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Question

Posted on: March 6 2013

Precipitation in a salicylic acid solution for topical use salicylic acid 4.00 g Sulpho resorcinol 3.00 g propylene glycol 10.00 g ethanol 96 ° 30.00 ml Aqua AD 100.0 ml

< br/> We made the preparation as follows:
  1. Salicylic acid in 30 ml disinfectant alcohol
  2. resorcine in water
  3. add propylene glycol to 2., then add 1.
The solution of salicylic acid in drinking became cloudy after adding the aqueous solution of resorcine. After warming up the precipitation was gone, but after 1 day of storage we found back a needle-like precipitation. The preparation with 3 G salicylic acid and without propylene glycol we have to make more often and succeed perfectly...

Answer

It is clear that 4g salicylic acid is too much for the mixture of alcohol-propylene glycol-water as prescribed. The most appropriate is to increase the alcohol concentration. But the required quantity is not known and difficult to calculate. Therefore, in a measuring cylinder in the mixture of 10g propylene glycol and 30ml of alcohol, we dissolve the salicylic acid and the Sulpho resorcinol and slowly dilute this solution with water. We will check if a cloudy and as soon as it appears we add extra 10ml of alcohol and further dilute it with water. If there is again a cloudy form then we add 1 or 2 times 5ml of alcohol of course the total volume in the eye.