K

Sn and

S - Sr at pH values below the isoelectric point, and

at pH values above the isoelectric point.

Table 5.8 gives solubility data for oxytetra-cycline (XVI) as a function of pH. Oxytetra-cycline has three pKa values: pKal = 3.27, pKa2 = 7.32, and pKa3 = 9.11, corresponding to the regions 1, 2 and 3 in the structure shown.

The equations for the solubilities of acidic, basic and zwitterionic drugs (equations 5.11, 5.16, 5.23 and 5.24) can all be used to calculate the pH at which a drug will precipitate from solution of a given concentration (or the concentration at which a drug will reach its maximum solubility at a given pH). This is especially important in determining the maximum allowable levels of a drug in infusion fluids or formulations. Some idea of the range of pH values encountered in common infusion fluids is given in Table 5.9. The variation in pH between preparations and within batches of the same infusion fluid (the monograph for Dextrose Infusion BP allows a pH ranging from 3.5 to 5.5) means that the fluids vary considerably in their solvent capacity for weak electrolytes.

Table 5.8 Oxytetracycline: pH dependence of solubility at 20°Ca

It is observed that the zwitterion has the lowest solubility (see Fig. 5.3 and Table 5.8), which we take to be S0.

Table 5.8 Oxytetracycline: pH dependence of solubility at 20°Ca

pH

Solubility (g dm~3)

0 0

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