In vitro pH effects

The titratable acidity or alkalinity of a system may be more important than pH itself in determining compatibility and stability. For example, an autoclaved solution of dextrose may have a pH as low as 4.0, but the titratable acidity in such an unbuffered solution is low. 

The solubility of calcium and phosphate in total parenteral nutrition (TPN) solutions is dependent on the pH of the solution. TPN solutions are, of course, clinically acceptable only when precipitation can be guaranteed not to occur. Dibasic calcium phosphate, for example, is soluble only to the extent of 0.3 g dm whereas monobasic calcium phosphate has a solubility of 18 g dm . At low pH the monobasic form predominates, while at higher pH values the dibasic form becomes available to bind with calcium and precipitates tend to form.  

Calcium solubility curves for TPN solutions containing 1.5% (w/v) amino acid and 10% (w/v) dextrose at pH 5.5 are shown in Fig. 10.1. The broken straight lines show the calcium and phosphate concentrations at 3 1 and 2 1 ratios. The dotted curve for Aminosyn solutions shows the concentrations at which precipitation occurs after 18 h at 25°C followed by 30 min in a water bath at 37°C. The full curve is for TrophAmine solutions, and represents calcium or phosphate concentrations at which visual or microscopic precipitation or crystallisation occurs. Compositions to the left of the curves represent physically compatible solutions. 

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