Diffusion of drugs in solution

Buffer solutions are widely used in pharmacy to adjust the pH of aqueous solutions to that required for maximum stability or that needed for optimum physiological effect. Solutions for application to delicate tissues, particularly the eye, should also be formulated at a pH not too far removed from that of the appropriate tissue fluid, as otherwise irritation may be caused on administration. The pH of tears lies between 7 and 8, with an average value of 7.4. Fortunately, the buffer capacity of tears is high and, provided that the solutions to be administered have a low buffer capacity, a reasonably wide range of pH may be tolerated, although there is a difference in the 

We have seen from Fig. 3.9 that the buffer capacity is at a maximum at a pH equal to the pK of the weak acid used in the formulation of the buffer system and decreases appreciably as the pH extends more than one unit either side of this value. If, instead of a single weak monobasic acid, a suitable mixture of poly-basic and monobasic acids is used, it is possible to produce a buffer which is effective over a wide pH range. Such solutions are referred to as universal buffers. A typical example is a mixture of citric acid (pJC i = 3.06, pK,2 = 4.78, pK,3 = 5.40), Na HPO (pK, of conjugate acid H2PO4 = 7.2), diethylbarbituric acid (pKji = 7.43) and boric acid (pK i = 9.24). Because of the wide range of pK, values involved, each associated with a maximum buffer capacity, this buffer is effective over a correspondingly wide pH range (pH 2.4-12). 

Diffusion is the process by which a concentration difference is reduced by a spontaneous flow of matter. Consider the simplest case of a solution containing a single solute. The solute will spontaneously diffuse from a region of high concentration to one of low concentration. Strictly speaking, the driving force for 

The relationship between the radius, a, of the diffusing molecule and its diffusion coefficient (assuming spherical particles or molecules) is given by the Stokes-Einstein equation as 

The diffusional properties of a dmg have relevance in pharmaceutical systems in a consideration of such processes as the dissolution of the dmg and transport through artificial (e.g. polymer) or biological membranes. Diffusion in tissues such as the skin or in tumours is a process which relies on the same criteria as discussed above, even though the diffusion takes place in complex media. 

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