This paper describes the Coulometry system implemented at National Metrology Institute (NMI) of Colombia for purity certification of potassium hydrogen phthalate (KHP). The system aims to become a primary measurement system. This study includes an evaluation of precision in terms of relative standard deviation (%RSD), and its associated uncertainty by repeatability and end-point detection such as type-A uncertainty; And by electric resistance, time, potential difference and sample mass such as type-B uncertainty sources. The implementation of this system will allow the certification in purity of different reference materials.