Ion activity concentrations

The transmembrane potential derived from a concentration gradient is calculable by means of the Nemst equation. If K+ were the only permeable ion then the membrane potential would be given by Eq. 1. With an ion activity (concentration) gradient for K+ of 10 1 from one side to the other of the membrane at 20 °C, the membrane potential that develops on addition of Valinomycin approaches a limiting value of 58 mV87). This is what is calculated from Eq. 1 and indicates that cation over anion selectivity is essentially total. As the conformation of Valinomycin in nonpolar solvents in the absence of cation is similar to that of the cation complex 105), it is quite understandable that anions have no location for interaction. One could with the Valinomycin structure construct a conformation in which a polar core were formed with six peptide N—H moieties directed inward in place of the C—O moieties but... 

In ion-selective electrode potentiometry, the cell potential reflects the dependence of the membrane potential on the primary ion activity (concentration). According to the Teorell-Meyer-Sievers (TMS) theory, the sum of... 

Lux acid-base reactions require data on the dependence of e.m.f. of the cell used on the equilibrium oxide-ion activity (concentration) in the melt. The calibration of the potentiometric cell is performed as described below. 

The ionic sites [tetrakis (p-chlorophenyl) borate. TpClPB ] work to reject counterions from being coextracted into the organic phase and are, therefore, requisite for the generation of analyte-ion activity (concentration)-dependent membrane potential changes. This molecular mechanism for the potential response process of ISEs was recently revealed experimentally by simultaneous measurements of laser-induced second-harmonic generation (SHG) and membrane potentials, in which it was found that the observed membrane potentials originate from the SHG-active surface-oriented charged chemical species of the order of a few molecular layers (Fig. 

In general, the maximum rate of corrosion does not exceed approximately 20 p,m per year if the dissolved ion concentration at the metal surface remains equal or inferior to 10 mol F. A rate of corrosion of this magnitude can often be tolerated. Thus, we define the protection potential (Fprot) a the reversible potential predicted by the Nernst equation for a dissolved metal-ion activity (concentration) of 10- mol r. ... 

According to Nemst equation the potential of calomel electrode is dependent on chloride ions activity (concentration). In practice, three cases are encountered normal calomel electrode (with KCl solution of concentration 1 mol dm ), 0.1 normal calomel electrode (with KCl solution concentration 0.1 mol dm ), and most often used saturated calomel electrode (with saturated KCl solution and the presence of KCl crystals), abbreviated as the saturated calomel electrode , SCE. A version with saturated NaCl solution ( sodium chloride saturated calomel electrode , SSCE) is also widely used in solution, where risk of a potassium salt precipitation occurs, e.g., in perchlorate ions containing solutions. 

In ISE potentiometry, the cell potential reflects the dependence of the membrane potential on the primary ion activity (concentration). According to the Teorell-Meyer-Sievers (TMS) theory, the membrane potential is the sum of three potential contributions namely the phase boundary potentials generated by ion-exchange processes at both interfaces, ((()i - c )n, i) and (([)n, 2 4>2). and the inter membrane diffusion potential, (c )ni i - <[)ni,2). If the membrane composition is constant and there are no concentration gradients within the membrane, then the membrane diffusion potential is zero and the membrane potential can be described by phase boundary potentials (see Figure 10.3b). This approach is also used to treat the response of ISE made with a range of membranes. 

This electrode, shown diagrammatically in Figure 4.4, is assigned zero potential when hydrogen gas at one atmosphere bubbles over platinised platinum in a solution of hydrogen ions of concentration 1 mol 1 (strictly, at unit activity). 

Equation 11.16 can be solved for the metal ion s concentration if its activity coefficient is known. This presents a serious complication since the activity coefficient may be difficult to determine. If, however, the standards and samples have an identical matrix, then yM + remains constant, and equation 11.16 simplifies to... 

A particular concentration measure of acidity of aqueous solutions is pH which usually is regarded as the common logarithm of the reciprocal of the hydrogen-ion concentration (see Hydrogen-ION activity). More precisely, the potential difference of the hydrogen electrode in normal acid and in normal alkah solution (—0.828 V at 25°C) is divided into 14 equal parts or pH units each pH unit is 0.0591 V. Operationally, pH is defined by pH = pH(soln) + E/K, where E is the emf of the cell ... 

