Activity in aqueous solutions

We could have tackled the problem in another way. If we defined the activity of the solute naphthalene using the Henry s Law standard state ptf (from Section 6.10) we would obtain 

As naphthalene in hexane follows Henry s Law (but not Raoul t s Law) the activity coefficient defined on this basis would be approximately unity, as the equation = fxf + RT nxt holds for this system. Whether the pure-liquid or the Henry s Law standard state is used in any thermodynamic calculation is purely a matter of convenience. There is no fundamental thermodynamic objection to using any standard state. In the particular problems we are considering it is sometimes helpful to have the activity coefficient equal to unity so we can apply the equation pLt = juf + RTIn x( to the system. On the other hand, if we wish to quantify the varying behaviour of naphthalene in different solvents the activity coefficients based on the pure-liquid standard state give us a convenient measure. 

Many chemical experiments are carried out in aqueous solutions and it is important to be able to define activities in these circumstances. However, the standard state we have used so far—the pure liquid at one atmosphere pressure—is singularly inappropriate. We usually wish to express concentrations in molality (moles per kilogram of solvent) and for an electrolyte, such as sodium chloride, the pure-liquid state at room temperature is not a suitable reference state. 

For an electrolyte solution the procedure is similar but we use unit molality, = 1 mol kg 1, as the concentration of the standard state. Then 

Now iuf is the chemical potential of an ion in a (hypothetical) solution of unit molality which behaves like a solution of infinite dilution (where yt = 1, trii = 1 mol kg-1, and ftt - tuf). 

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Entries 6 to 9 involve reactions conducted under catalytic conditions. Entry 6 uses a lanthanide catalyst that is active in aqueous solution. Entries 7 and 8 are examples of the use of (Cp)2Ti(03SCF3)2 as a Lewis acid. Entry 9 illustrates the TMS triflate-MABR catalytic combination. 

The nitrilase activity of Arthrobacter sp. F-73 retains substantial activity in aqueous solutions containing a significant concentration of organic co-solvent [84]. More than 10% of nitrilase activity remains at acetone concentrations up to 60%, whereas no activity... 

Moore and Hemmens [119] studied the photosensitization of primaquine and other antimalarial agents. The drugs were tested for in vitro photosensitizing capability by irradiation with 365 nm ultraviolet light in aqueous solutions. The ability of these compounds to photosensitize the oxidation of 2,5-dimethylfuran, histidine, trypotophan, or xanthine, and to initiate the free radical polymerization of acrylamide was examined in the pH range 2 12. Primaquine does not have significant photochemical activity in aqueous solution. 

Scheme 1. Speciation of Feni-TAML activators in aqueous solution (solid rectangle) and suggested mechanism of the H+-induced deme-talation (dashed rectangle) — — = free base ligand. From Ref. (13).

The alpha amylase of malted barley, the amylase of Aspergillus oryeae and pancreatic amylase all are thermolabile proteins that rapidly lose their amylase activities upon exposure to unfavorable temperatures, to unfavorable hydrogen ion activities, or to other unfavorable chemical environments. The loss of amylase activity in aqueous solutions increases with increasing temperatures and is exceedingly rapid for each of these amylases at 50°. The inactivation of each of these amylases at unfavorable temperatures or at unfavorable hydrogen ion activities may be retarded by the presence of suitable concentrations of calcium ions. 

W. Preibsch, K. D. Hofmann and W. Krafft, Method for determination of alkylating activity in aqueous solutions by reaction with NBP [4-(p-nitrobenzyl)pyridine]. Archiv fuer Geschwulstforschung, 1972, 40(3), 259-262. 

Lipases are serine hydrolases that catalyse the hydrolysis of lipids to fatty acids and glycerol [2]. In contrast to esterases, they work at the lipid-water interface and show only little activity in aqueous solutions. Studies of the X-ray structures of human lipase [3,4] and Mucor miehei lipase [5,6] revealed a change in conformation at the lipid-water interface, which explains the increase of activity. 

