There is a non-linearity in the curve of coulombs vs. concentration, i.e. two slopes exist for low and high concentration. This could be due to the strong adsorption of thiourea on the gold electrode. The anodic response of thiourea is expected to be strongly influenced by its adsorption isotherm, because the mechanism for anodic detection of sulfur compounds is believed to involve adsorption via sulfur atoms onto oxide-free metal atoms in the electrode surface 14