An extraction-spectrophotometric method for the determination of potassium by means of a new reagent, 4-[(4-(phenylamino)phenyl)azo]-2, 5-dichlorobenzenesulfonic acid (TOOCl
2) was proposed. The complexes of potassium ions with dibenzo-18-crown-6 (DB18C6) associate with TOOCl
2 anions to give colored ion pairs which can be extracted into benzene. The ion pairs in benzene exhibit a maximum absorption at 430 nm with an apparent molar absorption coefficient of 9300 dm
3 mol
-1 cm
-1. Potassium in river water was determined as follows: To a 50-cm
3 vessel with a stopper, 5 cm
3 of an aqueous TOOCl
2 solution (0.02 mol dm
-3), 5 cm
3 of an aqueous Li
2(EDTA) (0.001 mol dm
-3) solution, and 10 cm
3 of a water sample were transferred. The resulted solution was shaken with 20 cm
3 of a benzene solution of DB18C6 (0.01 mol dm
-3) for 30 min at 20°C. After the mixture was allowed to stand for 30 min, the absorbance of the benzene phase was measured at 430 nm against a reagent blank. Interferences from calcium (II) and magnesium (II) were avoided by complexing with EDTA. Sodium ions up to 8 ppm gave virtually no interference. Potassium concentrations determined by means of the proposed method agreed closely with those of atomic absorption spectrophotometry.
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