Development of simple pocket test tools for fast determination of dissolved sulfate in waters

A rapid field method for routine checks on dissolved sulfate in surface, running, and potable waters is presented. The method uses reagent indicator paper strips and a thermometric unit. The RIS-Sulfate-Test was developed by immobilization of an Arsenazo III-barium complex together with buffer and masking reagents on cellulose paper. It was adapted to sulfate-ion determination with the use of visual, densitometric, and photometric techniques, the lower detection limit being 0.05– 0.1 g L⁻¹. The sensing principle is based on the reaction, which gives a colorless barium sulfate precipitate with a heat of formation of ΔH = 4.6 kcal M⁻¹. After the strip is immersed into a sample solution for 1 s, the color changes from black–blue to pink–violet because of sulfate-induced complex decomposition. Color changes are monitored with the use of a standard color scale and a miniaturized reflectometer with 660-nm light diode. A miniaturized calorimeter was used for determining high concentrations of sulfate. A 0.3-ml aliquot of sample solution is placed on the bottom of a 5-ml polyethylene vessel, and 0.1–0.2 ml of 0.3-M barium chloride is introduced into the vessel with a 1-ml syringe. The lower part of the syringe with the reagent is immersed into the sample solution for a quicker thermal equilibration. After 10 min, the reagent is injected into the solution and readings are taken with a small voltmeter. A linear dependence of the instrument readings on sulfate concentration was observed in the 0.2 to 5-g L⁻¹ range (P = 0.95, n = 3, RSD = 20%). Both methods were checked by standard addition and dilution procedures, and their reliability was confirmed by flow-injection analysis of seawater with the use of spectrophotometric detection of a turbidimetric indicator reaction. © 2000 John Wiley & Sons, Inc. Field Analyt Chem Technol 4: 147–153, 2000