An electrically powered apparatus for generating a solute such as chlorine to sanitise a body of water such as a pool or spa, a by-product of such generation being an explosive gas such as hydrogen, said apparatus including: a) an electrolytic cell (1) adapted to operate in a substantially vertical orientation through a range of 45 degrees either side of the vertical; b) a water inlet (13) and outlet (14) both located at the lower end of said electrolytic cell (1); and c) a defined space (16) surrounding one or more electrodes (28) of said electrolytic cell (1), wherein, in, the event that water flow through said apparatus ceases and said electrolytic cell (1) continues to produce said explosive gas, said explosive gas will displace water in said defined space (16) until there is no water around said electrodes (28).