Buffers and Other Equilibria

Copyright 2007 by John Moore and Richard Langley. Click here for terms of use. 

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The general features discussed so far can explain the complexity of these reactions alone. However, thermodynamic and kinetic couplings between the redox steps, the complex equilibria of the metal ion and/or the proton transfer reactions of the substrate(s) lead to further complications and composite concentration dependencies of the reaction rate. The speciation in these systems is determined by the absolute concentrations and the concentration ratios of the reactants as well as by the pH which is often controlled separately using appropriately selected buffers. Perhaps, the most intriguing task is to identify the active form of the catalyst which can be a minor, undetectable species. When the protolytic and complex-formation reactions are relatively fast, they can be handled as rapidly established pre-equilibria (thermodynamic coupling), but in any other case kinetic coupling between the redox reactions and other steps needs to be considered in the interpretation of the kinetics and mechanism of the autoxidation process. This may require the use of comprehensive evaluation techniques. 

If measurements are to be carried out at low activities (for example in studying complexation equilibria), standard solutions cannot be prepared by simple dilution to the required value because the activities would irreproducibly vary as a result of adsorption effects, hydrolysis and other side reactions. Then it is useful to use well-defined complexation reactions to maintain the required metal activity value [14, 50, 132]. EDTA and related compounds are very well suited for this purpose, because they form stable 1 1 complexes with metal ions, whose dissociation can be controlled by varying the pH of the solution. Such systems are often termed metal-ion buffers [50] (cf. also p. 77) and permit adjustment of metal ion activities down to about 10 ° m. (Strictly speaking, these systems are defined in terms of the concentration, but from the point of view of the experimental precision, the difference between the concentration and activity at this level is unimportant.)... 

The second novelty of our work is the elucidation of the role of pH-buffers in swelling equilibria and kinetics. It is interesting to note that most physical chemists choose not to use buffers because they complicate the system under study. On the other hand, any ultimate biological application of gels is likely to involve some sort of buffered medium. As we have shown, buffer properties such... 

The first and most often encountered separation mechanism in CE is based on mobility differences of the analytes in an electric field these differences are dependent on the size and charge-to-mass ratio of the analyte ion. Analyte ions are separated into distinct zones when the mobility of one analyte differs sufficiently from the mobility of the next. This mechanism is exemplified by capillary zone electrophoresis (CZE) which is the simplest CE mode. A number of other recognized CE modes are variations of CZE. These are micellar electrokinetic capillary chromatography (MECC), capillary gel electrophoresis (CGE), capillary electrochromatography (CEC), and chiral CE. In MECC the separation is similar to CZE, but an additional mechanism is in effect that is based on differences in the partition coefficients of the solutes between the buffer and micelles present in the buffer. In CGE the additional mechanism is based on solute size, as the capillary is filled with a gel or a polymer network that inhibits the passage of larger molecules. In chiral CE the additional separation mechanism is based on chiral selectivity. Finally, in CEC the capillary is packed with a stationary phase that can retain solutes on basis of the same distribution equilibria found in chromatography. 

Note that these equilibria and Equation 7.88 hold although there are other buffer systems in the blood. The pH is the result of all the buffers and the [HCO3"]/ H2CO3] ratio is set by this pH. 

Other authors have drawn attention to the fact that textbooks and teachers often present complex equilibria in a simplified way, to make these equilibria easier to handle for (beginning) students of chemistry. Hawkes (1998) has discussed this problem as a pedagogical dilemma that one either has to teach simplified ideas, which do not have much meaning in reality, or one has to adopt a rigorous treatment, which is normally beyond the scope of an introductory course. Hawkes concluded that, in any case, it is better to confront students with the complexity of phenomena, than to teach them simplified tmths. Obviously, problems associated with simplifications and idealisations can occur in many fields of application of the Equilibrium Law, such as solubility (Clark Bonicamp, 1998), pH and buffers, and electrochemical systems. 

Because Bicine is derived from glycine, it was expected already nearly 50 years ago that this buffer forms complexes with metal ions [216]. For Tris and Bistris the awareness of metal ion interactions is much lower, and the fact that also mixed ligand complexes may form [212,213], has hardly been reaUzed. Therefore, the stabilities of the ternary Cd " and some other metal ion complexes formed between these buffers and ATP are briefly summarized. The stability constants according to equilibria (28) and (29) are listed in Table 13 together with the stability differences defined in equation (30). 

Based on the above equilibria, the concentration of HOCl in the normal pH range varies inversely with the total concentration of cyanurate. Increased concentration of cyanuric acid, therefore, should decrease the biocidal effectiveness of FAC. This has been confirmed by laboratory studies in buffered distilled water which showed 99% kill times of S.faecalis at 20°C increasing linearly with increasing cyanuric acid concentration at constant av. Cl at pH 7 and 9 (45). Other studies in distilled water have found a similar effect of cyanuric acid on kill times of bacteria (46—48). Calculations based on the data from Ref. 45 show that the kill times are highly correlated to the HOCl concentration and poorly to the concentration of the various chloroisocyanurates, indicating that HOCl is the active bactericide in stabilized pools (49). 

