which the membrane is any type of surface modified membrane e g a membrane having a layer of electrocatalyst on at least one surface thereof to form a membrane electrode composite such as described in published u k patent application gb no 2 009 788a a membrane having thereon a gas and liquid permeable porous non electrode layer such as described in published u k patent application gb no 2 064 586a or a membrane having a roughened surface such as described in published u k patent application gb no 2 043 108a a particularly suitable arrangement for the practice of the invention is that wherein a membrane having such non electrode layer is used in a narrow gap configuration suitable ferrite solutions for use in the invention include those of sodium or potassium ferrite in strong aqueous caustic solution the caustic being e g 40 to 50 by weight sodium hydroxide or 40 to 52 by weight potassium hydroxide most suitably the solution is an aqueous solution such solutions can be made in known manner by heating a strong caustic solution with particles of iron any unreacted iron particles are easily removed by filtering or centrifuging the resulting ferrite solution is best stored out of contact with air to preclude destruction of ferrite ions by oxidation the chemical formula for sodium ferrite is sometimes shown as na 4 feo 3 and is sometimes shown as na 4 fe oh 6 the ferrite ion can further be represented as feo 2 or hfeo 2 there is moreover some discrepancy in the chemical literature as.