Selectivity constants

Selectivity constants are approximate, and those found experimentally may vary substantially from the listed values. ... 

Chaput, Jeminet and Juillard measured the association constants of several simple polyethylene glycols with Na", K", Cs", and Tl". Phase transfer catalytic processes and most biological processes are more likely to involve the first two cations rather than the latter two, so we will confine the discussion to these. Stability constants for the dimethyl ethers of tetra-, penta-, hexa-, and heptaethylene glycols were determined poten-tiometrically in anhydrous methanol solution and are shown in Table 7.1. In the third column of the table, the ratio of binding constants (Ks/K s) is calculated. Note that Simon and his coworkers have referred to this ratio as the selectivity constant. ... 

Concentration of Electrolyte Myer and Sievers"" applied the Donnan equilibrium to charged membranes and developed a quantitative theory of membrane selectivity. They expressed this selectivity in terms of a selectivity constant, which they defined as the concentration of fixed ions attached to the polymer network. They determined the selectivity constant of a number of membranes by the measurement of diffusion potentials. Nasini etal and Kumins"" extended the measurements to paint and varnish films. 

Here Yi and y2 are the activity coefficients of ions in solution, y, and y2 are the coefficients of resin activity, cx and c2 are ion concentrations in solution, ntj and m2 are fixed ion concentrations (exchange or weight concentrations) and Ks is the concentration constant of ion exchange, the selectivity constant. 

Since the mechanism of interaction between proteins polyfunctional with respect to ionogenic groups and CP is complex, an approximate method of calculation of sorption selectivity constants according to the inverse form of Langmuir isoterm should be used. Hence, the approximate values of AG, AH and AS obtained from Eq. (3.5) should be applied (Table 7). 

In accordance with Eq. (3.4) or Eq. (3.6), the concentration selectivity of ion exchange is variable depending on the degree of ideality of the solution and CP phase. For dilute solutions at a constant ionic strength, it is possible to take into account as a variable only the degree of non-ideality of the CP phase. For the systems considered here, it is convenient to study the effect of the molar fraction of organic counterions (NJ on the concentration selectivity constant. Fig. 14 shows the dependences of Ks on the molar fraction of oxytetracycline in CP. For CP... 

Y. Cauchois and H. Ilulubei, Tables de Constantes et Donnees Numeriques. 1, Con-stantes Selectionnees. Longuers d Onde des Emissions X et des Discontinuites d Absorption X. (Tables of Constants and Numerical Data. 1. Selected Constants. Wavelengths of X-ray Emissions and of X-ray Absorption Discontinuities.), Hermann Cie., Editeurs, Paris, 1947. 

In the above derivation we may assume that aA-(S) = aA- and aB (S) = aB, because by analogy with the build-up of an electrode potential (see pp. 26-27) the build-up of the ion-exchange potential will not significantly alter the original concentrations of A- and B in the solution under test. Hence in eqn. 2.80 the ratio aB-(n)/aA- n), which reflects the exchange competition of B versus A, still depicts the interference ratio of B in more straightforward manner than does the so-called selectivity constant (k), usually mentioned by ISE suppliers. 

When using the selectivity constant or coefficient (k) mentioned by ISE suppliers, one must be sure that if the ion under test and the interfering ion have different valence the exponent in the activity term according to Nikolski has been taken into account it has become common practice to mention the interferent concentration that results in a 10% error in the apparent ion concentration these data facilitate the proper choice of an ISE for a specific analytical problem. Often maximum levels for no interference are indicated. 

See data on interfering ions, selectivity constants and references in the original Table XV by Koryta39 and also data given by electrode suppliers32,37. 

The band-selective constant-time HMBC, or the ultra resolved 10-ppm HMBC, aiming at providing a better resolution in the 13C dimension,... 

Walters [24] examined the effect of chloride on the use of bromide and iodide solid state membrane electrodes, and he calculated selectivity constants. Multiple linear regression analysis was used to determine the concentrations of bromide, fluorine, and iodide in geothermal brines, and indicated high interferences at high salt concentrations. The standard curve method was preferred to the multiple standard addition method because of ... 

