Question
Posted on: March 6 2013Precipitation in a cough drink with promethtazine HCl and ascorbic acid.
R/promethazineHCl 100mg, codeine phosphate 400mg, sodium benzoate 4g, pseudoefedrineHCl 1.2 g, triprolidine 80mg, raspberry aroma, Sirupus simplex AD 200 ml
< br/> We dissolve prometazineHCl in some water with ascorbic acid (600mg). We also dissolve the other ingredients in water. When merging the two water phases, a precipitation arises. What can this be due?
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.