Aqueous equilibria buffered solutions

The linear trinuclear cation (100) and the tetranuclear cations (101) to (103)448-450 (Table 52) exist with mononuclear complexes in aqueous ethylenediamine buffer solutions in which equilibrium has been established by the catalytic effect of Cr2+ and charcoal under well-specified conditions.358 The cations were separated by ion exchange chromatography and fractional crystallization, and isolated as various salts (Table 52). The tetranuclear (rhodoso) compound (104)4S1 has been reprepared448 by a modification of Pfeiffer s method. 

Spectrophotomeric study of the voltammetric oxidation of [Ptj (pop)4] in aqueous phosphate buffer solution in the presence of an excessive amount of various halide anions (X" = Cl , Br", or I ) by use of the OTTLE cell technique indicated the quantitative formation of [Pt2(pop)4X2] with expected isosbestic points (228). The intermediate mixed-valence state was not detected. Cyclic voltammetric study employing similar conditions revealed that the oxidation potential depends significantly on the kind of coexisting halide ions. It was suggested that a small amount of [Pt2(pop)4X] in equilibrium with [Pt2(pop)4] in the vicinity of the electrode undergoes oxidation. 

Tin literature there are many studies on the adsorption of ionic surfactants at aqueous solution/alkane interface, such as [11, 26, 28-31]. A direct comparison of the adsorption behaviour of CnTAB with alkyl chain lengths 10, 12, 14 and 16 at the water/air and water/hexane interfaces was presented in [8] and it was shown that the FIC model described the experimental data for both interfaces quite well. The new model proposed in [6], leading to the set of Eqs. (7), (9) and (10), has shown to be superior over the FIC model as it allows to assume that the oil molecules provide not only a hydrophobic environment for the adsorbing surfactant molecules but the adsorb themselves at the interface. The equilibrium interfacial tension isotherms presented in Fig. 3 for four CnTABs (n = 10, 12, 14 and 16) adsorbed at the aqueous phosphate buffer solution/hexane interfaces allow to demonstrate the feasibility of the given physical picture of a co-adsorption of surfactant and alkane molecules. 

When gaseous CO2 is equilibrated with aqueous buffer solution in a closed vessel, a large portion of the CO2 is dissolved in the aqueous phase, mostly in the form of bicarbonate, maintaining the equilibrium of the following three phases ... 

As a measure of their thermodynamic stability, the pAfR+ values for the carbocation salts were determined spectrophotometrically in a buffer solution prepared in aqueous solution of acetonitrile. The KR+ scale is defined by the equilibrium constant for the reaction of a carbocation with water molecule (/CR+ = [R0H][H30+]/[R+]). Therefore, the larger p/CR+ index indicates higher stability for the carbocation. However, the neutralization of these cations was not completely reversible. This is attributable to instability of the neutralized products. The instability of the neutralized products should arise from production of unstable polyolefinic substructure by attack of the base at the aromatic core. 

It is unfortunate that there has been so little work devoted to quantitative measurements of cation-pseudobase equilibria in methanol and ethanol since these media have several advantages over water for the determination of the relative susceptibilities of heterocyclic cations to pseudobase formation. The enhanced stability of the pseudobase relative to the cation in alcohols compared to water is discussed earlier this phenomenon will permit the quantitative measurement of pseudobase formation in methanol (and especially ethanol) for many heterocyclic cations for which the equilibrium lies too far in favor of the cation in aqueous solution to allow a direct measurement of the equilibrium constant. Furthermore, the deprotonation of hydroxide pseudobases (Section V,B) and the occurrence of subsequent irreversible reactions (Sections V,C and D), which complicate measurements for pKR+ > 14 in aqueous solutions, are not problems in alcohol solutions. Data are now available for the preparation of buffer solutions in methanol over a wide range of acidities.309-312 An appropriate basicity function scale will be required for more basic solutions. The series of -(substituted phenyl)pyridinium cations (163) studied by Kavalek et al.i2 should be suitable for use as indicators in at least some of the basic region. The Hm and Jm basicity functions313 should not be assumed90 to apply to methoxide ion addition to heterocyclic cations because of the differently charged species involved in the indicators used to construct these scales. 

Section 19.1 discusses the Brpnsted theory of acids and bases, which extends the concepts of add and base beyond aqueous solutions and also explains the acidic or basic nature of solutions of most salts. Dissociation constants, the equilibrium constants for the reactions of weak acids or bases with water, are introduced in Section 19.2. The concept of the ionization of covalent compounds is extended to water itself in Section 19.3, which also covers pH, a scale of acidity and basicity. Section 19.4 describes buffer solutions, which resist change in their acidity or basicity even when some strong acid or base is added. Both the preparation and the action of buffer solutions are explained. Section 19.5 discusses the equilibria of acids containing more than one ionizable hydrogen atom per molecule. 

The hydrogen ion activity of the equilibrium aqueous phase was measured with Beckman pH and saturated calomel electrodes. The reference voltage Ea of the system was calculated using the Nemst Equation and the electrode measurement of standard buffer solutions. For all pH electrode measurements Ea varied only slightly between 414 and 417 mV. The data are shown in Table III and the calculated hydrogen ion activity [H+] is based on the average of the two values of electrode voltage (El and E2). 

An objective of preformulation scientist is to determine the equilibrium solubility of a drug substance under physiological pH to identify the BCS class of drug candidate for further development. For BCS classification the test conditions are strictly defined by the FDA. The pH solubility profile of the test drug substance should be determined at 37°C in aqueous media with a pH in the range of 1-7.5. Standard buffer solutions described in the USP are considered to be appropriate for use in these studies. A number of pH conditions are used bracketing the pKa value for the respective test substance. For example, for a drug with a Ka of 5, solubility should be determined at... 

Table 2 Equilibrium association constants (M 1) between dansyl guests and the indicated receptor in aqueous buffer solution (pH = 7) at 22°C...

The equilibrium solubilities of indapamide in aqueous buffer solutions over the pH range of 1 through 10 are shown in Table VI and Figure 12 (12). From the data it can be seen that the aqueous solubility of the compound is 0.1 mg/mL or less at pH values of 8 or below. The rapid increase in solubility beginning at about pH 8 is the result of the formation of the more soluble ionized species. 

The solubility measure describes the concentration reached in solution, when a pure phase of the material is allowed to dissolve in the solvent for a defined period of time, at a defined temperature (and pressure). Most often for pharmaceutical purposes, the pure phase is a solid, ideally a crystalline solid, and the liquid is water or a buffered aqueous solution, at a controlled temperature (often 25 or 37 °C) and ambient pressure. The purity of the solid can have a large effect on measured solubility. Solubility can be measured in water or in pH-controlled buffers. In water, the extent of solubility for ionizable compounds will depend upon the p fCa values and the nature of the counterion. In pH-controlled aqueous buffered solution, at equilibrium, the solubility will depend upon the compound s intrinsic solubility, its plQ, and the ionic strength. It may also depend upon the relative solubility of the initial added compound and the solubility of the salt formed by the compound with the buffer salts, with which the solid may equilibrate. In any buffer or solvent system, the measured solubility may depend on the time of sampling, as solubility kinetics... 

In this chapter we will continue the study of acid-base reactions with a discussion of buffer action and titrations. We will also look at another type of aqueous equilibrium—that between slightly soluble compounds and their ions in solution. 

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