The reactor coolant pH is controlled using lithium-7 hydroxide [72255-97-17, LiOH. Reactor coolant pH at 300°C, as a function of boric acid and lithium hydroxide concentrations, is shown in Figure 3 (4). A pure boric acid solution is only slightly more acidic than pure water, 5.6 at 300°C, because of the relatively low ionisation of boric acid at operating primary temperatures (see Boron COMPOUNDS). Thus the presence of lithium hydroxide, which has a much higher ionisation, increases the pH ca 1—2 units above that of pure water at operating temperatures. This leads to a reduction in corrosion rates of system materials (see Hydrogen-ION activity). 

The increased acidity of the larger polymers most likely leads to this reduction in metal ion activity through easier development of active bonding sites in siUcate polymers. Thus, it could be expected that interaction constants between metal ions and polymer sdanol sites vary as a function of time and the sihcate polymer size. The interaction of cations with a siUcate anion leads to a reduction in pH. This produces larger siUcate anions, which in turn increases the complexation of metal ions. Therefore, the metal ion distribution in an amorphous metal sihcate particle is expected to be nonhomogeneous. It is not known whether this occurs, but it is clear that metal ions and siUcates react in a complex process that is comparable to metal ion hydrolysis. The products of the reactions of soluble siUcates with metal salts in concentrated solutions at ambient temperature are considered to be complex mixtures of metal ions and/or metal hydroxides, coagulated coUoidal size siUca species, and siUca gels. 

Using excess voltage, ions can be pumped from the low concentration (activity) side of the electrolyte to the high activity side, during which the storage battery is charged. In another appHcation, the ion activity on one side can be fixed at a known value and the activity on the other side determined for various unknown conditions. 

Experimentally deterrnined equiUbrium constants are usually calculated from concentrations rather than from the activities of the species involved. Thermodynamic constants, based on ion activities, require activity coefficients. Because of the inadequacy of present theory for either calculating or determining activity coefficients for the compHcated ionic stmctures involved, the relatively few known thermodynamic constants have usually been obtained by extrapolation of results to infinite dilution. The constants based on concentration have usually been deterrnined in dilute solution in the presence of excess inert ions to maintain constant ionic strength. Thus concentration constants are accurate only under conditions reasonably close to those used for their deterrnination. Beyond these conditions, concentration constants may be useful in estimating probable effects and relative behaviors, and chelation process designers need to make allowances for these differences in conditions. 

Not unexpectedly, this procedure reveals some dependence on the particular type of base used, so no universal Hq scale can be established. Nevertheless, this technique provides a very useful measure of the relative hydrogen-ion activity of concentrated acid solutions which can be used in the study of reactions that proceed only at high acid concentration. Table 4.8 gives Hq values for some water-sulfuric acid mixtures. 

The extent to which B3O3 rings catenate into more complex structures or hydrolyse into smaller units such as [B(OH)4] clearly depends sensitively on the activity (concentration) of water in the system, on the stoichiometric ratio of metal ions to boron and on the temperature (7-A5). 

Where R is the gas constant, T the temperature (K), Fthe Faraday constant and H2 is the relative partial pressure (strictly, the fugacity) of hydrogen in solution, which for continued evolution becomes the total external pressure against which hydrogen bubbles must prevail to escape (usually 1 atm). The activity of water a jo is not usually taken into account in elementary treatments, since it is assumed that <7h2 0 = U nd for dilute solutions this causes little error. In some concentrated plating baths Oh2 0 I O nd neither is it in baths which use mixtures of water and miscible organic liquids (e.g. dimethyl formamide). However, by far the most important term is the hydrogen ion activity this may be separated so that equation 12.1 becomes... 

The use of a pH meter or an ion activity meter to measure the concentration of hydrogen ions or of some other ionic species in a solution is clearly an example of direct potentiometry. In view of the discussion in the preceding sections the procedure involved will be evident, and two examples will suffice to illustrate the experimental method. 

At low ionic strengths, Tm increases exponentially with ion activity. The effect of high concentrations of salts or miscible solvents depends on the influence they have on hydrogen-bonding and may increase or decrease Tm. In the case of xanthan gum, the value of Tm can be adjusted from ambient to over 200°C by the addition of appropriate salts. Table 7.2 presents Tm values for some industrial viscosifiers. 

A second messenger is an intracellular metabolite or ion whose concentration is altered when a receptor is activated by an agonist, considered to be the first messenger. ... 

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... 

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