Acid-base buffer systems described

The presence of H3 0+ or HO may alter drastically the observed reaction rate either because they catalyse the reaction (acid or base catalysis, see Section 3.2.3 for the Aldol reaction, and Chapter 11) or because of ionic strength effects. Proper pH control in an aqueous solution will require a buffer system which is described by the appropriate version of the Henderson-Hasselbach equation, according to whether the acid or base is the charged species ... 

In the same manner as described in Section 2.2.5, mathematical expressions for the buffer capacity of polyprotic weak acid/base systems can be developed. When one adds a strong base such as NaOH to an aqueous buffer solution containing a polyprotic weak acid (HnA), the electroneutrality would be ... 

The potentiometric mass titration method [657,658] produces results equivalent to those of the drift method described above. The same amount of base is added to three dispersions with different solid-to-liquid ratios and a constant ionic strength. The dispersions are titrated with acid, and the pH is recorded as a function of the amount of acid added. The intersection point of the obtained curves is taken as the PZC. In other words, the PZC is identified with the pH at which solid addition does not induce a change in pH. The drift method and mass titration are based on the same principle, the difference being that in potentiometric mass titration, the reagents are added in a different order. Potentiometric mass titration is affected by the acid or base associated with the powder in the same way as in the drift method and mass titration. The advantage of potentiometric mass titration over the drift method is that in the former the pH is measured only in buffered systems. 

As you can see, the two reactions that characterize this buffer system are identical to those for the common ion effect described in Example 16.1. The buffering capacity, that is, the effectiveness of the buffer solution, depends on the amount of acid and conjugate base from which the buffer is made. The larger the amount, the greater the buffering capacity. 

The dissociation equilibrium depends on the pH value of the aqueous phase and on the pA a value of the organic compound. Equation (34) (for acids) and Eq. (35) (for bases) describe the apparent partition coefficients log Papp of acids and bases at different pH values of the aqueous phase in an -octanol/buffer system ( Pu = partition coefficient of the neutral, uncharged form P = partition coefficient of the ionized,... 

As described earlier, a mobile phase is the liquid that is pumped through die column. Mobile phases are comprised of major and minor components. In RP sqiarations, components are typically considered major if they are present at levels of >5% in the mobile phase. Mmor components are present at <5% and are commonly referred to as mobile phase modifiers, lypical mobile phase modifiers (MPMs) are undiluted acids (e.g., phosphoric, trifluoroacetic, and acetic) and bases (e.g., trietlqrlamine, triethanolamine, and diethylamine) as well as buffer systems (e.g., phosphate and acetate or mixed such as trifluoroacetic acid/triethylamine) and ion-pair reagents (e.g., sodium dodecyl sul te and tetrabutylammonhun phosphate). 

Purely diffuse-layer models have become quite popular since the database for hydrous ferric oxide has been published by Dzombak and Morel [76]. The model calculations by these authors have shown that it is possible to describe a wide range of experimental sorption data within a relatively simple model framework. However, the description of acid-base properties of hydrous ferric oxide with this model is not convincing. Substantial failures with respect to true predictions can therefore be expected whenever dynamic systems involving the transport of protons are considered and variations of pH are possible (Lutzenkirchen et al., in preparation). Nevertheless, for conditions in which this is not the case (i.e., buffered systems), the database is very useful but should not be used in the context of mechanistic discussions. 

The first work in this field was reported by Winnick et al. in 1995 [4], In order to design a sodium/iron(II) chloride battery, they examined a l-ethyl-3-methyl-imidazolium chloride/aluminum chloride-based system. As described by Lipsztajn and Osteryoung for lithium it was first necessary to synthesize the acidic ionic liquid by adding an excess of AICI3 and then adding an equivalent amount of sodium chloride as a buffer to obtain again the neutral species. 

This chapter describes polyfunctional acid and base systems, including buffer solutions. Calculations of pH and titration curves are also described. 

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