This International Standard specifies two methods, i.e. flow injection analysis (FIA) and continuous flow analysis (CFA), for the determination of soluble silicate ions in various types of water (such as ground, drinking, surface, leachate and waste water). Both methods are applicable to the determination of a mass concentration of silicate (SiO2) ranging from 0,2 mg/l to 20 mg/l (with working ranges 0,2 mg/l to 2,0 mg/l and 2 mg/l to 20 mg/l). Other mass concentration ranges are applicable, provided they cover exactly one decade of concentration units (e.g. 0,02 mg/l to 0,2 mg/l in SiO2). These methods can be made applicable to seawater by changing the sensitivity and by adapting the reagent and calibration solutions to the salinity of the samples.