A new analytical method for the determination of the purity of CASARM Lewisite samples (Chemical Agent Standard Analytical Reference Materials) has been developed and tested. The method essentially replaces the volumetric iodimetric procedures currently in use with the in-situ electrochemical generation of iodine. Tests on both arsenious acid and Lewisite/CVAA solutions show that the method is capable of achieving high accuracy and precision. Sample preparation and analysis times are considerably less than the current titrimetric method. The coulometric iodimetric method has been shown capable of determining not only total organic trivalent arsenic, but also total trivalent arsenic merely by adjusting the pH. Organic arsenic is determined at pH 4, and total trivalent arsenic at pH 8.