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Determination of Potassium, Sodium, Calcium, and Magnesium in Natural Waters by Ion Chromatography¹

ABSTRACT

Simultaneous determination of alkali and alkaline earth metals in water samples is needed. Studies to evaluate the Dionex Model 10 Ion Chromatograph (IC) for simultaneous determination of K and Na or Ca and Mg in diverse types of natural waters, including soil extracts, showed that the results by the IC method closely agreed with those obtained by flame photometry for K and Na and by atomic absorption spectrophotometry for K, Na, Ca, and Mg. The IC method is based on separation of the monovalent and divalent cations by a separator column containing a low-capacity sulfonated divinylbenzene polystyrene copolymer, cation-exchange resin in the H form and on converting the various metal ions to their corresponding hydroxides by a suppressor column containing a high-capacity, anion-exchange resin in the OH^– form. The eluent used for determination of alkali metals was 5 mM HCl, and a solution containing 2.5 mM HCl + 2.5 mMm-phenylenediamine dihydrochloride was used for determination of alkaline earth metals. Electrical conductivity is used as the mode of detection. The method is precise, accurate, and can detect as little as 0.1 mg L^–1 of K, Na, Ca, or Mg in water samples. The coefficient of variation values ranged from 1.3 to 5.0% for K, from 0.63 to 9.1% for Na, from 1.9 to 3.5% for Ca, and from 0 to 2.4% for Mg. With the IC method, K and Na or Ca and Mg can be determined simultaneously in 2 mL of water in approximately 7 min. A single operator can perform about 40 analyses of alkali or alkaline earth metals in a normal working day. Successive analyses of waters containing high concentrations of metals (e.g., Fe) may lead to loss of resolution of alkali metals, but this can be restored by washing the separator column with 3 M HCl saturated with EDTA.

Key Words: alkali and alkaline earth metals • flame photometry • atomic absorption spectrophotometry • ion-exchange chromatography • conductimetric detection

¹ Journal Paper no. J-11632 of the Iowa Agric. & Home Econ. Exp. Stn., Ames. IA. Projects 2112, 2431, and 2601.

² Graduate research assistant and professor of Soil Chemistry, respectively. Dep. of Agronomy, Iowa State Univ., Ames, IA 50011.

Received for publication September 27, 1984.