ION-ASSOCIATION TITRATION OF ONIUM PHARMA CEUTICALS USING TETRABROMOPHENOLPHTHALEIN ETHYL-ESTER AS AN INDICATOR

Acrinol, cetylpyridinium chloride and benzalkonium chloride react with tetrabromophenolphthalein ethyl ester (TBPE) to form blue ion associa tes. Also, dl-methylephedrine hydrochloride forms a red ion associate. Both associates can be quickly extracted into 1,2-dichloroethane. Whe n sodium tetraphenylborate in a burette is dropped as a titrant, the o nium compound remaining in the equeous phase forms a colorless ion ass ociate (R4N+. TPB- or R3NH+. TPB-) which is extracted quantitatively. At the end point, the tetraphenylborate anion takes the onium compound away from the colored ion associate in the organic phase and TBPEH, h aving a maximum absorption near to 400 nm, is dissociated. Consequentl y, the color of the organic phase turns to yellow. The color change in the organic phase provides a clear end point.