A device (10) for producing a biocidal solution has a sealed and replaceable reservoir (16) with a low volume capacity which contains pre-prepared aqueous salt solution (18) with a pre-determined concentration, an electrolytic cell (20) for electrochemically treating the salt solution (18), a pump (28) for delivering a flow of accurately measured volumes of the salt solution (18) to the electrolytic cell (20), a restrictor valve (36) for controlling further the flow rate of the salt solution (18) to a cathode chamber (24) of the electrolytic cell (20), an electrical circuit (46) for connecting the pump (28) and the electrolytic cell (20) to a mains supply source (48) and including a power pack (50) for providing a substantially constant direct current of (7) to (12) amps to the electrolytic cell (20) and an operator-controlled switch (52) for completing the electrical circuit (46) when the switch (52) is operated to produce a biocidal solution (12) having a predetermined level of available free chlorine and pH.