Activity of solutions

The Nemst equation above for the dependence of the equilibrium potential of redox electrodes on the activity of solution species is also valid for uncharged species in the gas phase that take part in electron exchange reactions at the electrode-electrolyte interface. For the specific equilibrium process involved in the reduction of chlorine ... 

The chief limitation of Pourbaix diagrams is that they show only the dominant compound in any particular area. They do not show the presence of other compounds, which may be of comparable concentration nor the fact that the activities of solution species are continuous functions. 

Calculation of Activity of Solute from That of Solvent... 

As figure 8.20 shows, the boundary between condensed phases and solute species is dictated by the activity of solutes, T being equal. 

The activities of solute species in an aqueous solution in equilibrium with K-feldspar at the P and T of interest will be those dictated by equation 8.232. Let us now imagine altering the chemistry of the aqueous solution in such a way that the activities of the aqueous species of interest differ from equilibrium activities. New activity product Q ... 

As already stated, one of the important pieces of data for biotransformation processes is knowledge of phase equilibrium and the activity of solutes involved. Hence, assuming that gas and liquid phases are at thermodynamic equilibrium, we can write... 

The most important factor in zeolite synthesis in the laboratory, or factory, is the rate of crystallization. Composition and concentration of the liquid solution acting on the solids is important to the process as is the absolute necessity of maximum disorder of the Si-O-Al bonds in the initial solids reacted (Zhdanov, 1970). It is thus evident that not only bulk chemical (equilibrium) factors are important in the initial crystallization of zeolites but also the. relative free energies of the reactants. It is apparent that zeolite equilibria are essentially aqueous i.e., that silicate equilibrium or approach to it is attained through reaction with solutions, and thus the solubilities of the solids present are of primary importance. If materials are slow to enter into solution they are essentially bypassed in the rapid crystallization sequence (Schwochow and Heinze, 1970 Aiello, et al , 1970). In most studies the zeolites precipitated from solution appear to respond to the laws concerning chemical activity of solutions (Zhdanov, 1970). 

Just as in aqueous solutions, the activity of solute i (acl) in non-aqueous solutions is related to its (molar) concentration (sj by aCii = yCiiCi, where g is the activity coefficient that is defined unity at infinite dilution. For non-ionic solutes, the activity coefficient remains near unity up to relatively high concentrations ( 1 M). However, for ionic species, it deviates from unity except in very dilute solutions. The deviation can be estimated from the Debye-Hiickel equation, -log yci = Az2 /1/2/ (1+aoBf1 2). Here, I is the ionic strength and / (moll-1), a0 is the ion size parameter... 

The Pfeiffer effect is a term used to describe changes in the optical activity of solutions containing a chiral compound (the environmental substance ) on the addition of a racemic dissymmetric complex. The effect is generally attributed to a shift in the position of the equilibrium between d and l isomers for the racemic complex. The exact mechanism involved in mediating the chiral interaction is unknown. Perhaps surprisingly, both environmental substance and complex may simultaneously be cations. Studies of the Pfeiffer effect usually involve a moderately labile racemic complex [Cr(ox)3]3 is a popular choice for such studies, summarized in Table 82. Other studies of the optical activity of tris oxalates include work on photoinduced optical activity,898 photoracemization899 and the solid-state racemization of K3[Cr(ox)3]. 900 901... 

Let us consider first lipid-lipid interaction. Urry et al, showed the existence of a positive CD band at 218 m/x and a negative CD band at about 192 m/z in phosphatidyl choline and phosphatidyl ethanolamine dissolved in trifluoroethanol (86). The 192-m/z band was not characterized in detail, but the 218-m/z band is of such position and shape that the addition of lipid and protein CD bands could produce a composite CD band, and hence an ORD Cotton effect, which is red shifted. As noted by Urry, the 218-m/z CD extremum of lecithin must arise from n — 7T transitions in the fatty acid ester groups. Although the optical activities of solutions of deproteinized membrane phospholipids determined at the same concentration as in the intact membrane are negligibly small, in membranes an ordered array of lipids could greatly enhance rotation. Such an effect could yield information on the nature of lipid-lipid association. This can be tested experimentally. Halobacterium cutirubrum offers a unique system since Kates has shown that the lipids in this extreme halophile contain ether bonds rather than ester bonds (43, 44), Hence, the n — tt transition essential to the CD band at 218 m/z in phospholipids does not exist. Nevertheless, we found that the ORD... 

Thus, firstly, the choice of the pure solvent as the reference state for the definition of activities of solutes in fact impairs a fair comparison of the activity of dilute solutes such as general adds to the activity of the solvent itself. Secondly, the observed first-order rate constants k or k0 for the reaction of a solute with the solvent water are usually converted to second-order rate constants by division through the concentration of water, h2o = oA iho, for a comparison with the second-order rate coefficients HA. Again, it is questionable whether the formal h2o coefficients so calculated may be compared with truly bimolecular rate constants kUA for the reactions with dilute general acids HA. It is then no surprise that the values for the rate coefficients determined for the catalytic activity of solvent-derived acids scatter rather widely, often by one or two orders of magnitude, from the regression lines of general adds.74... 

