Ionization repression

The addition of the strong acid induces the protonation of the conjugate base of the weak acid. The weak acid is less ionized when the strong acid is present than when it is alone at the same concentration. This phenomenon is called ionization repression (of the weak acid). The weak acid dissociation is repressed. For example, in a solution of 10 " mol/L of hydrochloric acid and 10 mol/L of acetic acid, calculations based on the principles recalled above give the result [H3O+] = 4.71 X 10 " mol/L. In a solution of acetic acid alone at the same analytical concentration, C = 1.0 x 10 mol/L, calculations give 4.18 x 10 " mol/L. In the mixture of both acids, the acidity due to acetic acid can be calculated by taking away the hydrochloric acid s acidity from the total acidity ... 

There is also an ionization repression of the weak base. 

In the case of the titration of strong acids by strong bases, and vice versa, there is no significant error due to the addition of the indicator, at least when the concentration of the latter is less than 10% that of the titrand. A calculation performed on the same grounds as those that permitted us to study the ionization repression (recul d ionisation, - see Chap. 5) shows that a 10 mol/L of sodium hydroxide and 10 mol/L of methyl orange Ka = 1.6 10 ) exhibits a pH value pH of 10.00. This is exactly the same value as that exhibited by sodium hydroxide alone at the same concentration. 

The nitronium ions produced in this way tend to repress the selfdehydration of the nitric acid and therefore the net concentration of nitronium ions is not proportional to the concentration of the catalyst. When sufficient sulphuric acid has been added to make the self-ioniza-tion of nitric acid relatively unimportant, the nitronium ions will be produced predominantly from the above ionization, and the acceleration will follow a linear law. 

We saw in Chapter 13 that under these conditions, it is usually a good approximation to take [HB] = [HB]0. The approximation is even more accurate when a considerable amount of B-is added, as is the case with a buffer. By Le Chateliers principle, the reverse reaction occurs, the ionization of HB is repressed, and [HB] = [HB]0. 

The presence of H,0+ from an acid will repress this water ionization. The water will ionize less than it does in neutral solution, and the H,0+ generated by the water will be negligible in all but the most dilute acid. The OH" generated by the water will be equally small, but since it is the only OH" present, it still has to be considered. 

Ans. The added C2H,0 " will cause the ionization of HC2H302 to be repressed. The acid will not ionize as much as if the acetate were not present. 

Ans. (a) Na and C2H, 02. (b) The acetate ion should repress the ionization of the acetic acid, so that there would be less hydronium ion concentration in the presence of the sodium acetate, (c ) The hydronium ion concentration dropped from 1.9 x 10 to 2.4 x 10 as a result of the addition of the sodium acetate. 

Problem 21.7 (a) How does one repress the ionization of an a-amino acid (b) The pH at which [anion] = [cation] is called the isoelectric point. In an electrolysis experiment, do a-amino acids migrate at the isoelectric p>oint ... 

Additional base is needed to repress this ionization. The isoelectric point of lysine is basic (pH = 9.6). 

A more strongly acidic solution is needed to repress this ionization. Aspartic acid s isoelectric point is strongly acidic (pH = 2.7). (d) Cystine is a diaminodicarboxylic acid and behaves like a monoaminomonocarboxylic acid. The isoelectric point is slightly acid (pH = 4.6). (e) Tyrosine is a monoaminomonocarboxylic acid containing a phenolic OH, which however is too weakly acidic to ionize to any significant extent. The isoelectric point is slightly acidic (pH = S.6). 

Ionization or dissociation in general can be repressed by adding an excess of a product of the dissociation process. 

Zinc sulphide does not precipitate from an acidified solution because the S ion concentration is repressed by the H+ ions of the strong acid, H2S 2H+ + S, and the solubility product of zinc sulphide cannot be reached. Acetate ions, however, remove H+ ions, and, the hydrogen sulphide thus being allowed to ionize to a greater extent, the solubility product of zinc sulphide is exceeded and the white precipitate appears. (See Solubility Product, page 131, and Experiment 22, page 175.)... 

