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, ru nning, and potable waters is presented. The method uses reagent indicator p aper strips and a thermometric unit. The RIS-Sulfate-Test was developed by immobilization of an Arsenate III-barium complex together with buffer and m asking reagents on cellulose paper. It was adapted to sulfate-ion determina tion 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 Delta H = 4.6 kcal M-1. After the strip is immersed i nto a sample solution for 1 s, the color changes from black-blue to pink-vi olet because of sulfate-induced complex decomposition. Color changes are mo nitored with the use of a standard color scale and a miniaturized reflectom eter with 660-nm light diode. A miniaturized calorimeter was used for deter mining 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 solu tion for a quicker thermal equilibration. After 10 min, the reagent is inje cted into the solution and readings are taken with a small voltmeter. A lin ear dependence of the instrument readings on sulfate concentration was obse rved 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 relia bility was confirmed by flow-injection analysis of seawater with the use of spectrophotometric detection of a turbidimetric indicator reaction. (C) 20 00 John Wiley & Sons, Inc.