Spectroscopic behavior of acid-base indicators after immobilization on glass supports

This paper describes an extensive spectrophotometric study on several acid-base indicators immobilized, by means of a silylation process, on quartz powder and on controlled pore glass. The spectrophotometric properties of the immobilized indicators are at variance with those of the free indicators with respect to pH dependence and ranges. The different behavior of both types of indicators in the liquid and immobilized phases may be explained by both a modification in the electronic structure of the indicators and a distribution of the molecules on different sites. The differences among the spectrophotometric properties of the indicators, when these are immobilized on different solid supports, can be justified by taking into account the steric hindrance and the reactivity of the indicators. Particular attention was paid to the pH-sensitive bands and their position, keeping in mind that the investigated material could possibly be used as a transducer for pH fiber-optic sensors.