Patent GB-1308630-A (GB)
Title: Alkali metal specific measuring electrode
IPC: G01N27/333
Filed: 1970-06-03
Abstract: 1308630 Ion-selective membranes HYDRONAUTICS-ISRAEL Ltd 3 June 1970 [4 June 1969 25 May 1970] 26834/70 Heading B5B [Also in Division G1] A solid membrane suitable for use in a specific ion-measuring electrode consists of, (a) a porous support having a negative charge corresponding to an anion concentration in the range of 10<SP>-6</SP> to 10<SP>-3</SP> milliequivalents per gram. of solvent either as an integral part of the membrane or as a result of reaction between it and (c) below; (b) an electrically uncharged ion-carrier capable of selectively binding the ion that it is desired to measure and, (c) a solvent for the ion-carrier which is preferably hydrophobic and capable of plasticizing the material of the support. The support may be any suitable plastic material but is preferably cellulose acetate or cellulose acetate-nitrate. When the charge is derived from the support an electrically neutral solvent is used, e.g. bromobenzene, but when the charge derived from the solvent, as for instance in the core of some esters which contain traces of acid, the supporting material should be electrochemically neutral. The support may be given its charge by means of a chemical reaction; thus, for instance cellulose acetate may be oxidized by means of potassium permanganate to yield carboxyl groups, or reaction may be effected with dichloro-azinyl dye to yield sulphonic acid groups. The quantity of charge on the membrane is best regulated by mixing charged and neutral polymer in the correct proportions. The selectivity of the electrode is fed to the ion-carrier and, consequently, different ion-carriers may be used to provide membranes having differing selectivities. Examples of ion carriers are (i) valinomycin, which selects K<SP>+</SP> over Na<SP>+</SP> to the extent of 10<SP>4</SP> times and which selects (NH 4 )<SP>+</SP> to 10<SP>2</SP> times the extent of Na<SP>+</SP>, (ii) various cyclic polyethers, which can confer selectivity for Li<SP>+</SP>, Ru<SP>+</SP>, Cs<SP>+</SP> and Na<SP>+</SP> to the membrane, (iii) other substances such as other members of the valinomycin group; tetralactones; macrolide actins such as monactin, nonactin, dinactin and trinactin, enniatin A and B; cyclohexadepsipeptides, gramicidin, nigericin, nystatin, monensin and alamethicin, all of which have a selectivity for K<SP>+</SP> similar to that of valinomycin. Suitable solvents for the ion-carrier are alkylphosphates, alkylphthalates and alkylsebacates. The membranes may be made in either of two fashions: (i) by making a sheet of the appropriate membrane material having the required porocity and charge and imbibing it with a solution of the ion-carrier in the plasticizing solvent, (ii) by dissolving the components, polymer, plasticizing solvent and ion-carrier, in a common solvent such as acetone, casting the solution into a film and allowing the common solvent to evaporate. Membranes used in the examples can confer on electrodes the following selectivities: (a) ability to measure K<SP>+</SP> to an accuracy of 0À1% in a 10<SP>-3</SP> M solution in the presence of a 9000- fold excess of Na<SP>+</SP> or Ca<SP>++</SP> ions or a 60-fold excess of NH 4 <SP>+</SP> ions, the operation being unaffected by pH within the range of 1À1 to 9À6; (b) ability to measure Na<SP>+</SP> between 10<SP>-5</SP> and 1M in the presence of a 100-fold excess of K<SP>+</SP>; (c) ability to measure NH4<SP>+</SP> within a range of concentrations between 10<SP>-5</SP> to 1M in the presence of a 10-fold excess of K<SP>+</SP>.