A composition for use in a chloride ion electrode is provided herein. This composition is made up of a mixture of an epoxy resin, a vinyl chloride resin, one or more solvents, a quaternary ammonium salt and hardening agents for setting said epoxy resin; wherein said epoxy resin and the vinyl chloride resin is used at a ratio of 8:1 to 1:2 by weight. The present invention also relates to a response membrane produced from said composition and a method for producing the response membrane by setting or hardening the aforementioned composition. When the response membrane is used in the chloride ion selective electrode, it is capable of measuring a sample containing blood or protein and by use of the response membrane, it is possible to selectively determine chloride ions in a sample with a high degree of accuracy.