Ionic polymerisation as a means of end-point indication in non-aqueous thermometric titrimetry. Part XI. The reaction mechanism of iodimetric end-point indication and an evaluation of a copolymerisation indicator reaction

Abstract

The mechanism of the reactions marking the end-point when ethyl vinyl ether is used as the indicator reagent in the thermometric titration of iodinereactive analytes with solutions of iodine in dimethylformamide has been investigated by capillary gas chromatography - mass spectrometry. Three major constituents, ICH₂CH(OEt)CH₂CH(OH)(OEt), I[CH₂CH(OEt)]₂CH₂CH-(OH)(OEt) and I[CH₂CH(OEt)]₃CH₂CH(OH)(OEt) have been identified, confirming the occurrence of a polymerisation process initiated by the iodonium ion. The hydroxy group in the molecules arises from the use of aqueous thiosulphate to terminate the polymerisation.

An improvement in end-point sharpness using this catalytic thermometric procedure is effected by using a mixture of ethyl vinyl ether and 1,3-dioxolane as the indicator reagent instead of ethyl vinyl ether alone. The temperature rise in this copolymerisation reaction is greater than in the ethyl vinyl ether homopolymerisation, and the end-point inflection is sharper.