In this work, we propose a new general method based on the use of cubic spline functions, suitable for the numerical determination of extended semi integrals and semi differentials. The procedure combines these interpolation functions with different numerical integration and differentiation standard methods, including commercial software. The method has been applied to the electrochemical reduction of the meso-tetra(4-N-methylpyridyl)porphyrin in acidic aqueous solution, where this compound exhibits a two-electron wave in cyclic voltammetry. In this work, we determine the semi differential of the current and demonstrate that the signals obtained correspond to an EE process, where both electrochemical steps are reversible and their formal potentials are very close.