Abstract: The invention relates to an ion-selective electrode and to a procedure for producing the electrode. The electrode is a single-piece all-solid-state electrode comprising a conducting or semiconducting solid substrate (6) coated with a composite membrane (4). The electrode is produced by dissolving a conjugated polymer or oligomer together with the components of a conventional ion-selective membrane to produce a solution, applying this solution to an electronically conducting or semiconducting substrate and allowing the solvent to evaporate, with the result that a composite membrane is formed on the substrate.

Abstract: An electrochemical method for determining the total carbonate content of fluids, especially biological fluids. According to the invention, using an ion-selective electrode, the concentration of certain ions dissociable from carbon acid or affecting the dissociation balance is first measured for the fluid. The fluid is then mixed with another fluid containing a reactive component that changes the ionic concentration just measured, whereupon the concentration is again measured by means of an ion-selective electrode, thus producing two different measurement results, from which the total carbonate content can be determined. The measurement preferably relates to the pH of the fluid, while metallic ions such as Cu.sup.2+, forming a complex with hydrocarbonate ions, are used as the component to be added to alter the pH.