Studies of Acid-Base Equilibria

Conductimetric Studies of Acid-Base Equilibria Determination of Autoprotolysis Constants 

The autoprotolysis constant Ksh[= a(SH2 )a(S )] of an amphiprotic solvent SH is related to the conductivity of the pure solvent k by  

The procedures typically employed for a weak acid HA are described below [39, 40]  

The molar conductivity of HA is measured as a function of its concentration, ca, and the data are analyzed appropriately, according to the mechanism of the acid dissociation  

1 (a) Fernandez-Prini, R., in Physical Chemistry of Organic Solvent Systems (Eds A.K. Covington, T. Dickinson), Plenum Press, New York, 1973, Chapter 5.1 (b) Spiro, M., ibid., Chapters 5.2, 5.3. 

---

The recording of absorption spectra has also been applied to studies of acid-base equilibria, since radicals with dissociable functional groups usually show spectral shifts with pH which can be utilized for the determination of p -values. Most of the reported values of dissociation constants have, in fact, been determined in this manner. Several examples of spectral shifts can be found in Table 2 and a detailed discussion of acid-base equilibria of organic radicals is given in Section 6. 

V.L. Cherginets, O.V. Demirskaya and T.P. Rebrova, Potentiometric Study of Acid-Base Equilibria in the KCl-LiCl Eutectic Melt at Temperatures in the Range 873 to 1073 K, J. Chem. Thermodyn. 36 (2004) 115-120. 

K.W. Herrmann, Non-ionic-cationic micellar properties of dimethyldodecylamine oxide, J. Am. Chem. Soc., 1962, 66, 2895-300 P. Holt and B. Tamami, Relation between pH and viscosity of some poly(alkylvinylpyridine N-oxides) in aqueous solution, Die Makromol. Chem., 1972, 155, 55-60 L. Chmurzynski, A. Liwo and P. Barczynski, A potentiometric study of acid-base equilibria of substituted TV-oxides in nitrobenzene, Anal. Chim. Acta, 1996, 335, 147. 

J. Berdys, M. Makowski, M. Makowska, A. Puszko, and L. Chmurzyriski, Experimental and theoretical studies of acid-base equilibria of substituted 4-nitropyridine N-oxides, J. Phys. Chem. A 107 (2003), pp. 6293-6300. 

Some studies of acid-base equilibria in molten alkaline halides have been performed by Rybkin et al. The empirical acidity scales have been constructed on the base of potentiometric studies in molten KCf and Csl. The practical significance of the mentioned works consists in the proposition to use some buffer solutions - SO/ /SjOg, , P0/ /P207 as reference standards for the indicator electrode calibration and determinations of pO in molten alkaline halides. 

This reference electrode was used in the potentiometric study of acid-base equilibria in PC using a pH glass electrode, in which the potential of the reference electrode should be very stable [178]. 

E. F. C. Cain, Diss. Abs., 27B, 67 (1966). The Study of Acid-base Equilibria in Mixed Aqueous Solvents. Theoretical equations are developed to predict changes in acid and base strength as a function of dielectric constant of the solvent. 

E. Chufan, F. Suvire, R. Enriz, and J. C. Pedregosa, A potentiometric and spec-trophotometric study on acid-base equilibria in ethanol-aqueous solution of acetazolamide and related compounds, Talanta 49 (1999), 859-868. 

However, the practice of investigations of acid-base equilibria consists in studies of the reactions with the transfer of definite particles (acid or base) in different solvents, irrespective of the fact whether they are prone to dissociation with the formation of given acid (base), or not. In order to give the most general formulation of the solvosystem concept we should divide the solvent in relation to the initially chosen definition of acids and bases (definition of acidic and basic particles). 

Carbonates are often used for studies in molten halides, which is connected with their basic character, i.e. they are donors of O2-. Since alkaline carbonates are suitable for Lux acid neutralization they are often used as Lux bases (oxide ion donors) in molten halide melts to study the acid-base equilibria [165, 178, 185]. Quantitative estimations of the carbonate ion s basicity in different melts are performed on the assumption of its dissociation according to equation (2.3). 

The fact that the ion activities are measured rather than concentrations is ambivalent from the point of view of practical measurements. It is a great advantage for speciation purposes and for study of acid—base complexation and precipitation equilibria in solution. On the other hand, it is a drawback when the total content of an analyte in a sample is to be foimd, as great care must be taken to compensate or correct for interactions of the analyte with the sample matrix and the ambient atmosphere during calibration and measurement itself (cf. the problem of complexation of fluoride with iron(III) and alumi-num(III) ions in analyses of natural waters, or oxidation of sulfide by atmospheric oxygen). 

Everyday laboratory experience suggests that, with very few exceptions, reactions between acids and bases are extremely fast, since no time lag is observable in the dissociation of acids or bases, buffer action, hydrolysis, etc. In fact, for many purposes proton-transfer reactions involving simple acids and bases are fast enough to be treated as equilibrium processes. However, there are two reasons why the rates of these processes are of interest. In the first place modem techniques have made it possible to measure the rates of extremely fast reactions, with half-times down to about 10" second, and hence to obtain information about the mechanism of such reactions. In the second place, when proton-transfer reactions are coupled with other chemical processes they may lead to slow observable changes, in particular to the catalysis of reactions by acids and bases. The latter type of approach is historically the older, but it is more logical to consider first the direct observation of reactions between simple acids and bases, as will be done in this chapter. Some general features of the experimental results will be described, but detailed consideration of the relations between rates, equilibria, and structures will be deferred until Chapter 10, so as to include the information obtained less directly from studies of acid-base catalysis, described in Chapters 8 and 9. ... 

