Ss

the maximum value is given by ph = pKa + log|

(b) The concentration of solution is 1.4 mg cm 3, which is 1.4 g dm 3. S0 = 0.5 g dm 3. At pH values below 7, the pH at which S is the maximum solubility is given by

At pH values above 7, the pH at which S is

Thus at pH values between 3.05 and 7.56 the solution containing 1.4 mg cm 3 will precipitate.

5.3 Measurement of solubility

A simple turbidimetric method for the determination of the solubility of acids and bases in buffers of different pH can be used. 9 Solutions of the hydrochloride (or other salt) of a basic drug, or the soluble salt of an acidic compound, are prepared in water over a range of concentrations. Portions of each solution are added to buffers of known pH and the turbidity of the solutions is determined in the visible region. Typical results are shown in Fig. 5.4. Below the solubility limit there is no

Figure 5.4 (a) Plot of extinction against concentration of amitriptyline hydrochloride at pH 9.78 (O) and pH 9.20 (•). (b) Relationship between hydrogen ion concentration and solubility of pecazine (•) and amitriptyline (O).

Reproduced from reference 9.

Figure 5.4 (a) Plot of extinction against concentration of amitriptyline hydrochloride at pH 9.78 (O) and pH 9.20 (•). (b) Relationship between hydrogen ion concentration and solubility of pecazine (•) and amitriptyline (O).

Reproduced from reference 9.

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