M

: (BH+)aq + (Cl-)aq to be pushed to the left-hand side, with a resultant decrease in solubility. The solubility of XIX as the hydrochloride decreases from 24 x 10 5 mol dm 3 in 1.3 mmol dm 3 chloride ion, to 3 x 10 5 mol dm 3 in 40 mmol dm 3 chloride ion concentration. It should be noted that the stomach contents are rich in chloride ions. The common ion effect will be apparent in many infusion fluids to which drugs may be added, and therefore the effect of pH as well as electrolyte concentrations must be considered.

Consideration of Table 5.14 suggests that the hydrochloride salts of tetracyclines are always more readily dissolved than the base. The situation is more complex than at first appears, however. In dilute HCl at pH 1.2, the free base dissolves more than the hydro-chloride, probably owing to the differences in crystallinity. The amount of compound

Table 5.14 Aqueous solubilities of tetracycline, erythromycin and Chlorhexidine salts

Compound

Solubility in water

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