Constants for Weak Monoprotic Acids and Bases

O Solutions containing a universal indicator. A universal indicator shows a wide range of colors as pH varies. The pH values are given by the black numbers. These solutions range from quite acidic [upperleft) to quite basic [lowerright). 

O Universal pH paper has been soaked in a universal indicator, allowing a wide range of pH s to be measured easily by comparing the test strip to the color scale on the container. 

18-4 Ionization Constants for Weak Monoprotic Acids and Bases 

Several weak acids are famihar to us. Vinegar is a 5% solution of acetic acid, CHjCOOH. Carbonated beverages are saturated solutions of carbon dioxide in water, which produces carbonic acid. 

The pH of some common substances is shown by a universal indicator. Refer to F ure 18-2 to interpret the indicator colors. 

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Use ionization constants for weak monoprotic acids and bases... 

IONIZATION CONSTANTS FOR WEAK MONOPROTIC ACIDS AND BASES... 

The answer is in the expression for the ionization constant, K l or Kb, where the ratio of the conjugate acid and base concentrations is found. In the case of a weak monoprotic acid, HA, we have the following ... 

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 theory of titrations between weak acids and strong bases is dealt with in Section 10.13, and is usually applicable to both monoprotic and polyprotic acids (Section 10.16). But for determinations carried out in aqueous solutions it is not normally possible to differentiate easily between the end points for the individual carboxylic acid groups in diprotic acids, such as succinic acid, as the dissociation constants are too close together. In these cases the end points for titrations with sodium hydroxide correspond to neutralisation of all the acidic groups. As some organic acids can be obtained in very high states of purity, sufficiently sharp end points can be obtained to justify their use as standards, e.g. benzoic acid and succinic acid (Section 10.28). The titration procedure described in this section can be used to determine the relative molecular mass (R.M.M.) of a pure carboxylic acid (if the number of acidic groups is known) or the purity of an acid of known R.M.M. 

Figure 4.1 Solubility versus pH for a monoprotic weak base. A constant amount of weak base is considered and the pH is adjusted with a strong acid or strong base.

When two immiscible solvents are placed in contact with each other and a non-ionizable compound is dissolved in one of the solvents, the compound distributes itself between the two solvents. This distribution is referred to as partitioning. The ratio of the concentrations of the compound in each phase is a constant for a specific set of solvents, pH, buffers, buffer concentrations, ionic strength and temperature. This ratio is referred to as a partition coefficient or distribution coefficient and is equal to the ratio of the solubilities in the two solvents. When the compound is a weak acid or base, the distribution of the compound can be shown to be given by the following equation for a monoprotic compound ... 

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 ... 

Figure 5.32 shows the pH-pseudo-ftrst-order rate constant profile of the degradation of aspartame (an ampholyte, pKal = 3.19 and pKa2 = 8.14). In the hydrolysis of aspartame, the protonated/undissociated form predominates at low pH (<3), while the deprotonated/dissociated form exists at high pH (>8). As demonstrated for monoprotic weak acids and weak bases, certain terms in the numerator of Equation (5.173) become negligible. For example, the hydrolysis of the protonated/undissociated form by OH-, of the protonated/dissociated form by H+ and OH-, and the deprotonated/dissociated form by H+ are not likely to occur. Then, Equation (5.173) for the ampholytic drug is written as ... 

Thevaluesof k2+ = 2.05xl0 5sec-1M-1 and k0 = 1.50 sec 1M 1 are estimated. Substituting these values into Equation (5.174) followed by nonlinear regression analysis gives other k values. Intuitively from the profile of pH 2 to 4, one can assume kj 0 = 0. As demonstrated for monoprotic and diprotic weak acids and weak bases, the pH-rate constant profile is dependent on the kinetic pathway of the hydrolysis. It can be seen from Figure 5.32 that at pH 3 to 4 and pH 4 to 9, k2>0 and k0 0 are the predominant processes of the hydrolysis of aspartame, respectively. 

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. 

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- ... 

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