/AgCl (3 M KCl) reference electrode in flotation. The reduction of Eh was noticeable, from 157 mV in the baseline flotation to 51 mV with the addition of 10 ppm free cyanide. The addition of 10 ppm cuprous cyanide also led to a decrease of Eh to 67 mV. Therefore, the reducing condition caused by cyanide species was identified. In fact, it is always observed that the decrease of potential co-exists with the depression of pyrite flotation (Kocabag and Guler, 2007 and Wet et al., 1997). In the study by Kocabag and Guler (2007) using singe mineral flotation, there was a critical pyrite potential of about 100 mV (SHE) below which pyrite flotation was strongly depressed. It was also observed for a gold ore that the pyrite potential was decreased from 195 mV to 125 mV (SHE) with pyrite flotation being almost completely depressed (Wet et al., 1997). However, the critical potential above which pyrite floats can only apply to oxygenated atmosphere that used in the present study. Hydrophobic pyrite surface could be established at low potentials and low pH in a nitrogen atmosphere (Miller et al., 2002).