Ionic polymerisation as a means of end-point indication in non-aqueous thermometric titrimetry. Part XII. Acetals and cyclic ethers as end-point indicators in the titration of organic bases

Abstract

Dimethoxymethane, 1,3,5-trioxane, 1,3-dioxolane and alicyclic ethers with three-to seven-membered rings were evaluated as thermometric end-point indicators in the non-aqueous titration of weak organic bases including some basic drugs, in the free form and as hydrochlorides, phosphate and tartrate, with perchloric acid in acetic acid.

All the indicators except the three and four-membered-ring cyclic ethers give sharp end-point inflections in titrations of quinuclidine and 8-hydroxyquinoline, but for caffeine only the acetal indicators are satisfactory. Relative standard deviations obtained in replicate determinations of 0.1-mequiv. amounts range from 0.10 to 0.66%, with sample and titrant delivery errors making a significant contribution.

In the drug titrations, the acetal indicators give the sharpest end-point inflections for chloroquine phosphate. Only 1,3-dioxolane is a suitable indicator for the adrenaline hydrogen tartrate titration. The perchloric acid titrant is effective only in the presence of acetic anhydride, when it functions as acetyl perchlorate. Boron trifluoride etherate in 1,4-dioxane can also be used as the titrant/catalyst but is less effective in terms of end-point sharpness.