This International Standard specifies a method for the determination of dissolved sodium and potassium by flame emission spectrometry (FES). It is intended for the analysis of raw and drinking waters. The method is applicable to water samples with a mass concentration of sodium and potassium of up to 10 mg/l. For samples containing higher concentrations of sodium and potassium, a smaller test portion is taken for analysis. The lower limits of determination are less than 0,l mg/l for both sodium and potassium.