A novel polyvinyl chloride (PVC) membrane electrode for use in a potentiometric sensor for oxalate is described. The sensor comprises a surfactant-modified zeolite A (SMZ) as a modifier, dioctyl phthalate (DOP) as a plasticizer, and PVC matrix in the ratio 10:60:30 (modifier:DOP:PVC) (w/w). The membrane sensor showed a suitable response to oxalate in the concentration range of 1.0 x 10(-6) to 3.0 x 10(-1) M (r = 0.9997), with a detection limit of 3.2 x 10(-7) M and a Nernstian slope of -29.9 +/- 0.6 mV per decade(-1) of oxalate concentration. The electrode response to oxalate remained constant in the pH range of 3.2-10.8. The selectivity coefficients for oxalate relative to a number of potential interfering substances were also determined. The sensor was highly selective for oxalate over a wide variety of other anions and exhibited a rapid response time of 5s over a period of 2 months with good reproducibility. Thus, this novel sensor may be applied as an indicator electrode in the potentiometric titration of oxalate and Ca2+ ions. (C) 2011 Elsevier Ltd. All rights reserved.