Lactic acid is very good at penetrating the skin. Because ofthis, it is used in a number of products for skin care, such asexfoliating creams and chemical peels, and in formulations fortransdermal delivery of medications, such as topical ointments andextended release patches for delivery through the skin. Lactic acidhas a pKa of 3.86 in water at 25o C.
After graduation, you land your dream job at a small biotechfirm specializing in novel antimicrobial agents. They have justdeveloped a new drug, Poly-XSeption-L®, to help preventantibiotic-resistant infections during local surgeries.Poly-XSeption-L® is to be applied to the skin at the site of thesurgery in a liquid formulation containing lactic acid.
(a) The initial formulation being tested is a 1.0 wt% solutionof Poly-XSeption-L® in a 0.015 M solution of aqueous lactic acid.Neglecting the effect of the antibiotic on the solution, what wouldyou estimate the pH of this solution to be? State any assumptionsyou make.
(b) In clinical studies, it was found that the acidity of theinitial formulation caused some problems with local irritation, andthat Poly-XSeption-L® was not as effective below a pH of 4.2. It isproposed that sodium lactate be added to create a buffered solutionwith pH = 4.2. What concentration of sodium lactate should be usedto achieve this pH?
