Determination of phosphate and sulphite in natural waters in the presence of high sulphate concentrations by ion chromatography under isocratic conditions

• Published in: Journal of Chromatography A 1996-07, Vol.739 (1), p.207-215
• Main Authors: de Medina, Hilda Ledo, Gutiérrez, Elizabeth, de Vargas, Marinela Colina, González, Graciela, Marín, Julio, Andueza, Eduardo
• Format: Article
• Language: English
• Subjects: Environmental analysis; Inorganic anions; Phosphate; Phosphorus; Sulfate; Sulfite; Water analysis
• ISSN: 0021-9673
• DOI: 10.1016/0021-9673(96)00336-6

Phosphorus in aquatic systems is generally considered the limiting nutrient in most of cases. Its concentration may be below 0.01 mg/l. The sensitivity of the standard method for phosphate with ascorbic acid does not allow its determination in these kind of samples. Ion chromatography (IC) provides a suitable way for phosphate quantification. However, when the sulphate concentrations are significantly high, interference of the phosphate signal does occur. This can be avoided by precipitating sulphate as lead sulphate with lead perchlorate solution performing a potentiometric titration using a lead ion selective electrode (ISE). The samples are then filtered through a 0.45-μm membrane filter and analyzed by IC. A Dionex Model 2000i/SP IC, with an anion pre-column (Dionex AG4A), an anion separator column (Dionex AS4A), a suppressor column (Dionex AMMS-II), and a conductivity detector was used. The mobile phase (2 ml/min) was 1.7 m M NaHCO 3-1.8 m M Na 2CO 3. An automatic Titroprocessor Metrohm with Dosimat E678 and a lead ISE Orion 94-82 coupled to a double junction reference electrode Orion 90-02 was used for the potentiometric titration of the samples. In samples (e.g. interstitial waters) in the presence of sulphite, it is not possible to quantify this ion using IC with isocratic conditions, since it co-elutes with sulphate ion. In this case, sulphate may be determined potentiometrically, 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 sulphate concentrations between 10.000 and 200 mg/l and phosphate concentrations between 0.020 and 0.090 mg/l were analyzed. The detection limit for phosphate was 1 μg/l, and the relative standard deviation (R.S.D.) was less than 5 percent. The detection limit for sulphate was 3 μg/l and the R.S.D. was less than 5 percent too. The objective of this paper is to present an economical, sensitive and accurate way to avoid sulphate as interference with phosphate and sulphite in isocratic conditions by IC.