I If the ethanoate in the buffer was replaced by citrate, and there were no other competing equilibria, what would be the effect on the retention of the two acids ... 

Ru(edta)(H20)] reacts very rapidly with nitric oxide (171). Reaction is much more rapid at pH 5 than at low and high pHs. The pH/rate profile for this reaction is very similar to those established earlier for reaction of this ruthenium(III) complex with azide and with dimethylthiourea. Such behavior may be interpreted in terms of the protonation equilibria between [Ru(edtaH)(H20)], [Ru(edta)(H20)], and [Ru(edta)(OH)]2- the [Ru(edta)(H20)] species is always the most reactive. The apparent relative slowness of the reaction of [Ru(edta)(H20)] with nitric oxide in acetate buffer is attributable to rapid formation of less reactive [Ru(edta)(OAc)] [Ru(edta)(H20)] also reacts relatively slowly with nitrite. Laser flash photolysis studies of [Ru(edta)(NO)]-show a complicated kinetic pattern, from which it is possible to extract activation parameters both for dissociation of this complex and for its formation from [Ru(edta)(H20)] . Values of AS = —76 J K-1 mol-1 and A V = —12.8 cm3 mol-1 for the latter are compatible with AS values between —76 and —107 J K-1mol-1 and AV values between —7 and —12 cm3 mol-1 for other complex-formation reactions of [Ru(edta) (H20)]- (168) and with an associative mechanism. In contrast, activation parameters for dissociation of [Ru(edta)(NO)] (AS = —4JK-1mol-1 A V = +10 cm3 mol-1) suggest a dissociative interchange mechanism (172). 

The anthocyanins exist in solution as various structural forms in equilibrium, depending on the pH and temperature. In order to obtain reproducible results in HPLC, it is essential to control the pH of the mobile phase and to work with thermostatically controlled columns. For the best resolution, anthocyanin equilibria have to be displaced toward their flavylium forms — peak tailing is thus minimized and peak sharpness improved. Flavylium cations are colored and can be selectively detected in the visible region at about 520 nm, avoiding the interference of other phenolics and flavonoids that may be present in the same extracts. Typically, the pH of elution should be lower than 2. A comparison of reversed-phase columns (Ci8, Ci2, and phenyl-bonded) for the separation of 20 wine anthocyanins, including mono-glucosides, diglucosides, and acylated derivatives was made by Berente et al. It was found that the best results were obtained with a C12 4 p,m column, with acetonitrile-phosphate buffer as mobile phase, at pH 1.6 and 50°C. 

It is seen from Table I that Reactions (2) and (3) satisfy the definition of buffered equilibria quite well in that and hence Poaic pEq. 0)3, are quite insensitive to the equilibrium positions or the exact product compositions for even the highly underbalanced explosives. Although values of N differ by as much as 74%, values of M by as much as 33%, and values of Q by as much as 107% for individual explosives as these equilibria are shifted from one set of extremes to the other, values of p q. (4)3 show changes no greater than 7%. Where these equilibria do not lie completely to the extremes, as is more likely to be the case in ruby computations or in actual detonations, differences between < arb and < ruuy or acutni should be even smaller. 

Buffering results from two reversible reaction equilibria occurring in a solution of nearly equal concentrations of a proton donor and its conjugate proton acceptor. Figure 2-19 explains how a buffer system works. Whenever H+ or OH- is added to a buffer, the result is a small change in the ratio of the relative concentrations of the weak acid and its anion and thus a small change in pH. The decrease in concentration of one component of the system is balanced exactly by an increase in the other. The sum of the buffer components does not change, only their ratio. 

The aqueous mobile phases used in RPLC allow the use of buffers in the mobile phase. This may lead to improved selectivity and efficiency. Secundary (ionic) equilibria other than acid-base dissociation may also be used (see section 3.3.2). 

Another method widely used is based on conductometric measurements at short times after an irradiation pulse. This method determines the electrical charge of the species studied and is thus useful for radicals where several dissociations can take place. This technique can be complicated by buffering effects if the parent compound itself undergoes acid-base equilibria in the region of interest for the study of the radical (see e.g. Bhatia and Schuler, 1973b). It is therefore, a prerequisite to know all the other p/ -values which may be involved. Another limitation of this technique lies in the fact that only p f-values between about 2 and 12 can be studied and high background conductivity decreases the sensitivity of the measurements. 

Reaction (5) is most likely to represent the major gas redox buffer because of the comparable abundances of the two sulfur-bearing gas species. The other effective geochemical buffer is the Fe0-Fc203 rock buffer, which can affect the redox equilibria above in magma, and as gas and wall-rock interact. 

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