as being between about 0 9 and 25 milliequivalents of halogen per liter of the sensitizing solution preferably about 2 5 to 9 milliequivalents e g 0 3 1 0 gm l for cupric bromide increasing the proportions of the halide ions is usually undesirable as such increase appear to gradually diminish the sensitizing effect of the treatment below what is obtainable with the optimum amount also the proportion of these halide ions expressed as equivalents is less than that of the cupric or other reducible non noble metal cations in the treating solution for instance the ratio of equivalents of such metal ions to halide ions is usually in the range of at least 2 1 and preferably about 4 1 to 100 1 in one embodiment the eptfe materials membranes of fig1 and 2 are treated with a wetting agent by immersing it in a solution of 75 methanol 25 ethanol and of 1 weight copolymer of tetrafluoroethylene and vinyl alcohol at room temperature for about 30 seconds thereafter the wetted membranes are dipped in a catalytic treating solution prepared by adding the following components to one liter of d i water ______________________________________component amount gms ______________________________________2 6 di sodium anthraquinone di.