Determination of Total Phosphorus Content in Soil by H₂PO₄^- Ion Selective Electrode with Hydrophobic Ionic Liquid Sensitive Membrane of BmimPF₆

An ion selective electrode for H₂PO₄^- (H₂PO₄^--ISE) was prepared by using 1-butyl-3-methylimidazolium hexafluorophosphate (abbrev. as BmimPF₆) as hydrophobic ionic liquid sensitive membrane. And its potential response to H₂PO₄^- ion in 0.1 mol·L^-1 HAc-NaAc buffer solution of pH 4.66, at 30 ℃ with Ag|AgCl|BmimCl as reference electrode was studied. As shown by the experimental results, linear relationship was found between values of potentials and -lgH₂PO₄^- with concentration of H₂PO₄^- in the range of 1.0×10^-1 to 1.0×10^-5mol·L^-1. Slope of the curve found was -25.4 mV·dec^-1, with time of response of 3 min. This method was applied to the determination of total phosphorus in soil samples. The soil sample (0.300 0 g) which had passed through sample sieve of 0.150 mm, was moistened with small amount of water and 8 mL of H₂SO₄ with 10 dreps of HClO₄ were added. The mixture was digested by heating until the solution turned to white, and the digestion was continued for 20 min. After cooling, the solution was made up to 100 mL with water and stood overnight. An aliquot of 1.0 mL of the supernatant was taken, and 1.0 mL of pH 4.66 HAc-NaAC buffer solution was added to the supernatant, and mixed well. The H₂PO₄^--ISE was placed into the solution, with the 2 terminals of the ISE connected to the potentiometer, after stablizing for 3 min, the potential was recorded. Five soil samples were analyzed by this method, giving values of total phosphorus checked quite well with the results given by the molybdate-photometric method.