Aqueous solution Acid-base equilibria

Reviewed herein are some of the fundamental concepts associated with chemical equilibrium, chemical thermodynamics, chemical kinetics, aqueous solutions, acid-base chemistry, oxidation-reduction reactions and photochemistry, all of which are essential to an understanding of atmospheric chemistry. The approach is primarily from the macroscopic viewpoint, which provides the tools needed by the pragmatist. A deeper understanding requires extensive treatment of ihe electronic structure of matter and chemical bonding, topics that are beyond the scope of this introductory text. This book can be used for either self-instruction, or as the basis for a short introductory class... 

It is well known that the rates of all azo coupling reactions in aqueous or partly aqueous solutions are highly dependent on acidity. Conant and Peterson (1930) made the first quantitative investigation of this problem. They demonstrated that the rate of coupling of a series of naphtholsulfonic acids is proportional to [OH-] in the range pH 4.50-9.15. They concluded that the substitution proper is preceded by an acid-base equilibrium in one of the two reactants, which was assumed to be the equilibrium between the diazohydroxide and the diazonium ion, in other words, that the reacting equilibrium forms are the undissociated naphthol and the diazohydroxide. 

C17-0025. Write the acid-base equilibrium that determines the pH of aqueous solutions of each of the following salts, and state whether the resulting solution is acidic, basic, or neither (a) NH4 I (b) NaClOq and (c) NaCHg CO2. ... 

For cryptands in which the molecular cavity is larger than in the case of the [l.l.l]-species [78], proton transfer in and out of the cavity can be observed more conveniently. Proton transfer from the inside-monoprotonated cryptands [2.1.1] [79], [2.2.1] [80], and [2.2.2] [81 ] to hydroxide ion in aqueous solution has been studied by the pressure-jump technique, using the conductance change accompanying the shift in equilibrium position after a pressure jump to follow the reaction (Cox et al., 1978). The temperature-jump technique has also been used to study the reactions. If an equilibrium, such as that given in equation (80), can be coupled with the faster acid-base equilibrium of an indicator, then proton transfer from the proton cryptate to hydroxide ion... 

Before examining the equilibrium behavior of aqueous solutions of weak bases, let s look at the behavior of water itself. In the initial discussion of acid—base equilibrium above, we showed water acting both as an acid (proton donor when put with a base) and a base (proton acceptor when put with an acid). Water is amphoteric, it will act as either an acid or a base, depending on whether the other species is a base or acid. But in pure water the same amphoteric nature is noted. In pure water a very small amount of proton transfer is taking place ... 

Generally, it has been found that the organic acids and bases do exist in aqueous solution as equilibrium mixtures of their respective neutral as well as ionic forms. Thus, these neutral and ionic forms may not have the same identical partition coefficients in a second solvent therefore, the quantity of a substance being extracted solely depends upon the position of the acid-base equilibrium and ultimately upon the pH of the resulting solution. Hence, extraction coefficient (E) may be defined as the ratio of the concentrations of the substance in all its forms in the two respective phases in the presence of equilibria and it can be expressed as follows ... 

In aqueous solutions, the peak potentials of the oxidation of thiols vary with pH (Aiip/ApH = 60 mV), reflecting the position of the acid-base equilibrium affecting the SH group. In basic solutions. 

HPTS is a highly water-soluble, pH-sensitive dye with a pK of 7.5 in aqueous solution [8], When in alkaline medium, pH > 7.5, acid-base equilibrium is totally displaced toward the anion form (3sPyO ) of the dye. The electronic character of 3sPyO remains unchanged after photo-excitation, and corresponds to a singlet-excited state [9], Fluorescence from this state undergoes a fast 0.4 ps Stokes shift and has a maximum at 515 nm and a lifetime of 5.3 0.1 ns [10],... 

Chlorine in aqueous solution can be present as different species depending on the acidity (Scheme 4.5). At 25°C and below pH ss 4, the predominant species is Cl2(aq), and Cl3—(aq) when Cl- is present in the range pH ss 4-7.5, hypochlorous acid (HOC1) is the major species, which is in acid-base equilibrium with its conjugate base, hypochlorite (CIO-) hypochlorite is the predominant species above pH 7.5. 

In aqueous solution, organic acids and bases exist in equilibrium mixtures in their neutral and ionic forms. Because the neutral and ionic forms will not have the same partition coefficient, the amount extracted depends on the acid-base equilibrium. For an efficient extraction, the analyte should be at least 95% in the extracable form. This would usually mean either as its free acid or free base. Figure 2.1 is a nomogram relating pK values to percentage of ionization at various pH values [21]. In most cases, pH adjustment of the sample to pH = pK — 2 for acidic compounds or pH = pK + 2 for basic compounds is sufficient. 

Carboxylic acids are weak acids in aqueous solution, forming an equilibrium between the free acids and the carboxylic ion. In the presence of a base like sodium hydroxide or sodium hydrogen carbonate, they ionise to form water-soluble salts and this provides a method of separating carboxylic acids from other organic compounds. 

Fig. 4-2. One-dimensional Gibbs energy diagram for the acid/base equilibrium reaction between the ammonium ion and trimethylamine in the gas phase (top) and in aqueous solution (bottom) [115].

In this chapter we have encountered many different situations involving aqueous solutions of acids and bases, and in the next chapter we will encounter still more. In solving for the equilibrium concentrations in these aqueous solutions, you may be tempted to create a pigeonhole for each possible situation and to memorize the procedures necessary to deal with each particular situation. This approach is just not practical and usually leads to frustration Too many pigeonholes are required, because there seems to be an infinite number of cases. But you can handle any case successfully by taking a systematic, patient, and thoughtful approach. When analyzing an acid-base equilibrium problem, do not ask yourself how a memorized solution can be used to solve the problem. Instead, ask yourself this question What are the major species in the solution, and how does each behave chemically ... 

Trihydroxybenzoic acid and some substituted salicyclic acids are decarboxylated much faster and their rates can be studied using dilute aqueous solutions of strong acids. In these examples, the first-order rate coefficient, k = rate/Cs, is found to be proportional to the fraction of non-ionized acid, ArCOOH, in the acid—base equilibrium of the substrate [239, 244, 245], viz. 

In aqueous solution, water is both an acid and a base and we deal with a double acid-base equilibrium... 

Since water is a much stronger base than alcohol, all hydrogen ions remain in the form of H2OH+ even after the addition of alcohol to an aqueous solution. Alcohol can, however, displace the acid-base equilibrium in aqueous solutions and this influence which alcohol exerts upon the color of an indicator depends not only upon the nature of the indicator but also upon the kind of acid-base system found in the solution. 

The concepts and equations of acid-base dissociation have referred chiefly to aqueous solutions. Recently, interest in the behavior of acids and bases in solvents other than water has increased considerably. The classical definition of an acid and a base, which is satisfactory for water solutions, is too limited for other solvents. Because of the great importance of the general question of the acid-base equilibrium, the clear and fruitful views of Bronsted are exhaustively considered in a special (fourth) chapter. ... 

The importance of water as a medium for inorganic reactions stems not only from the fact that it is far more readily available than any other solvent, but also because of the abundance of accurate physicochemical data for aqueous solutions compared with the relative scarcity of such data for solutions in non-aqueous solvents. This chapter is concerned mainly with equilibria and we begin by reviewing calculations involving acid-base equilibrium constants. 

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