An electrocatalytic detection on a potassium hexacyanoferrate (II) modified polypyrrole electrode has been studied. The pyrrole electropolymerization was carried out on a Pt disk of 3 mm diameter, using constant potential chronoamperometry in the presence of the modifier. The electrochemical control of the electropolymerization and modification permit us to know the polymerisation charge, Qp, as well as the number of moles of K4Fe (CN)6incorporated into the polymer at its generation step using Faraday's Law. Finally a determination method for vitamin C (ascorbic acid) using scan and differential pulse voltammetry was developed. This method was applied to determination of vitamin C in several samples, such as tablets and orange drinks, using the standard addition method.