An acidic dye, alizarin S, PVC-matrix ion selective electrode was prepared by using its salts of large cation as a sensing element. The effects of various quaternary ammonium salts, quaternary phosphonium salts, basic dye, ferroin ion and various mediator solvents have been discussed in detail. Trioctylmentyl ammonium chloride and dibutyl phthalate are preferred. The optimum membrane content of sensing element is 0.12%. The response to alizarin S ion is linear with a Nernstian slope 57 m V/decade over the concentration range of 2X10-4-5X10-7M in the pH range of 2-4.2. The limit of detection is 2X10-7M. The selectivity coefficients of various common anions were determined. ClO4^-, CNS- and I- anions show remarkable interference. As alizarin S is a metallochromic indicator, the variations of potential in the presence of some metals were also tested. The electrode potential drift is less than 2mV for the duration of eight hours period. The stability and reproducibility are satisfactory. The life time of the membrane is more than four months. This electrode can be used as an indicator electrode in the potentiometric titration with EDTA and ARS. It is necessary to titrate Sc at proper pH in order to avoid interference and to get good precision. There is no interference from alkali and alkaline earth metals and the amounts of La as much as 30 times the amounts of Sc and Y 10 times of Sc do not interfere at designated pH. It is a good practice to determine Sc potentiometrically with alizarin S as a titrant. The variation coefficient is 0.4%. The present method is suitable for the determination of Sm in Co-Sm alloys with good results.

Key words: APPLICATION, PREPARATION, ION SELECTIVE ELECTRODE, ALIZARINE