[en] An accurate and rapid electrometric method for the determination of praseodymium and thorium as their meta-arsenates has been investigated. It consists in titrating praseodymium chloride and thorium nitrate solutions pH-metrically against a standard solution of sodium meta-arsenate, using a wide range glass electrode in conjunction with S.C.E. A marked change in pH is observed at the end-point corresponding to the precipitation of Pr(AsO₃)₃ and Th₃(AsO₃)₄ in the neighbourhood of pH 4.8 and 4.2, respectively. The curves have a regular form and a pronounced maximum in dpH/dV occurs at the end-point. The accuracy and reproducibility of the results have been found to be excellent even at low concentrations (5x10sup(-4)M) of the reactants. pH titration offers a simple, rapid and accurate method for the determination of praseodymium and thorium as their meta-arsenates. Similar titrations were also performed for estimation of samarium but they failed to provide any dependable results. (author)