The hydrogen ion concentration of the solution in which an ophthalmic drug is dissolved may alter the therapeutic results of the drug itself. A variation of the pH often determines the speed and the quantity of absorption of the drug and the amount of irritation which the patient experiences on instillation.
However, not only the pH but the osmotic pressure of the resultant solution is important. The lacrimal fluid is alkaline, with a pH of 7.4, and is isotonic with a 1.4 per cent solution of sodium chloride. This means that the two fluids have the same osmotic pressure and in this instance the same freezing point, at 0.9 C. A hypotonic solution will cause passage of fluid into the ocular tissues, bringing about congestion in an attempt to reach an osmotic balance. A hypertonic solution, on the other hand, will cause the removal from the tissues
ELVIN NC. THE pH AND TONICITY OF OPHTHALMIC SOLUTIONS. Arch Ophthalmol. 1943;29(2):273–277. doi:10.1001/archopht.1943.00880140119007
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