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 × 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 × 10⁵ 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 μM). 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 μM was observed. Background anions (in an order of SO₄²⁻ > NO₃⁻ > Cl⁻) have greater effects than cations (Ca²⁺ > Mg²⁺ > Na⁺) on the reagent blank and the molar absorptivity of the color product.