Monoprotic Acid-Base Equilibria

Roberts, and J. D. Watson. Molecular Biology of the Cell, 2nd ed. (New York Garland Publishing. 1989.] 

Rosenzweig, and Z Rosenzwetg, Synthesis, Characterization, and Application of Fluorescence Sensing Upobeads for Intracellular pH Measurements. And. Chem. 2001, 

What could be easier than calculating the pH of 0.10 M HBr HBr is a strong acid, so the reaction 

K Example Activity Coefficient in a Strong-Acid Calculation 

Raman spectrum of aqueous HN03 at 25 C. Signals at 1 360,1 049. and 720 cm 1 arise from N03 anion. Signals denoted by asterisks are from undissociated HN03. The wavenumber unit, cm, is 1/wavelength. 

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Monoprotic Acid/Base Equilibria in Mixed Solvents... 

Monoprotic Acid—Base Equilibria in Water Solutions... 

In case of a weak monoprotic acid-base analyte, two different species are retained the protonated acid form (HA, uncharged or cationic) and the deprotonated conjugate basic form (A, uncharged or anionic). In other words, if HA is uncharged, A is anionic if HA is charged, A is uncharged. The ionization of the analyte is described by the following equilibrium ... 

Method development in MLC can be largely potentiated by extending the modeling of the retention along the pH scale (Fig. 8.1). Arunyanart and Cline-Love [4], and Rodgers et al. [5,6], studied the effect of pH on the MLC retention of weak acids and bases. These authors combined the equations that describe the acid-base equilibrium for a monoprotic system in water ... 

Weak Monoprotic Acid-Bases. It has been traditional in elementary texts to use molarity equilibrium constant expressions and to make approximations for the numerical values of the concentrations of all species except the one to be calculated, usually H. This is a valid approach, but difficult for students because of the variety of assumptions used in making the approximations. Let us start by looking at the complete treatment and then showing under what conditions certain approximations will be justified. Until experience is achieved, the best method is to try the approximations and then check the result in the complete equation. 

The simplest example of the application of graphical representation of equilibrium data is that for acid-base equilibria involving a monoprotic acid, such as the acid HA, for which the equilibrium expression for solution in water may be written in terms of a concentration acidity constant, that is, an acidity constant valid at the appropriate temperature and corrected for activity by, f3r example, the Guntelberg approximation ... 

Strategy Determining the equilibrium concentrations of the species of a diprotic acid in aqueous solution is more involved than for a monoprotic acid. We follow the same procedure as that used for a monoprotic acid for each stage, as in Example 15.8. Note that the conjugate base from the first stage of ionization becomes the acid for the second stage ionization. 

Strategy Follow the same procedure for each ionization as for the determination of equilibrium concentrations for a monoprotic acid. The conjugate base resulting from the first ionization is the acid for the second ionization, and its starting concentration is the equilibrium concentration from the first ionization. 

An acid dissociation constant is a particular example of an equilibrium constant. For the specific equilibrium between a monoprotic acid, HA and its conjugate base A , in water,... 

Besides equilibrium constant equations, two other types of equations are used in the systematic approach to solving equilibrium problems. The first of these is a mass balance equation, which is simply a statement of the conservation of matter. In a solution of a monoprotic weak acid, for example, the combined concentrations of the conjugate weak acid, HA, and the conjugate weak base, A , must equal the weak acid s initial concentration, Cha- ... 

Consider again the titration of a monoprotic weak acid, ITA, with a strong base. At any point during the titration the weak acid is in equilibrium with 1T30+ and A ... 

The dependence of the equilibrium constants of polyprotic weak acids and weak bases on ionic strength allows them to be treated in exactly the same manner as monoprotic weak acids and weak bases. The first, second, and third ionization constants for triprotic weak acids in terms of activity coefficient are given by ... 

The constant Kaz is very small, and so the equilibrium lies significantly to the right. Because Kai, hydrolysis of HP04 is suppressed by the OH from the first step, and the pH of P04 can be calculated just as for a salt of a monoprotic weak acid. However, because Kaz is so small, Kf, is relatively large, and the amount of OH- is not negligible compared with the jnitial concentration of P04 - (Cb-), and the quadratic equation must be solved, that is, P04 is quite a strong base. 

The strength of an acid or a base is quantified by the value of the equilibrium constant for the reaction describing its ionization in water. As discussed on page 737, a monoprotic Bronsted-Lowry acid may be represented by the... 

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