Physical Constants Recommended value of selected constants a... 

Source A. J. Bard, R. Parsons, and J. Jordan (eds.), Standard Potentials in Aqueous Solution (prepared under the auspices of the International Union of Pure and Applied Chemistry), Marcel Dekker, New York, 1985 G. Chariot etal. .Selected Constants Oxidation-Reduction Potentials of Inorganic Substances in Aqueous Solution, Butterworths, London, 1971. 

Ion to be monitored Type of elctrode Concentration range, M Optimum pH Approximate values of selectivity constants for interfering ions, kx 2 ... 

Y. Cauchois and C. Senemand, Wavelengths of X-ray Emission Lines and Absorption Edges, Volume 18 of International Tables of Selected Constants, Pergamon Press, Oxford, 1978. 

The competihve hydrogenation of alkyl-substituted arenes was also performed with lr(0) nanoparhcles [49]. Using toluene as a standard substrate, several toluene/ benzene and toluene/monoalkylbenzene hydrogenation experiments were conducted in order to determine the selectivity constants of the transition-metal nanoparticles. These selechvity constants can be used to predict the relative reactivity of any other couple of monoalkylbenzenes. A series of initial reaction... 

It is, in this case, possible to speak in a formal sense about a selectivity constant having the value... 

The selectivity constant for ion K with respect to ion depends on the exchange constant /Texch- This constant is given by the ratio of the stability constants of the associates of the given ions with the ion exchanger active site X ... 

Inspection of Eq. (13.6) shows that the selectivity behavior of a liquid membrane is specified completely by the membrane selectivity constant, Ky, which in turn is dependent on the equilibrium constant of Eq. (13.5) and on the mobility of ions i and j within the membrane. For the case in which the membrane consists of a neutral carrier [129], the exchange reaction can be presented as ... 

As can be gathered from Table 6, theory predicts for all resolvable special cases involving liquid membranes that contain the electrically neutral ligand S a selectivity constant ... 

EMF-measurements are, therefore, a method for directly assessing the relative stability of the complexes of the valinomycin group antibiotics in water and water-like solvents (cf. Fig. 11 and Ref. (87)). The selectivity constants of the liquid membranes are in this case independent of the ion-selective behavior of the membrane solvents used. 

EMF-measurements may also be used to estimate relative complex formation constants in membrane solvents. To this end, the selectivity constant Ryot of the pure membrane solvent must be measured, in addition to Kyot, the selectivity constant for the membrane plus ligand. Using Table 6 and Eq. (9) one finally obtains ... 

In spite of the simple theoretical treatment of the electrochemical alkali ion selectivity for liquid membranes incorporating electrically neutral ligands, nothing comparable has been achieved for negatively charged ligands. As shown in Table 6 for this type of membrane, two completely different expressions for the selectivity constant can be derived (27, 25, 26), depending on which assumptions are made ... 

This deficiency of the theory can be explained as follows The electrically neutral complexes IS and JS do not influence the diffusion potential in the same manner as do the charged complexes, for which the theory works so nicely. In practice, as neither of the two limiting cases defined by Eqs. (13) and (14) will be realistic (compare with the stipulations in Table 6), it seems more sensible to formulate the selectivity constant as follows ... 

This shows that the parameter derived from EMF-measurements, Kyot, used to describe the monovalent-divalent ion selectivity, depends not only on the ligand and the membrane solvent but is also a function of the activities in the outside solution (26)1). Therefore, any numerical values pertaining to a so-called selectivity constant are meaningful only as long as the exact composition of the outside solution is cited. 

Kyot selectivity constant of the membrane for zt zy. selectivity function... 

The reaction is characterized by an equilibrium constant 3, which referring to the competition between two analytes, is defined selectivity constant, and assuming that the acitivity coefficients are equal to the unit, can be written as... 

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