Medium activity coefficient

See also (b) O. Popovich, Transfer activity coefficients (medium effects) in Treatise on Analytical Chemistry, Part 1, 2nd ed., (Eds. ... 

The "ionic medium activity scale, on the other hand, is so defined that the activity coefficient, yA = A / [A] approaches unity as the solution approaches the pure solvent (in this case the ionic medium)—Le., when the concentrations go toward zero for all other species than water and the medium ions. 

Besides the standard free energy of transfer AG , also the medium activity coefficient or transfer activity coefficient yf is used quite often to relate the ion activities aj and aj of an ion referred to the standard states in the two solvents ( and ") ... 

The relationship between medium activity coefficient and the free energy of transfer in the standard state is ... 

This free energy change (AUe) represents the electrostatic contribution to the chemical potential of the ion, that is, the electrical woik necessary to charge the ideal solution, and it is responsible for deviations of the solution from ideal behavior. The activity coefficients of single ions are not measurable experimentally [35] for an electrolyte EpHq, the medium activity coefficient is... 

G = 1,3-propanedial an20 = activity of water aSo — activity of organic solvent component y(Cr(OH2)63+) = medium activity coefficient for Cr(OH2)63+ y(Cr(OH2)6-nSon3+) = medium activity coefficient for Cr(OH2)6 nSo3+ Kn = equilibrium constant ... 

Overall rate coefficients and activation parameters for the Claisen rearrangement of predominantly ort/io-substituted allyl aryl ethers are given in Table 26. There are some changes in the solvent medium for these rearrangements, but as seen later this should not greatly affect the comparisons. 

Activity coefficients defined within the infinite dilution activity scale cannot be formulated theoretically for the ionic medium of seawater. Since the oceans contain an ionic medium of practically constant composition, the ionic medium activity scale might be used advantageously in studying acid-base and other equilibria in seawater (see also Appendix 6.2 in Chapter 6). 

Transport of the gaseous species from the donor to the acceptor stream depends on several parameters, e.g., temperature, ionic strength, surface tension, contact time between the solutions and the semi-permeable medium, the characteristics of this medium (active surface, porosity, thickness), the partition coefficient between the fluid and the membrane,... 

Measurements of rate coefficients and activation parameters were carried in some cases and the results are given in Table 16. Other kinetic studies were made at one temperature, for purpose of evaluation of medium and/or structural effects some of the relevant rate coefficients are gathered in Table 17. For the reaction between FiC CCU and butadiene derivatives, partial rate coefficients attributable to every double bond were also calculated, taking into account the product analyses , and are given in parentheses. All reactions were found to be first-order with respect to both reactants. 

The brackets represent the thermodynamic activity. At low concentrations in water, the concentration value can be used for activity at higher concentrations or with other ions in solution, the concentration value must be multiplied by the activity coefficient. The activity of the ions on the ion-exchange medium is not readily available the mole fraction, defined as the moles of an individual component divided by the total moles of all components in the phase, has been used in its place (Rieman and Walton 1970). 

Medium activity coefficients, cf. Section 5.5, can be used to discuss these effects. Fig. 15 provides a summary of the changes in solvation energy of ions and neutral molecules of various types in solvents which are representative for the solvent classes of Table I. The energy scale, RTln j y, with methanol as the reference solvent is taken from Ref. The non-measurable medium activity coefficient of the activated complex can be estimated from similar stable mol niles or ions. 

These equilibrium reactions are needed to generate the H, O, N, and OH radicals at high temperature, before the slower kinetic reactions can be initialized. The coefficients and activation temperatures used in (13.4) and (13.9) for a medium-speed DI diesel engine are summarized in a study by Tanner and Srinivasan [49]. 

Medium activity coefficient Measure for the change in Gibbs energy when an electrolyte is transferred from one solvent to another solvent at infinite dilution. 

Separation into ionic medium activity coefficients is executed with the help of the equation... 

Grunwald and B. J. Berkowitz, J. Am. Chem. Soc., 73, 4939 (1951). The term medium activity coefficient has also been suggested, and is perhaps more descriptive. 

The relevant equation for the dissection of solvent effects in terms of the Gibbs energy may be deduced from the Pronsted-Bjerrum equation for the reaction of Q and R through a transition state Tr(3-5,32,33). Here, k is the rate constant in a standard solvent and k is the rate constant in any other solvent. The terms and are primary medium activity coefficients. Equation can be recast as equation and the latter simplified by defining the... 

As the medium is still further diluted, until nitronium ion is not detectable, the second-order rate coefficient decreases by a factor of about 10 for each decrease of 10% in the concentration of the sulphuric acid (figs. 2.1, 2.3, 2.4). The active electrophile under these conditions is not molecular nitric acid because the variation in the rate is not similar to the correspondii chaise in the concentration of this species, determined by ultraviolet spectroscopy or measurements of the vapour pressure. " ... 

Phenol Coefficient Test. The first important attempt at standardizing testing methods was known as the phenol coefficient test (96). It has been modified several times, and is an official AO AC screening test recognized by EPA and PDA. The phenol coefficient test compares the activity of disinfectants to that of phenol, under specific conditions, to give a number that measures the activity of the chemical tested with respect to that of phenol, ie, the phenol coefficient. The AO AC method employs visual examination of bacterial growth in a nutrient medium. The Kelsey-Sykes test (1969) is a modified method popular in British circles. 

The diffusion current Id depends upon several factors, such as temperature, the viscosity of the medium, the composition of the base electrolyte, the molecular or ionic state of the electro-active species, the dimensions of the capillary, and the pressure on the dropping mercury. The temperature coefficient is about 1.5-2 per cent °C 1 precise measurements of the diffusion current require temperature control to about 0.2 °C, which is generally achieved by immersing the cell in a water thermostat (preferably at 25 °C). A metal ion complex usually yields a different diffusion current from the simple (hydrated) metal ion. The drop time t depends largely upon the pressure on the dropping mercury and to a smaller extent upon the interfacial tension at the mercury-solution interface the latter is dependent upon the potential of the electrode. Fortunately t appears only as the sixth root in the Ilkovib equation, so that variation in this quantity will have a relatively small effect upon the diffusion current. The product m2/3 t1/6 is important because it permits results with different capillaries under otherwise identical conditions to be compared the ratio of the diffusion currents is simply the ratio of the m2/3 r1/6 values. 

If the activity coefficient term on the right of equation (13) stays constant with changing medium, then a linear correlation of log kx with -H0 should follow for the A-l mechanism. The above diagnosis of the A-l mechanism is known as the Zucker-Hammett hypothesis3. 

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