The inclusion of activity coefficients into the simple equations was briefly considered by Purlee (1959) but his discussion fails to draw attention to the distinction between the transfer effect and the activity coefficient (y) which expresses the non-ideal concentration-dependence of the activity of solute species (defined relative to a standard state having the properties of the infinitely dilute solution in a given solvent). This solvent isotope effect on activity coefficients y is a much less important problem than the transfer effect, at least for fairly dilute solutions. For example, we have already mentioned (Section IA) that the nearequality of the dielectric constants of H20 and D20 ensures that mean activity coefficients y of electrolytes are almost the same in the two solvents over the concentration range in which the Debye-Hiickel limiting law applies. For 0-05 m solutions of HC1 the difference is within 0-1% and thus entirely negligible in the present context. Of course, more sizeable differences appear if concentrations are based on the molality scale (Gary et ah, 1964a) (see Section IA). 

Rather than using a diagram such as that in Fig. 5, to describe an electrochemical cell, a standard simplified diagram is used. Vertical lines separate the various phases in the cell. For the separation between two liquid phases (by a porous barrier), a dotted or dashed vertical line is used. The terminals of the cells are placed on the ends of the diagram, with the anode on the left. Any metals attached to the terminals are written next to them. Gas or insoluble materials in contact with the metals are written next, and the electrolytic solution of the cell is described in the center of the diagram. To completely define the cell, the concentrations or activities of solutions and the pressures of gases are included. The simplified diagram for the cell illustrated in Fig. 5 is therefore... 

Since the first report in 1965, on the catalytic activity of solutions of [(C6H5)3P] 3RhCl, extensive mechanistic studies have been carried out on this system (1,8,9). However, the mechanistic picture is still somewhat clouded. The various pathways which have been suggested to be operating are shown in Fig. 2. Of the four catalytic routes illustrated, two (A- B- O G + H- I- products and A-+ E-> F > G-> H > I-> products) consider Wilkinson s catalyst... 

In dilute solutions, the concentration of water is very close to that of pure water, and the activity of pure water, by convention, is taken to be 1.0. Furthermore, in dilute solutions, the activity of solutes may be approximated by their concentrations so we may write an expression for a practical acid dissociation constant ... 

We have made a number of assumptions in this calculation, the most notable being that the ionic solutions are ideal, in that there are no interactions (attractive or repulsive) between solute molecules. It is most unlikely that this is the case, especially in moderately concentrated solutions of ions. In order to correct for nonideality (interactions between solute molecules), we need to substitute the activities of solute molecules for their concentrations in all thermodynamic calculations. The activity (a) of a solute molecule is related to its concentration (C) by an activity coefficient (y). 

Although other methods using such experimental data have been documented,theoretical and/or empirical considerations have led to various expressions involving a dependence on ionic strength, I, which is a major factor influencing the activity of solutes in aqueous solution. 

In certain cases we can adequately describe the chemical properties of species / by using the concentration of that solute, Cj. Owing to molecular interactions, however, this usually requires that the total solute concentration be low. Molecules of solute species j interact with each other as well as with other solutes in the solution, and this influences the behavior of species /. Such intermolecular interactions increase as the solution becomes more concentrated. The use of concentrations for describing the thermodynamic properties of some solute thus indicates an approximation, except in the limiting case of infinite dilution for which interactions between solute molecules are negligible. Where precision is required, activities—which may be regarded as corrected concentrations—are used. Consequently, for general thermodynamic considerations, as in Equation 2.4, the influence of the amount of a particular species / on its chemical potential is handled not by its concentration but by its activity, aj. The activity of solute j is related to its concentration by means of an activity coefficient, y ... 

To compare activities of solutes in different solvents, a single reference state for the solute must be chosen. Although from some points of view it is awkward, water is a logical choice for a single reference solvent in which the behavior of solutes in other solvents can be compared. To make comparisons of solute activities among solvents, it is convenient to consider separately the effect of dilution within a given solvent and the difference in the usual reference states of a solute at infinite dilution in different solvents. The activity coefficient yt of a species i in a solvent may be considered the product of two terms... 

Many models have been developed to estimate the thermodynamic activities of solutes in natural waters (e.g., see Millero, 1984). Of these, the ion pairing and specific interaction theories are the most widely used. A combined model that uses the Pitzer equations to represent specific interactions between ions, together with a thermodynamic description of chemical equilibria, has proved successful in estimating the activities of both major and minor components of seawater (Dickson et al., 1988 Harvie et al., 1984). 

Isomerisation is the process of conversion of a dmg into its optical or geometric isomers. Since the various isomers of a dmg are frequently of different activity, such a conversion may be regarded as a form of degradation, often resulting in a serious loss of therapeutic activity. For example, the appreciable loss of activity of solutions of adrenaline at low pH has been attributed to racemisation - the conversion of the therapeutically active form, in this case the levorotary form, into its less-active isomer. 

Loss of activity of solutions of some dmgs such as the tetracyclines can occur because of epimerisation of the dmg molecule, while others such as vitamin A lose activity because of geometrical isomerisation. 

Know how the activities of solutes and solvent water are defined. 

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