This chapter will be concerned mainly with the relation between the equilibrium constants of acid-base reactions and their forward and reverse rates. Relations between equilibrium constants and structure have already been considered in Chapter 6, so that the present discussion also implies relations between rates and structure. Moreover, there are many cases in which rates are easier to measure (though more difficult to interpret) than equilibria and can be compared directly with structures. We shall first consider the general basis and experimental evidence for this type of relation, followed by its molecular interpretation, with special reference to exceptional cases. We have seen in the two preceding chapters that the rates of proton-transfer reactions can be measured either directly, or indirectly through the study of acid-base catalysis, and in the following discussion information from both sources will be used indifferently. 

Micellar Effects on Chemical Equflibria.—A few studies have been made of acid-base equilibria in micelles. Hydronium ion activity in anionic micelles has been measured conductimetrically using hydrophilic indicators, it being found that a plot of mn+ versus [H ]-t-[Na ] is linear with a slope of 0.82. The quantity mH+ is defined as the number of micellized hydrogen ions per surfactant head group, namely mH = [H ]tot—[H ]w/ [D]tot c.m.c., where [DJtot is the total catalyst concentration. The use of fluorescent indicators (21a) and (21b) in anionic, neutral, and cationic surfactantspermitted the evaluation of the electrical potential at the micellar surface as a function of added electrolytes. Indicator pK values for mixed micelles and pK values of weak... 

Another important type of equihbrium, which we will study in the second half of the chapter, involves the dissolution and precipitation of shghtly soluble substances. These processes are examples of heterogeneous equilibria that is, they pertain to reactions in which the components are in more than one phase. But first we will conclude our discussion of acid-base equilibria by considering buffer solutions and taking a closer look at acid-base titrations. 

Strauss UP, Vesnaver G (1975) Optical probes in polyelectrolyte studies. 1. Acid-base equilibria of dansylated copolymers of maleic-anhydride and aUtyl vinyl ethers. J Phys Chem 79(15) 1558-1561. doi 10.1021/jl00582a017... 

B. Pilarski, A Dobkowska, H. Foks, T. Michatowski, Modeling of acid-base equilibria in binary-solvent systems A comparative study, Talanta, 80(3) (2010) 1073-1080. 

Studies of Acid-Base, Hydration-Dehydration and Keto-Enol Equilibria in Aqueous Solutions of a-Ketoacids by Polarography, Linear Sweep Voltammetry and Spectroscopy... 

Using these assumptions and conventions, Imoto and co-workers have correlated a number of series of reactions of thiophenes and furans. The reactions studied are the acid-base equilibria pK values) and the acid catalyzed methylations (thiophenes only) of thiophene-and furan-carboxylic acids and the alkaline hydrolyses of their ethyl esters the side-chain bromination of the a-acetylthiophenes, and the a-mercuration of thiophenes and the polarographic half-wave potentials of the methyl esters of thiophene- and furan-carboxylic acids and of nitrothiophenes. The pK values were determined and the ester hydrolyses studied for all three substitution orientations in the thiophene series. For the 4-R-2-Y and 5-R-2-Y series, the p-values do not appear significantly different and the data could probably be combined into a single series unfortunately, however, no limits of accuracy are reported for the p-values, and some of the raw data are not readily available so recalculation is not easily possible. For the 5-R-3-Y series the p-values deviate considerably from the other values however, whereas they are higher for the pK values, they are lower for the ester hydrolyses, and it is possible that the differences are neither systematic nor significant. 

Many studies of such acid-base equilibria of substituted benzenediazonium ions have been reported since the first investigation by Wittwer and Zollinger (1954), namely by Lewis and Suhr (1958 a), Littler (1963), Ritchie and Wright (1971 a, 1971 b), Machackova and Sterba (1972 a), Beranek et al. (1973), Jahelka et al. (1973 a), Vir-tanen and Kuokkanen (1977), BagaPs group (Luchkevich et al., 1986 and references therein), and in DMSO by Petrov et al. (1973). 

The authors studied, as they call it, "acid-base equilibria in glacial acetic acid however, as they worked at various ratios of indicator-base concentration to HX or B concentration, we are in fact concerned with titration data. In this connection one should realize also that in solvents with low e the apparent strength of a Bronsted acid varies with the reference base used, and vice versa. Nevertheless, in HOAc the ionization constant predominates to such an extent that overall the picture of ionization vs. dissociation remains similar irrespective of the choice of reference see the data for I and B (Py) already given, and also those for HX, which the authors obtained at 25° C with I = p-naphthol-benzein (PNB) and /f B < 0.0042, i.e., for hydrochloric acid K C1 = 1.3 102, jjrfflci 3 9. IQ-6 an jjHC1 2.8 10 9 and for p-toluenesulphonic acid Kfm° = 3 7.102( K ms 4 0.10-6) Kmt = 7 3.10-9... 

Acid-base equilibria Partly Near-surface studies tend to investigate fresh or moderately saline water, which creates quite different conditions for acid-base equilibria. Studies of ocean geochemistry come closest to approximating deep-well conditions. 

R. Narayanaswamy and F. Sevilla III, Reflectometric study of the acid-base equilibria of indicators immobilized on a styrene/divinylbenzene copolymer, Anal. Chim. Acta 189,365-369 (1986). 

---