Tonicity of a solution and pH appear to be independent but there is some bearing of one over the other. Drugs in solutions may ionize and dissociate at alkaline pH. Solutions having a pH value between 6.5 to 7.5 are very well tolerated by the eye.
However, solutions having a pH as low as 4 and as high as 10 are known to be prescribed and instilled without extraordinary pain and irritation. Pain and irritation in the eye due to acidic drug solutions are immediately felt on instilling the solution and these results into tear secretion which is slightly alkaline.
The alkaline tear fluid gradually neutralizes the acidity resulting in the reduction of irritation and pain.
The degree of the tear secretion depends on the extent of acidic nature and the volume of the solution instilled. Buffer solutions are included in the eye solutions sometimes which serve many a purpose maximizing therapeutic activity of the medicament, bringing about stability of the solution, reducing pain etc.
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Lot of investigations has been recently carried out with regard to the choice of buffers to be included in eye drops and eye washes. However, this area relates more to the manufacturer than the practicing pharmacist as he faces lesser problems of stabilizing than the manufactured products.
Sodium EDTA (Sodium ethylene diamine tetraacetate) in a concentration of 0.1% acts as a good preservative of eye solutions as performs antibacterial action as well. Very often it is used in conjunction with boric acid.
Salts of the alkaloid-pilocarpine are more stable in presence of phosphate buffer. Sodium borate another substance employed for preservation of eye solutions.