Simposio de Metrología 2008 Santiago de Querétaro, México, 22 al 24 de Octubre Centro Nacional de Metrología SM2008-M210-1057-1 Constant-Current Coulometry Studies for Assay of Primary Reference Materials to Develop Standards for Titrimetric Analysis Paulo P. Borges, Isabel Cristina S. Fraga, Sidney P. Sobral, Bianca de Souza R. Marques, Júlio C. Dias, Valnei S. Cunha. Instituto Nacional de Metrologia, Normalização e Qualidade Industrial Av. Nossa Senhora das Graças, 50 Xerém, 25250-020, Rio de Janeiro, Brasil. dquim@inmetro.gov.br ABSTRACT Coulometry is a primary method for measuring the amount of substance of high-purity compounds and its solutions. Coulometric titration is used to determine the purity of primary reference materials for acid/base, precipitation, redox and complexometric titrations. This work aims at presenting the primary system of coulometry from Inmetro, which was used for the characterization of hydrochloric acid solutions from nominal values of 1.0 mol·kg −1 and 0.01 mol·kg −1 , as well as the preliminary studies and the results of the key comparison CCQM-K48, which Inmetro has recently participated to determine the purity of potassium chloride (KCl).The reference materials characterized by the Inmetro’s primary system of coulometry will be used to guarantee the traceability and reliability to the measurement results from titrimetric analysis carried out by the Brazilian chemical analysis laboratories. 1. INTRODUCTION Metrology in chemistry has been currently acknowledged as a relevant area in researches of analytical and measuring chemistry. Its importance comes from the need to produce reliable and uniform measurements in science, technology and international trade. In the analytical chemistry area, a great number of methods use titration as the main technique to determine the concentration of a substance in a specific analyte. To carry out this analysis with accuracy, the laboratories need to use primary standard. Coulometry has the potential to become a primary method [1] by the Consultative Committee for Amount of Substance (CCQM). Its main function in the National Metrology Institutes (NMI) is to characterize primary standards, e.g., to determine the purity of primary reference materials. After being certified, these reference materials can be used as primary standards in acid/base, precipitation, redox and complexometric titration techniques [2] to determine the concentration of diverse chemical substances. Inmetro, the NMI of Brazil has established its primary system of coulometry [3] to characterize and certify high-purity reference materials such as primary standards to guarantee the reliability of the chemical analysis performed by Brazilian laboratories. This work aims to present the studies which have been developed in the Electrochemical Laboratory of Inmetro using the primary system of coulometry to characterize primary standards. Initially, the characterization of hydrochloric acid solutions from nominal values of 1.0 mol·kg −1 and 0.01 mol·kg −1 will be shown and afterwards the preliminary studies to determine the purity of potassium chloride (KCl) from NIST and the results of the key comparison CCQM-K48, which Inmetro has recently participated in will be focused. 2. EXPERIMENTAL 2.1. Primary System of Coulometry Fig. 1 shows the primary system of coulometry from Inmetro [3]. It consists of a coulometer (current source, indication unity and valve unit), argon gas unit, burette (dispenser), microbalance, water bath, magnetic stirrer, glass cells, electrodes and a microcomputer. The coulometric system is similar to the one in Slovak Institute of Metrology (SMU).