Interference studies were performed by spiking some cations, Ca2+, Mg2+, Co2+, Ni 2+, Cr3+, Cu2+ and Mn2+ into Fe(III) 10 ppm solutions at the final concentration of 1, 5 and 10 ppm (10:1, 2:1 and 1:1 concentration ratios of Fe(III):interference ion).