DETERMINATION OF PHOSPHATE AND SULFITE IN NATURAL-WATERS IN THE PRESENCE OF HIGH SULFATE CONCENTRATIONS BY ION CHROMATOGRAPHY UNDER ISOCRATIC CONDITIONS

Phosphorus in aquatic systems is generally considered the limiting nut rient in most of cases. Its concentration may be below 0.01 mg/l. The sensitivity of the standard method for phosphate with ascorbic acid do es not allow its determination in these kind of samples. Ion chromatog raphy (IC) provides a suitable way for phosphate quantification. Howev er, when the sulphate concentrations are significantly high, interfere nce of the phosphate signal does occur, This can be avoided by precipi tating sulphate as lead sulphate with lead perchlorate solution perfor ming a potentiometric titration using a lead ion selective electrode ( ISE). The samples are then filtered through a 0.45-mu m membrane filte r and analyzed by IC. A Dionex Model 2000i/SP IC, with an anion pre-co lumn (Dionex AG4A), an anion separator column (Dionex AS4A), a suppres sor column (Dionex AMMS-II), and a conductivity detector was used. The mobile phase (2 ml/min) was 1.7 mM NaHCO3-1.8 mM Na2CO3. An automatic Titroprocessor Metrohm with Dosimat E678 and a lead ISE Orion 94-82 c oupled to a double junction reference electrode Orion 90-02 was used f or the potentiometric titration of the samples. In samples (e.g. inter stitial waters) in the presence of sulphite, it is not possible to qua ntify this ion using IC with isocratic conditions, since it co-elutes with sulphate ion. In this case, sulphate may be determined potentiome trically, using a reduction solution to obtain hydrogen sulphide, then sulphite concentration is determined by the difference between IC and potentiometric results. Water samples of estuary and lagoon with sulp hate concentrations between 10.000, and 200 mg/l and phosphate concent rations between 0.020 and 0.090 mg/l were analyzed. The detection limi t for phosphate was 1 mu g/l, and the relative standard deviation (R.S .D.) was less than 5 percent. The detection limit for sulphate was 3 m u g/l and the R.S.D. was less than 5 percent too. The objective of thi s paper is to present an economical, sensitive and accurate way to avo id sulphate as interference with phosphate and sulphite in isocratic c onditions by IC.