A phosphate-selective electrode based on surfactant-modified zeolite (SMZ) particles into carbon-paste has been proposed (SMZ-CPE). The electrode was fully characterized in terms of composition, response time, ionic strength, thermal stability and usable pH range. The electrode containing 20% SMZ exhibited linear response range to phosphate species in the range of 1.58 x 10(-5) to 1.00 x 10(-2) M with a detection limit of 1.28 x 10(-5) M and a Nernstian slope of 29.9+/-0.9 mV per decade of phosphate concentration. The electrode response to phosphate remains constant in the pH range of 4-12 and in the presence of 1 x 10(-4) to 4 x 10(-3) M NaNO(3). The response of the electrode reaches equilibrium within several seconds after immersing the electrode in phosphate solution. Common anions such as Cl(-), Br(-), I(-), NO(3)(-), SO(4)(2-) and Cr(2)O(7)(2-) have little effect on the determination of phosphate but AsO(4)(3-) shows some interference. A successful application of the electrode for determination of phosphate in a fertilizer, using direct potentiometry, is presented. The electrode was also used for the potentiometric titration of phosphate. The validation of the obtained results in each case was proved by statistical methods.
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