Bundgaard, H. and M. Johansen. 1982. Kinetics of hydrolysis of tpHiaJe (An ureid J-Mannich base with platelet antiaggregant activity) in aqueous solution and in plasfon ]. Pharm. Chem., Sci. Ed. 10 139-145. 

Contrary to expectations that enzymes are only active in aqueous solution, activity in almost anhydrous organic solvents was already demonstrated in the 1930s and rediscovered in 1977. It was not water-miscible hydrophilic solvents such as methanol or acetone that proved to be the best reaction media, but hydrophobic water-immiscible solvents such as toluene or cyclohexane. Supposedly, the cause is the partitioning of water between the enzyme surface and the bulk phase of the organic solvent. As comparably hydrophilic solvents such as methanol or acetone can take up basically infinite amounts of water, they strip the remaining water molecules off the enzyme surface. As a consequence, the enzyme is no longer active because it requires a small but measurable amount of water for developing its activity... 

Activities in aqueous solution are generally based on the 1 molality standard state. 

In electrochemistry we make it a rule that the standard chemical potential ju. of hydrogen ions is set zero as the level of reference for the chemical potentials of all other hydrated ions. The standard chemical potentials of various hydrated ions tabulated in electrochemical handbooks are thus relative to the standard chemical potential of hydrogen ions at unit activity in aqueous solutions. Table 9.3 shows the numerical values of the standard chemical potential, the standard partial molar enthalpy h°, and the standard partial molar entropy. 5 ,° for a few of hydrated ions. 

From this experiment and the preceding one we should conclude that the non-metallic elements fall in the order F, Cl, Br, 0, I, S (fluorine being strongest) with respect to their activity in aqueous solutions containing free acid. This is approximately the order of the electromotive series for these non-metals. If the solution is made neutral the electromotive potential of oxygen is lowered so that the oxygen is no longer able to displace iodine. 

Fig. 1 Experimentally derived binding free energies for the substrate (S) and transition state (TS) out of aqueous solution to form the ODCase substrate (E S) and ODCase transition state (E TS) complexes (AGSbmd and AGTSbind) and free energies of activation in aqueous solution and ODCase (AG q and AGoDCase), all in kcal mol-1.

The thermodynamics of methanol and ethanol production help to explain why methanol is not manufactured by fermentation processes whereas ethanol is. The overall stoichiometries for converting glucose (C6H12O6) to methanol and ethanol by alcoholic fermentation under physiological conditions (pH 7, 25°C, unit activity in aqueous solution) are... 

Eugster R, Rusterholz B, Schmid A, Spichiger UE, Simon W. Characterization procedure for ion-selective electrode assays of magnesium activity in aqueous solutions of physiological composition. Clin Chem 1993 39 855-9. 

The calculation of Ecell for the reaction of Eq 2.32 can be used as an example. The reaction is rewritten as follows to show the activities in aqueous solution, aHcl and aFeCl2 ... 

Giner-Segui et al. (2006) studied the evolution of polygalacturonase (PG) (EC 3.2.1.15) activity in aqueous solution of commercial enzyme preparation. Up to 76.5% reduction of the PG activity could be achieved at 38 kV cm and 1100 J,s EF intensity and treatment time, respectively. However, an enhancement of PG activity at soft PEF treatment conditions (up to 110.9% at = 15kV cm and 300 J,s) was observed. A maximum of 80% of pectin methyl esterase activity in orange juice was inactivated at 35 kV cm and 1500 J,s EF strength and treatment time, respectively (Elez-Martinez et al., 2007). 

Emulsifiers for styrene-butadiene, styrene-butadine-acrylonitrile, and neoprene rubbers is the next important area of rosin use. Due to its unique properties of being surface active in aqueous solution and tacking in coagulated rubber, disproportionated rosin finds ready acceptance in this application. However, due to slow growth of SBR, which is by far the most important factor among all rubbers, the consumption of rosin here will rise only very slowly, if at all, in the future. It is significant to note that there appears to be an industry-wide acceptance of a mixed disproportionated rosin-fatty acid emulsifier, which is lower in manufacturing cost, to replace the traditional disproportionated rosin acid and soaps. 

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