Surfactant-sensitized malachite green method for trace determination of orthophosphate in aqueous solution.

A surfactant-sensitized spectrophotometric method for determination of trace orthophosphate has been developed using anion surfactant (Ultrawet 60 L) with molybdate and malachite green in low acidic medium (pH(T) 1.0). The method detection limit (3 x standard deviation of blank, n=10) was 8 nM and the calibration curve was linear over a range of 10-400 nM (r2=0.997). The molar absorptivity was 1.26 x 10(5) L mol(-1) cm(-1) at 600 nm with the background correction at 530 nm. The precision of method was 3.4% at 50 nM and 2.4% at 100 nM orthophosphate (n=10). The hydrolysis of eight organic phosphorus and polyphosphate compounds was less than 2% of the total phosphorus present (5-10 microM). This method showed less arsenate interference than previous methods, with only 3% even in the presence of orthophosphate in the samples. No interference of silicate up to 40 microM was observed. Background anions (in an order of SO4(2-)>NO3->Cl-) have greater effects than cations (Ca2+>Mg2+>Na+) on the reagent blank and the molar absorptivity of the color product.