Activity of an ion

One can write acid-base equilibrium constants for the species in the inner compact layer and ion pair association constants for the outer compact layer. In these constants, the concentration or activity of an ion is related to that in the bulk by a term e p(-erp/kT), where yp is the potential appropriate to the layer [25]. The charge density in both layers is given by the algebraic sum of the ions present per unit area, which is related to the number of ions removed from solution by, for example, a pH titration. If the capacity of the layers can be estimated, one has a relationship between the charge density and potential and thence to the experimentally measurable zeta potential [26]. 

The activity of an ion is related to its molality through the mean activity coefficient 7+ therefore... 

Regenerant The solution used to restore the activity of an ion exchanger. Acids are employed to restore a cation exchanger to its hydrogen form brine solutions may be used to convert the cation exchanger to the sodium form. The anion exchanger may be rejuvenated by treatment with an alkaline solution. 

Regeneration Restoration of the activity of an ion exchanger by replacing the ions adsorbed from the treated solution by ions that were adsorbed initially on the resin. 

It should be noted again that ISEs sense the activity, rather than the concentration of ions in solution. The term activity is used to denote the effective (active) concentration of the ion. The difference between concentration and activity arises because of ionic interactions (with oppositely charged ions) that reduce the effective concentration of the ion. The activity of an ion i in solution is related to its concentration, c by... 

The activity of an ion reflects the proportion of the molecules that are actually ionized at the time. 

Because potentiometric measurements reflect the activity of an ion rather than its concentration, it is usually necessary to calibrate the system with standard solutions of known activity. It is possible to calculate the concentration of an... 

We will now look at the effects of Ej on thermodynamic calculations, and then decide on the various methods that can be used to minimize them. One of the most common reasons for performing a calculation with an electrochemical cell is to determine the concentration or activity of an ion. In order to carry out such a calculation, we would first construct a cell, and then, knowing the potential of the reference electrode, we would determine the half-cell potential, i.e. the electrode potential E of interest, and then apply the Nemst equation. 

At higher ionic strength the limiting Debye-HOckel treatment no longer adequately describes the effect of the ionic atmosphere on the activity of an ion, and this deficiency has been the subject of much theoretical investigation. 

The equivalent circuit corresponding to this interface is shown in Fig. 6.1b. The charge-transfer resistances for the exchange of sodium and chloride ions are very low, but the charge-transfer resistance for the polyanion is infinitely high. There is no direct sensing application for this type of interface. However, it is relevant for the entire electrochemical cell and to many practical potentiometric measurements. Thus if we want to measure the activity of an ion with the ion-selective electrode it must be placed in the same compartment as the reference electrode. Otherwise, the Donnan potential across the membrane will appear in the cell voltage and will distort the overall result. 

Ion channels play an essential role in medical diagnostics and drug development. Such applications require the integration of ion channels together with a lipid bilayer into an artificial microstructured polymer membrane (Fig. 4). The polymer membrane is attached to a metal coated optical prism. The membrane contains micropores of approximately one micrometer in diameter. Lipid bilayers are stretched across the pores. The bilayers host the receptor molecules. After activation of an ion channel, thousands of ions stream into the cavity below the ion channel. The change of ion concentration can easily be detected by SPR measurements. 

The Nernst equation is applicable also to interfacial ionic equilibria where the activity of an ion is equal to a (1) at one side of the interface and a (2) at the other side, as follows ... 

Activities of Electrolytes.—When the solute is an electrolyte, the standard states for the ions are chosen, in the manner previously indicated, as a hypothetical ideal solution of unit activity in this solution the thermodynamic properties of the solute, e.g., the partial molal heat content, heat capacity, volume, etc., will be those of a real solution at infinite dilution, i.e., when it behaves ideally. With this definition of the standard state the activity of an ion becomes equal to its concentration at infinite dilution. 

The activity coefficients given by the Debye-Hiickel treatment presumably represent deviations from the dilute solution behavior, i.e., from Henry s law, and are consequently based on the standard state which makes the activity of an ion equal to its mole fraction at infinite dilution ( 37b, III B). In the experimental determination of activity coefficients, however, it is almost invariably the practice to take the activity as equal to the molarity or the molality at infinite dilution. The requisite corrections can be made by means of equation (39.13), but this is unnecessary, for in solutions that are sufficiently dilute for the Debye-Hackel limiting law to be applicable, the difference between the various activity coefficients is negligible. The equations derived above may thus be regarded as being independent of the standard state chosen for the ions, provided only that the activity coefficients are defined as being unity at infinite dilution. 

For the practical application of ISEs for measurement of ion activities in real samples there exist several concerns. We have already addressed the importance of liquid junction potentials and the need to either minimize these values or keep them constant between the calibrating solutions and unknowns. Also, in using ISEs one must fully realize the difference in measuring ion activities versus ion concentrations. The activity of an ion in solution is given by the expression... 

An emf measurement only gives the ratio of the activities of an ion at two total salt concentrations In order to obtain individual values for and a/2, and therefore for a itself, at any dilution (i/c), Lewis makes the reasonable assumption that at very great dilution, beyond i/iooo N, the value of a is sensibly identical with the value of y both of them approximating to umty in the limit It is clear that we are regarding both a and y as functions of the dilution... 

The balance between the binding of the activating cations to the two sides of the enzyme in the membrane and the activity of the enzyme is probably the basis for the activity of an ion pump that reflects the ion concentrations in the intra- and extracellular fluids. The linkage between the enzyme activity and the ion translocation is not known, but our results, together with those on lipid activation, suggest that mechanical coupling between the two surfaces may be critical to the proper functioning of the enzyme. 

The minimum uncertainty for simple matrices is achieved with use of electrochemical sensors, especially for the assay of organic cations and organic and inorganic anions in food and clinical analysis. The selectivity and sensitivity of these sensors are adequate to detect numerous pharmaceutical products, without any prior separation. The ability of electrochemical sensors to determine continuously the activity of an ion in solution has made their use possible in in vitro and in vivo dissolution tests of drugs. 

The enhanced chemical activity of an ion results in a chain of ion-molecule reactions with the colliding neutrals, and, in the first microsecond of the life of an air ion, a charged molecular cluster called the cluster ion is formed. According to theoretical calcula-... 

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