Physicochemical Properties of Drugs in Solution

Abstract

In this chapter we will examine some of the physicochemical properties of drugs in aqueous solution which are of relevance to such liquid dosage forms as parenteral solutions, eye drops and irrigations. Some basic thermodynamics will be introduced and the concept of thermodynamic activity, which is an important parameter in determining drug potency, will be discussed. It is important that parenteral solutions are formulated with osmotic pressure similar to that of blood serum; in this chapter we will see how to adjust the tonicity of the formulation to achieve this aim. Most drugs are at least partially ionised at physiological pH and many studies have suggested that the charged group is essential for biological activity. We will look at the influence of pH on the ionisation of several types of drug in solution and consider equations that allow the calculation of the pH of solutions of these drugs.