other features and advantages of the invention will become apparent from the following description of a method for preparing a membrane consisting of a polyantimonic acid powder and an organic binder according to the invention . this description is given only as an example and does not limit the invention . in order to prepare a membrane , eight gram of polyantimonic acid are wetted by means of n - methylpyrolidone . for this , it is possible to use polyantimonic acid powder with a grain size smaller than five micrometer and a specific area , called b . e . t . area of 25 m 2 / g , which is obtainable on the market under the name of polyan from applied research , 1080 brussels . the wetted powder is ground until the grain size is smaller than 1 micrometer and preferably smaller than 0 , 1 micrometer . a suspension is formed by means of the wet ground powder and twenty milliliter of n - methylpyrolidone . a solution is formed of 2 gram polyarylethersulfone in twenty milliliter of n - methylpyrolidone . for this , it is possible to use the polysulfone that is obtainable on the market under the name of udel from union carbide . this solution is added to the suspension and the whole quantity is mixed at a temperature of approximately 70 ° c . in this way there is obtained a viscous liquid , which is then poured on a plate . the solvent , that is to say the methylpyrolidone , is allowed to partially evaporate in an oven at a temperature of approximately 70 ° c ., but only until the viscous mass on the plate is sufficiently dry to show at the edges a change of colour from gray to white . after this , the plate is immersed in water at a temperature of 0 ° to 2 ° c ., so that the solvent further lixiviates . finally , the plate with the membrane produced thereon is put into streaming water , which allows the membrane to be removed . it is necessary to ensure that the evaporation takes place only partially , as described hereinabove , and to lixiviate the remaining solvent . if one removes the solvent completely through evaporation , one obtains a membrane which is transparent and brittle . apart from this , the partial removal of the solvent through lixiviation is necessary for obtaining a flat membrane with the required pore distribution . the membrane as prepared according to the example which has been described has a thickness of approximately 0 , 1 to 0 , 3 mm . it is conducting anions in an acid medium and conducting cations in an alkaline medium . in a concentrated alkaline medium , the chemical stability is ensured up to a temperature of 120 ° c . the gas separation is sufficient for allowing to use the membrane in an electrolytic cell . when applying the membrane for electrolysing water in 5 % by weight koh and in 30 % by weight koh , at 70 ° c . in both cases , the following results have been obtained . to the membrane were attached as a cathode a perforated nickel plate catalytically activated by means of nis x and as an anode a perforated nickel plate catalytically activated by means of nico 2 o 4 . the cell voltage u c as expressed in terms of the current density is varied as follows . ______________________________________ u . sub . c ( v ) u . sub . c ( v ) i ( kam . sup .- 2 ) 5 % by weight koh 30 % by weight koh______________________________________10 2 . 37 2 . 298 2 . 24 2 . 186 2 . 13 2 . 064 2 . 01 1 . 932 1 . 87 1 . 761 1 . 74 1 . 660 . 5 1 . 68 1 . 590 . 25 1 . 63 1 . 54______________________________________ the measurements in 5 % by weight koh were carried out with a membrane with 80 % by weight polyantimonic acid -- 20 % by weight polysulfone and the measurements in 30 % by weight koh were carried out with a membrane with 67 % by weight polyantimonic acid -- 33 % by weight polysulfone . although the resistivity of the free electrolyte increases from 0 . 8 ωcm for 30 % by weight koh at 70 ° c . to 2 . 7 ωcm for 5 % by weight koh at 70 ° c ., no noticeable increase in resistance of the membrane and cell voltage is experienced , because the quantity of polyantimonic acid in the membrane has been increased from 67 % by weight to 80 % by weight . with a membrane produced according to the invention with 67 % by weight polyantimonic acid , thus not yet optimalized as far as the polyantimonic acid concentration is concerned , current efficiencies have been measured in the chlorine electrolysis which are comparable with the ones obtained with a nafion membrane ( 7 mil , ew 1200 ), as there appears from what follows . when electrolysing a 20 % to 25 % by weight nacl solution , the higher the cationic permselectivity of the membrane , the higher is the production of naoh . the smaller the quantity of oh - ions migrating to the anode through the membrane , the higher is the current efficiency that is defined as follows : ______________________________________ ## str1 ## current efficiencynaoh (%) concentration membrane according nafion ® ( x )( normality ) to the invention 7 mil , ew 1200______________________________________0 . 2 94 960 . 5 88 951 80 921 . 5 72 . 5 872 67 782 . 5 61 68______________________________________ ( x ) w . g . f . grot et al ., &# 34 ; perfluorinated ion exchange membranes &# 34 ;, the 141 . sup . st national meeting the electrochem . society , houston , texas , may 7 - 11 ( 1972 ). the invention is not at all limited to the example described hereinabove and quite many changes can be made to the example described without going outside the scope of the present patent application . the binding agent must not necessarily be polyarylethersulfone ; it may also be another polysulfone ; generally speaking , any given organic binder apt to bind the polyantimonic acid and to be put into solution in a solvent which can be evaporated and lixiviated may be used . as organic binders , amongst others , the following are also to be taken into consideration : polyvinylidene fluoride , known on the market as solef from the solvay company , the product known on the market as hypalon from the dupont company and the product known on the market as halar from the allied chemical company . the solvent which is used to form a solution with the binding agent and to form a suspension with the polyantimonic acid must not necessarily be methylpyrolidone ; any liquid in which the binder is soluble and which can be evaporated and lixiviated may be used . the lixiviation must not necessarily take place with water , but may , for instance , be carried out in an aqueous medium , for instance in 10 - 2 or 10 - 3 n hcl . generally speaking , the lixiviation can be carried out with any liquid which is apt to be mixed with the liquid with which the solution of the binder and the suspension of the polyantimonic acid have been prepared .