Salts of weak bases

Neutralisation reactions, or addimetry and alkalimetry. These include the titration of free bases, or those formed from salts of weak acids by hydrolysis, with a standard acid (addimetry), and the titration of free acids, or those formed by the hydrolysis of salts of weak bases, with a standard base (alkalimetry). The reactions involve the combination of hydrogen and hydroxide ions to form water. 

Cations of weak bases (i.e. Bronsted acids such as the phenylammonium ion C6H5NH3) may be titrated with strong bases, and the treatment is similar. These were formerly regarded as salts of weak bases (e.g. aniline (phenylamine), Kb = 4.0 x 10 10) and strong acids an example is aniline hydrochloride (phenylammonium chloride). 

Solutions which prevent the hydrolysis of salts of weak acids and bases. If the precipitate is a salt of weak acid and is slightly soluble it may exhibit a tendency to hydrolyse, and the soluble product of hydrolysis will be a base the wash liquid must therefore be basic. Thus Mg(NH4)P04 may hydrolyse appreciably to give the hydrogenphosphate ion HPO and hydroxide ion, and should accordingly be washed with dilute aqueous ammonia. If salts of weak bases, such as hydrated iron(III), chromium(III), or aluminium ion, are to be separated from a precipitate, e.g. silica, by washing with water, the salts may be hydrolysed and their insoluble basic salts or hydroxides may be produced together with an acid ... 

The preparation of salts of organic compounds is one of the most important tools available to the for-mulator. Compounds for both IM and IV solutions may require high solubility in order for the drug to be incorporated into acceptable volumes for bolus administration (see Table 1). Sodium and potassium salts of weak acids and hydrochloride and sulfate salts of weak bases are widely used in parenterals requiring highly soluble compounds, based on their overall safety and history of clinical acceptance. 

The unequal basicities of the three nitranilines can be illustrated by the following experiment. It is a general property of the salts of weak bases—as well as of weak acids—that in aqueous solution they are stable only if an excess of acid (or alkali) is present. When such solutions are diluted with water hydrolysis occurs as a result of the operation of the law of mass action. In the present case this phenomenon shows itself in the appearance of the yellow colour characteristic of the bases and finally, since the nitranilines are sparingly soluble in water, in their precipitation in crystalline form. The weaker the base the smaller is the amount of water which must be added in order to make the hydrolysis perceptible. 

If a salt consists of the cation of a weak base and the anion of a strong acid, only the cation reacts with water. The solution has a pH that is less than 7. Salts of weak bases and strong acids dissolve in water and form acidic solutions. 

Acids and bases react to give salts and (usually) water. That is one definition of a salt. So, when we say salt solution, we are really talking about solutions that contain the conjugate acid or base of some other acid or base. The pH of a salt solution depends on the acid/base strength of the acid or base from which it was derived. There are three permutations on the problem salts of strong ac-ids/bases, salts of weak acids, and salts of weak bases. Let s consider them each in turn. 

Salts of Strong Acids/Bases 241 Salts of Weak Acids 242 Salts of Weak Bases 244 Extreme Concentrations 245 Other Acidic Species 247 Buffers 248... 

Note 2. Hydroxylamine hydrochloride is a salt of weak base and strong acid and undergoes hydrolysis at pH 5-7. Thus, the reaction mixture is adjusted to pH 4 with hydrochloric acid to prevent the hydrolysis of this salt. 

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... 

Similarly, salts of weak bases (the conjugate acid of weak bases) react with %vater to produce the weak parent base and H ions. 

The salts of weak bases and strong acids (for example, NH4C1) hydrolyze. Their solutions are acidic. 

Monoprotic Acids and Bases 18-10 Salts of Weak Bases and Weak... 

Apply acid-base equilibrium concepts to salts of weak bases and weak acids... 

Salts of weak bases and weak acids are the fourth class of salts. Most are soluble. Salts of weak bases and weak acids contain cations that would give acidic solutions and anions that would give basic solutions. Will solutions of such salts be neutral, basic, or acidic They may be any one of the three depending on the relative strengths of the weak molecular acid and weak molecular base from which each salt is derived. Thus, salts of this class may be divided into three types that depend on the relative strengths of their parent weak bases and weak acids. 

Salts of weak bases and weak acids for which is greater than are always basic because the anion of the weaker acid hydrolyzes to a greater extent than the cation of the stronger base. 

Weak Bases Plus Salts of Weak Bases... 

Example 19-3 Weak Base/Salt of Weak Base Biffir Solution... 

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