The aluminium content in a water supply is determined by a method which involves forming the aluminium in a sample stream of the water into a complex with added fluoride and sensing the fluoride content electrically. Equipment for carrying out the invention comprises means (10) to mix a sample stream (in 1) with a standard sodium fluoride solution (in 9), warm it (in 12) and sense the PD generated between electrodes (13 and 14). Solutions of known aluminium concentration can be drawn from reservoirs (3) and (4) to calibrate the fluoride ion selective electrode (14).
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