Aqueous systems, unit activity

Surface-active agents and hquids immiscible in water can form tiny dispersed units called reverse micelles. These can extract biochemicals from water or permit complexing or reacting in ways not possible in simple aqueous systems. 

At unit activities of the oxidant and reductant, the potential depends only on pH the slope of the line for a plot of potential versus pH is governed by the ratio m/n. Potential-pH diagrams are a concise means to display the redox properties of a system. We will take uranium as an example. The +6, +5, +4, and + 3 oxidation states are known in aqueous solution. The determination of +6 uranium by coulometric titration has been investigated by many workers and the lower oxidation states have all been used as coulometric titrants. Hydrolyzed uranium species exist in a noncomplexing solution, but the chemistry is simplified considerably if the discussion is limited to solutions more acidic than about pH 4. Some of the half-reactions to be considered are listed next with E° vs. NHE ... 

In summary, thermodynamic models of natural water systems require manipulation of chemical potential expressions in which three concentration scales may be involved mole fractions, partial pressures, and molalities. For aqueous solution species, we will use the moial scale for most solutes, with an infinite dilution reference state and a unit molality standard state (of unit activity), l or the case of nonpolar organic solutes, the pure liquid reference and standard states are used. Gaseous species will be described on the partial pressure (atm — bar) scale. Solids will be described using the mole fraction scale. Pure solids (and pure liquids) have jc, = 1, and hence p, = pf. 

For aqueous systems, a unit activity is expected for the solid species (i.e., we assume that the chemical reactivity of a solid in water is unchanging as long as there is solid in equilibrium with the solution). Also, for dilute concentrations, we assume that the activities are equal to the concentrations of the species. With these assumptions, we can reduce the solubility product constant equation to... 

Ideally this system would consist of three phases only. Two of these phases would be liquid, water and cooled hydrocarbons, and the third would be the cracked gas stream. Unfortunately, solids may accumulate in this vessel. Coke fines and small particles from the furnace may be entrained into this unit, and activated olefins such as butadiene or styrene may polymerize to form insoluble particles. All of this behavior promotes foaming. In this aqueous system, foam formation may be controlled by addition of a polyglycol. 

In aqueous environments, enzymatic activity is sensitive to the pH of the bulk solution. One may therefore suspect that the SCCO2, which is dissolved in the microwater layer of an enzyme in an essentially nonaqueous system, would change the pH of that layer and affect enzyme activity. Kamat et al. clearly showed that this effect is negligible [5]. Increasing the CO2 pressure by a factor of 100 decreases the pH of bulk water by only one unit in an unbuffered system. Enzymes are normally lyophilized from a buffered solution prior to their use as catalysts in SCFs. In lyophilization, the enzyme is first dissolved in water where the pH is adjusted for maximum enzymatic activity. The enzyme/buffer salt solution is then freeze-dried under vacuum to remove almost all of the water. In a typical phosphate buffer solution, the pH may be 7.8. Kamat et al. calculated that at 100 bar CO2 pressure the new pH of the buffer solution would be 7.75, and at 1010 bar the pH would be 7.66. Lyophilization increases the buffer salt concentration in the residual water in the enzyme considerably. Thus, the effect of CO2 on the pH of the remaining microaqueous layer in enzymes becomes even smaller. 

When a bright platinum plate is immersed in a solution containing the oxidised and the reduced forms of a system, of equal unit activity (concentration) eg Fe and Fe in aqueous solution, the potential of the Pt electrode can be measured by coupling it with a... 

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