Thermodynamic control buffer solutions

The comparison of I —> N and N —> I may also be explained by the buffered pH in the diffusion layer and leads to an interesting comparison between a process under kinetic control versus one under thermodynamic control. Because the bulk solution in process N —> I favors formation of the ionized species, a much larger quantity of drug could be dissolved in the N —> I solvent if the dissolution process were allowed to reach equilibrium. However, the dissolution rate will be controlled by the solubility in the diffusion layer accordingly, faster dissolution of the salt in the buffered diffusion layer (process I—>N) would be expected. In comparing N—>1 and N —> N, or I —> N and I —> I, the pH of the diffusion layer is identical in each set, and the differences in dissolution rate must be explained either by the size of the diffusion layer or by the concentration gradient of drug between the diffusion and the bulk solution. It is probably safe to assume that a diffusion layer at a different pH than that of the bulk solution is thinner than a diffusion layer at the same pH because of the acid-base interaction at the interface. In addition, when the bulk solution is at a different pH than that of the diffusion layer, the bulk solution will act as a sink and Cg can be eliminated from Eqs. (1), (3), and (4). Both a decrease in the h and Cg terms in Eqs. (1), (3), and (4) favor faster dissolution in processes N —> I and I —> N as opposed to N —> N and I —> I, respectively. 

What is actually observed, however, is that [Rh(phi)2(phen)]3+ intercalated into DNA quenches the intensity of [Ru(phen)2(dppz)]2+ much more effectively than it quenches the two lifetimes, as summarized in Table II. This effect is most pronounced when the DNA helix is a short oligonucleotide. The direct comparison of quenching in the absence of DNA cannot be accomplished because the ruthenium(II) complex does not luminesce in aqueous solution however, electron transfer from [Ru(phen)3]2+ to [Rh(phi)2(phen)]3+ in buffered solution provides a control with the same thermodynamic driving force (40). 

The number and variety of reported studies on the bro-mination of steroid ketones is so great [133] that it would be impossible to survey the field adequately in a small space. The situation is complicated by uncertainty in some early work as to whether the bromo-ketones are products of kinetic or thermodynamic control. It often happens that the initial product of electrophilic attack on an enol is the unstable epimer (kinetic control), which rearranges under acidic conditions into the more stable epimer or may even undergo positional isomerisation under thermodynamic control (see p. 385). Only under experimental conditions which inhibit subsequent rearrangements is it possible to be sure of isolating the primary product. In recent years the products of kinetically-controlled bromination have been obtained by permitting the enol acetate of the ketone to react with bromine in a buffered solution. Few free ketones are brominated at a useful rate under such mild conditions,... 

Equations (3-6) for the potentiometric and spectrophotometric methods will provide thermodynamic pKa values. For the solubility-pH dependence method [Eqs. (7-8)], the values obtained are apparent values (pKg )/ which are relevant to the ionic strength (7) of the aqueous buffers used to fix the pH value for each solution. If the ionic strength of each buffer solution is controlled or assessed, then the apparent value can be corrected to a thermod)mamic value, using an activity coefficient from one of the Debye-Hiickel equations (Section 2.2.5). If the solubility-pH dependence is measured in several buffer systems, each with a different ionic strength, then the Guggenheim approach can be used to correct the result to zero ionic strength [Eq. (17)]. 

The maximum rates of the reactions of most aldehydes and ketones with semi-carbazide occur in the pH range of 4.5-5.0. For the purpose of making derivatives of carbonyl compounds (Sec. 25.7), semicarbazide is best used in an acetate buffer (CH3CO2H/CH3CO2 ) solution, which maintains a pH in the maximum rate range of 4.5-5.0. However, to demonstrate the principle of kinetic and thermodynamic control of reactions, buffers that maintain higher pHs, and thus produce lower rates, are more desirable. Parts A-C of the experimental procedure involve a phosphate buffer system, whereas the bicarbonate system is used in Part D. It is then possible to compare how the difference in rates in the two buffer systems affects the product ratio. Analysis of the products from the various parts of these experiments provides strong clues as to which of the semicarbazones is the product of kinetic control and which is the product of thermodynamic control. 

The chemical concept of buffering is linked with pH control in homogeneous solutions of acids and bases in open systems. Buffer reactions absorb changes in intensive thermodynamic parameters (e.g. masses of H", Na+ etc.) in response to changes in extensive parameters, such as temperature, pressure etc. Buffering in... 

---