Abstract: The concentration ratio of lithium ions to sodium ions can be determined very accurately and excellently reproducible by using an electrode for the determination of lithium ions and an electrode for the determination of sodium ions and by contacting the ion sensitive members of said electrodes with the sample solution. The two electrodes are electrically connected via an instrument for the determination of the voltage and accordingly the potential difference .DELTA. V between said two electrodes is determined. From said potential difference the concentration ratio of lithium ions to sodium ions in the sample solution can be calculated. It is furthermore possible to connect electrically the lithium electrode as well as the sodium electrode via an instrument for the determination of the voltage with a single external reference-electrode or a common-electrode and then from the two measured potential differences the potential difference .DELTA.

Abstract: Keto compounds in which to the carbon atom in the neighboring position to the keto compounds there is bonded at least one strongly electron attracting substituent and/or in which there is conjugating with the carbon-oxygen-double bond of the keto group a .pi.-electron system have the ability of interacting selectively with the anions of oxa acids yielding adducts, chelates or complexes.Provided that in the corresponding keto compounds the keto group is a part of a chromophore, then the shifting of the absorption in the visible range or the ultraviolet range of the wave lengths is observed, compared with the corresponding absorption of the free keto compound.The corresponding keto compounds are used for the determination of the anions of the oxa acids and they e.g. can be the ion sensitive component of a corresponding ions sensitive part for the determination of said anions. Several of said keto compounds can be also used as color indicator for the anions of the oxa acids.

Abstract: Shaped bodies of plastic material have a high elasticity and high adhesive characteristics if they contain, referred to the percentage of the plastic material of said shaped bodies, a higher percentage of a benzophenonetetracarboxylic acid tetraester. In said benzophenone-tetracartoxylate the ester forming alcohol component is an alkanol, alkenol or alkynol having 4-24 carbon atoms. The shaped bodies can be adhered to a substrate by pressing them onto it. The inventive shaped bodies are preferably strands, fibers, filaments or foils, for example foils for the packaging of products. Preferred shaped bodies are ion sensitive membranes for the determination of the concentration of ions and said membranes contain furthermore an ion selective component for the ion to be determined.Solutions of the inventive shaped bodies in a volatile organic solvent can be applied to a substrated and after the solvent is evaporated there remains on the substrate a firmly adherent elastic film.