Potentiometric measuring chains

There are several possibilities to generate the reference potential in potentiometric measuring chains. Based on research results of Le Blanc [2] at first, the standard hydrogen electrode (SHE)... 

Membranes as location for the potential determining reaction in indicator electrodes as part of potentiometric measuring chains can be divided in crystalline and noncrystalline membranes. 

The stability constants of zinc complexes of 4,6-dimethyl-2-thiopyrimidine have been determined by potentiometric measurements. The crystal structure shows infinite zigzag chains of ZnL2 units with each zinc coordinated by an N3S2 donor set in a trigonal-bipyramidal geometry.853... 

Values of/x = Ac/A may be calculated from Kohlrausch s measurements of electrical conductivity of hydrochloric acid solutions. /h and fci can be evaluated from the potentiometric measurements on hydrochloric acid solutions performed by Scatchaed. These data are very reliable since the concentration chain was so arranged as to eliminate diffusion potentials. In this way, ScATCHARD determined the mean activity coefficient V/h/ci instead of the individual ion activities. If we assume that in a potassium chloride solution/ = /ci— which is plausible when we recall that both ions have the same structure—and that fci is the same in hydrochloric acid solutions and potassium chloride solutions of the same concentration, then we can calculate/h and fci in hydrochloric acid solutions. Naturally these values are not strictly correct since the effect of the potassium ions on the activity of the chloride ions probably is different from that of the hydrogen ions at the same ionic strength. In the succeeding table are given values of /x, /h, and fci calculated by the above method. 

Sprensen made this last suggestion because he had observed that potentiometric measurements realized with the help of a glass electrode (which responds to the hydroxonium ions) were not exactly matched with their [HsO ] concentration. The notion of a potentiometric chain enters in the full definition of pH. The pH definition in terms of activities, which cannot escape it, brings great complications since the activity of an ion cannot be measured (see Chap. 3). 

An example of a potentiometric measurement is one in which the pH (or number of protons bound versus those bound at some reference condition) is measured as a function of the denaturing condition. Such an approach would require a difference in the pKa of one or more amino acid side chains in the native and unfolded state. Usually, several such amino acid side chains are in a protein. However, the potentiometric approach requires technical skill, and it is difficult to use in combination with high concentrations of chemical denaturants or temperatures far from ambient. 

The methods mentioned above do not measure the micellar aggregation number itself but rather some micellar size. A direct determination of aggregation numbers can be performed by analysing some physico-chemical parameter in terms of the equilibria involved equilibrium analyses on the basis of potentiometric data have been pioneered by Danielsson and co-workers who studied short-chain, not typically micelle-forming, amphiphiles in the presence of added electrolyte (see above). 

Redox potential is measured potentiometrically with electrodes made of noble metals (Pt, Au) (Fig. 12). The mechanical construction is similar to that of pH electrodes. Accordingly, the reference electrode must meet the same requirements. The use and control of redox potential has been reviewed by Kjaergaard [218]. Considerations of redox couples, e.g. in yeast metabolism [47], are often restricted to theoretical investigations because the measurement is too unspecific and experimental evidence for cause-effect chains cannot be given. Reports on the successful application of redox sensors, e.g. [26,191], are confined to a detailed description of observed phenomena rather than their interpretation. 

The results of tj,.., measurements on the importance of the surfactant concentration 1 X 10 3 mol dm3 are in complete agreement with previous potentiometric and calorimetric findings. Below 1 X 10 3 mol dm-3, the polyelectrolyte behaves as an uncomplexed rod, while above 1 X 10 5 mol dm 3, extensive coiling of the chain takes place. Again, the distinction in binding of CP+ to PA and PSS is evident. The reduction of [T red] is greater for PA than for PSS chain. On the basis of our earlier experience we can say that this is in part due to the specific nature of the interaction between... 

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