Pore size distribution for membranes for electrolysis of brine

Microporous sheet 0.2-2mm thick for use as a diaphragm in a low voltage electrolytic cell characterised by having (a) a porosity of 70-90% and (b) a pore size distribution effectively in the range 1 millimicron-1mm of which at least 90% of pores have standard pore diameters, as detd. by porosimeter, in the range 0.1-100 microns dia., with at least 80% lie in the range 0.12-33 micron dia., and (c) the ratio no. of pores of 0.9-0.3 micron dia./no. of pores in the range 0.8-20 micron dia., is less than about 50. Used for porous membranes of PTFE or similar material for electrolytic desalination of sea water to produce Cl2, H2 and NaOH without concomitant subreaction to form sodium hypochloride, using cells operated at less than 5V, e.g. 2.6-4V and 0.1-0.3 A/sq.cm at 85-95% efficiency based on NaOH. Close specification of the pore size distribution eliminates electrolyte contamination due to oversize pores of excessive electrochemical resistance due to trapping of ions by undersized pores.