Implementation of Coulometric Titration System at Constant Current for Certification of Primary Reference Materials R.O. Cristancho 1 , A.F. Peralta 1 and H. Torres 1 1 Instituto Nacional de Metrología, Bogotá, D.C., Colombia E-mail: rcristancho@inm.gov.co . Postal code: 111321. Phone: (0571) 2542222. Abstract. Coulometry is a primary method for measuring high purity substances. This is used to certify the primary reference materials required in a chemical analysis process. This paper describes the coulometric titration system developed by the National Metrology Institute (NMI) of Colombia for the certification of hydrochloric acid 0,1 mol·kg -1 . In addition, it also shows preliminary studies for future development of and ethylenediaminetetraacetic acid disodium salt (EDTA-Na 2 ) certification. Introduction Coulometry is a high-level electroanalytical technique for measuring the amount of substance in compounds and solutions. This technique has been used to determine the purity of primary reference materials required by industry and in chemical testing laboratories [1]. Coulometry is one of the most widely used analytical techniques in research laboratories and National Metrology Institutes (NMI) worldwide due to its high precision and accuracy for measuring the trace level or purity of different materials [2]. Coulometric analysis uses electrolysis to generate a titrant that reacts stoichiometrically with an analyte, thus making possible a determination of the amount of substance. For this reason, The Consultative Committee for Amount of Substance (CCQM) of the International Bureau of Weights and Measures-(BIPM) has considered coulometry to be a primary method that can be used to determine the purity of primary reference materials. Electroanalytical methods for analyzing compounds are supported by current, potential and charge measurements of a system. When an analyte is reduced or oxidized, these electrical signals are directly related to the amount of analyzed substance present in the system, which has a direct relation to the mole unit of the International system-SI. With coulometry, this relation consists of a determination of the amount of substance with electric charge added to the electrochemical system described by Faraday’s law: ൌ ொ ௭ி ൌ ׬ ூௗ௧ ௭ி (1) Where n is the amount of substance, Q is electric charge, I is electric current, t is electrolysis time, z is the charge number of reaction and F is Faraday constant. In accordance with Faraday’s law, there are two forms of coulometry: (i) controlled-potential, in which electric current decreases exponentially over time; and (ii) controlled-current, in which electric current is constant. The latest form is used for determining analyte content in the matrix, such as salts or metals in solution. Coulometry provides high selectivity and good sensitivity for the identification and quantification of trace-level, which is hard to determine by other electroanalytical methods [3,4]. Additionally, it is able to provide good results with low uncertainty and has been implemented in a larger number of NMIs [5]. The goal of this paper is to provide constant current coulometry studies as developed for the Electrochemical Analysis Working Group at the National Metrology Institute (NMI) of Colombia. First, characterization studies of aqueous hydrochloric acid solutions 0,1 mol·kg -1 will be presented. Then, preliminary studies to determine the purity of EDTA-Na 2 will be presented. 1. Method Coulometry system development for the NMI of Colombia is similar to the system used by Ural Scientific and Research Institute for Metrology of Russia [6]. This coulometry system consists of: (i) a titration vessel with two independent compartments (these compartments