Reference electrodes, aqueous solutions ionic liquids

The second problem concerns an understanding of the sharing of transport duties (e.g., the carrying of current) in pure liquid electrolytes. In aqueous solutions, it was possible to comprehend the relative movements of ions in the sense that one ionic species could drift under an electric field with greater agility and therefore transport more electricity than the other until a concentration gradient was set up and the resulting diffusion flux equalized the movements when the electrodes were reached. In fused salts, this comprehension of the transport situation is less easy to acquire. At first, it is even difficult to see how one can retain the concept of transport numbers at all when there is no reference medium (such as the water in aqueous solutions) in which ions can drift. 

A mixture of two liquid ion exchangers, respectively cation and anion, containing mobile ionogenic groups dissolved in a diluent, functions as a liquid membrane across which ions can move selectively. When a liquid membrane of this kind is interposed between two aqueous solutions, one of which contains the specimen while the other is a reference solution (see Fig. 20), and two identical reference electrodes are immersed, one in each solution, the magnitude of the electromotive force (EMF) between them can be correlated with the activities of the ionic species in the sample solution. 

By the proposed reference method for the measurement of ionized calcium in serum, plasma or whole blood, the amount of substance concentration of ionized calcium in the water phase of plasma may be reliably determined on the basis of primary reference materials. These are aqueous solutions whose compositions are established by convention to contain known amount of substance concentrations of ionized calcium and which have a constant ionic strength of 0.160 mol/kg which value is commonly used for normal plasma. The proposed IFCC reference method for ionized calcium measurement in plasma is based on the use of a cell consisting of an external reference electrode (Ri) with a concentrated potassium chloride liquid/liquid junction in combination with an ion-selective electrode with an inner reference electrode (R2) according to the scheme ... 

This paper reviews efforts to establish single ion activities for aqueous electrolytes. Nevertheless, a closely related problem, that of the energies of transfer of single ionic species from one solvent to another, has received much attention. Among the chief approaches on which these efforts are based are the following choice of a reference electrode the potential of which may be independent of the solvent, such as Rb /Rb or the ferrocinium/ferrocene couple assumption of the equality of the transfer energies of certain large ions such as tetraphenylarsonium and tetraphenylborate and efforts to nullify the liquid-junction potential between ionic solutions in different solvents. 

For the study of the solvent effect, comparable equilibrium constants have to be determined in water and in solution made with non-aqueous solvents or solvent mixtures. Potentiometric (usually pH-metric) equilibrium measurements are used for this purpose in polyfunctional systems. The solvent effect makes the application of potentiometry somewhat difficult. The substitution of water by organic solvents results in changes of the autoprotolysis constant of the solvent changing the pH scale. The lower relative permittivity of the system favours association processes which have to be considered, e.g., in the determination of the ionic strength of the solution. Diffusion potentials at the liquid junctions connecting the galvanic cell with the reference electrode may falsify the measured data. 

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