Ionization of Acetylene. Acetylene is known to possess acidic properties and when it is shaken with heavy water the density of the latter becomes less in alkaline solution indicating an exchange between the hydrogen of the acetylene and the deuterium ions of the water. The exchange does not take place with measurable velocity in acid solutions where the dissociation of the weak acetylene acid is repressed. 

Amine Stoichiometry (Figure 7). It is necessary that a slight excess of amine be employed to achieve high molecular weight. The reasons for this are not at all clear. [A referee suggests this requirement for excess amine may reflect the effect of dissolved TEA HC1 on the polymerization. It may well be that the ionization of phenolic hydroxyls, viz., [—OH] + [TEA] = [—O"] + [TEA H+] is repressed by the increasing TEA.HCl concentration. Additional TEA would be expected to overcome this effect.]... 

OH)2D3 concentrations. Low l,25-(OH)2D3 results in little to no intestinal calcium absorption for soft tissue needs. Thus, blood calcium can be low and secondary hyperparathyroidism develops. The inability to regulate renal handling of phosphate by PTH leads to phosphate-mediated repression of ionized calcium. Serious osteodystrophic lesions occur because of high PTH activity on bone resorption resulting in osteitis fibrosa and osteosclerosis. 

The presence of the strong acid represses the ionization of the weaker acid, just as predicted by LeChatelier s principle. In fact, the presence of any stronger acid will repress the ionization of a weaker one, even if the stronger acid is not strong. I... 

Acids that contain more than one ionizable hydrogen atom per molecule are called polyprotic acids. These acids ionize in steps. The second (or third) proton has a much lower dissociation constant than does the prior proton because it is harder to remove a hydrogen ion the more negatively charged the Brpnsted acid (Table 19.4). Also, the prior ionization produces hydronium ions that repress the further ionization, in accord with LeChatelier s principle. Any acid ionizes less in the presence of a stronger acid (see Example 19.20). Thus, the hydronium ion in a solution of a polyprotic acid comes mainly from the first step in the ionization. 

We conclude that the hydronium ion concentration provided by the second step of the ionization (also equal to 1.1 X 10 M) is totally insignificant compared with that provided by the first step. There are two reasons for that (1) The value of the equilibrium constant is much smaller, and (2) the hydronium ion provided by the first step represses the ionization of the second. 

Polyprotic acids (Section 19.5) ionize stepwise, and the hydronium ion from each step represses the ionization of later steps. The second (and third) steps are essentially weaker because it is harder to remove a proton from a negatively charged species than from a neutral one. However, polyprotic acids follow the usual rules of equilibrium. For example, LeChatelier s principle can be used to predict and explain their behavior. 

We can readily see that hie excess H3O from hie HCl has repressed the ionization of the HC2H3O2 (compare wihi Example 19.7). Any weaker acid is affected in this way in hie presence of a stronger acid. 

The worst offenders, as far as ionization interference goes, are the easily ionizable alkali metals. For example, in the determination of calcium, the absorbance is increased when sodium is added to the mixture since the ionization of sodium represses the ionization of calcium. Obviously, the apparent concentration of calcium will increase as the sodium content increases. The customary method of minimizing this effect is to swamp the system with an easily ionizable element. Typically, both samples and standards are prepared so that the flnal solutions will... 

Ans. (a) Na+ and C2H302 (b) The acetate ion should repress the ionization of the acetic acid, so that there... 

Salts of weak bases or weak acids are hydrolyzed in aqueous solution (see Chap. XI) and they behave as if they contained excess of strong acid and strong base, respectively. According to the law of mass action the presence of one acid represses the ionization of a weaker one, so that the effective conductance of the water, which is due mainly to carbonic acid, is diminished. The solvent correction in the case of a salt of a weak base and a strong acid should thus be somew hat less than the total conductance of the w ater. For solutions of salts of a w eak acid and a strong base, vhich react alkaline, the correction is uncertain, but methods of calculating it have been described they are based on the assumption... 

The reason for this is that the common anion, i.e., the acetate ion, represses the dissociation of the acetic acid. With further addition of alkali, however, the conductance of the highly ionized salt soon exceeds that of the weak acid which it replaces, and so the specific conductance of the solution increases. After the equivalence-point there is a further increase of conductance because of the excess free alkali the curve is then parallel to the corresponding part of Fig. 24. 

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