Potassium ion transfer

Finally, a simple measurement of a diffusion potential allows us to determine the potassium ion transference number as shown in the cell of Figure 20.7. 

Beattie, P. D., A. Delay, and H. H. Girault, Investigation of the kinetics of assisted potassium ion transfer by di-benzo-18-crown-6 at the micro-ITIES by means of steady state voltammetry, /Secfroano/ Chem,Yol. 380, (1995) p. 167. 

Pipette 25.0 mL of the potassium ion solution (about 10 mg K + ) into a 50 mL graduated flask, add 0.5 mL 1M nitric acid and mix. Introduce 20.0 mL of the sodium tetraphenylborate solution, dilute to the mark, mix, then pour the mixture into a 150mL flask provided with a ground stopper. Shake the stoppered flask for 5 minutes on a mechanical shaker to coagulate the precipitate, then filter most of the solution through a dry Whatman No. 40 filter paper into a dry beaker. Transfer 25.0 mL of the filtrate into a 250 mL conical flask and add 75 mL of water, 1.0 mL of iron(III) nitrate solution, and 1.0 mL of sodium thiocyanate solution. Titrate with the mercury(II) nitrate solution as described above. 

Three kinds of equilibrium potentials are distinguishable. A metal-ion potential exists if a metal and its ions are present in balanced phases, e.g., zinc and zinc ions at the anode of the Daniell element. A redox potential can be found if both phases exchange electrons and the electron exchange is in equilibrium for example, the normal hydrogen half-cell with an electron transfer between hydrogen and protons at the platinum electrode. In the case where a couple of different ions are present, of which only one can cross the phase boundary — a situation which may exist at a semiperme-able membrane — one obtains a so called membrane potential. Well-known examples are the sodium/potassium ion pumps in human cells. 

The Prussian blue/Prussian white redox activity with potassium as the countercation is observed in cyclic voltammograms as a set of sharp peaks with a separation of 15-30 mV. These peaks, in particular the cathodic one, are similar to the peaks of the anodic demetallization. Such a set of sharp peaks in cyclic voltammograms correspond to the regular structure of Prussian blue with homogeneous distribution of charge and ion transfer rates throughout the film. This obvious conclusion from electrochemical investigations was confirmed by means of spectroelectrochemistry [10]. 

Facilitated diffusion within organisms takes place when carriers or proteins residing within membranes—ion channels, for instance—organize the movement of ions from one location to another. This diffusion type is a kinetic, not thermodynamic, effect in which a for the transfer is lowered and the rate of diffusion is accelerated. Facilitated diffusion channels organize ion movements in both directions, and the process can be inhibited both competitively and noncompetitively. It is known that most cells maintain open channels for K+ most of the time and closed channels for other ions. Potassium-ion-dependent enzymes include NaVK+ ATPases (to be discussed in Section 5.4.1), pyruvate kinases, and dioldehydratases (not to be discussed further). 

There is reason to beheve that cardiac glycosides, like other inotropic substances, act on the contractibility of the heart by affecting the process of calcium ion transfer through the membrane of myocardiocytes. The effect of cellular membranes in electric conductivity is mediated by transport of sodium, calcium, and potassium ions, which is a result of indirect inhibitor action on the (Na+-K+) ATPase of cell membranes. 

Lehn and coworkers have profitably employed tartaric acid-containing crown ethers as enzyme models. The rate of proton transfer to an ammonium-substituted pyridinium substrate from a tetra-l,4-dihydropyridine-substituted crown ether was considerably enhanced compared to that for a simple 1,4-dihydropyridine. The reaction showed first order kinetic data and was inhibited by potassium ions. Intramolecular proton transfer from receptor to substrate was thus inferred via the hydrogen bonded receptor-substrate complex shown in Figure 16a (78CC143). 

Fig. 2.3 Standard Gibbs energies of transfer of the potassium ion from AN to other solvents and standard potentials of the hydrogen electrode, both plotted against the donor number of solvents [13].

Sodium or potassium ions can also participate in the phase-transfer process when they are converted to lipophilic cations by complexation or by strong specific solvation. A variety of neutral organic compounds are able to form reasonably stable complexes with K+ or Na + and can act as catalysts in typical phase-transfer processes. Such compounds include monocyclic polyethers, or crown ethers (1), and bicyclic aminopolyethers (cryptates) (2). They can solubilize inorganic salts in nonpolar solvents and are particularly recommended for reactions of naked anions. Applications of these compounds have been studied.12,21-31... 

This ascribes the photoregeneration reaction to an electron transfer from amide to potassium ion occurring in the excited state of the ion pair. Such a picture is consistent with the immediate appearance (r <10 sec.) of the 650 m/z cation-centered monomer band, on flashing potassium ethylamide in ethylamine. The monomer is stabilized by the irreversible removal of EtNH radicals in the reaction forming the 265 m/z band. 

We may now assemble the foregoing information into a molecular description of a few biological processes in which the interaction between water and metal ions plays an important role. First some problems related to signal transfer in nerve cells are discussed. This is followed by some comments on the mechanism operating at nerve synapses in which, in addition to the sodium and potassium ions, a specific transmitter substance and calcium ions take part. 

Addition of potassium ions to the fibers leaves the fibrous structure intact but destroys the helicity [128]. Sandwich complexes between the cation and 114d are observed. With higher concentrations of potassium, the sandwich complexes break down and isolated 4K+114d are observed (Scheme 61). In both complexed forms the salt blocks the chirality transfer from the side chains to the supramolecular assemblies. Such fibers with controllable chirality can be interesting materials for molecular switches or in sensors devices. 

A solvent softness scale, dependent on the thermodynamics of transfer of ions from water to the target solvent, has been proposed (Marcus 1987). Since soft ions prefer soft solvents and hard ions hard solvents, and since silver ions are soft, whereas sodium and potassium ions are hard, the difference ... 

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