Question
Posted on: March 6 2013Precipitation in a cough drink with promethtazine HCl and ascorbic acid.
Answer
The pH will of course play an important role here and it is therefore useful to know the influence of sodium benzoate on the pH. The concentration of sodium benzoate is approximately 0.14 M The pH can be calculated with the following formula:
pH = pKw-1/2 (pKw – pKa – Log [Bz-]) = 1/2 (pKw + pKa + log [Bz-]) = about 8.6 Of course, the effective pH will be slightly lower due to the presence of acid-reactive salts but I suspect that 600mg ascorbic acid is insufficient to make this solution slightly acidic and thus avoid the precipitation of promethazine. In addition, it is desirable to use as little water as possible.