Copolymeric hexyl acrylate-methacrylic acid microspheres – surface vs. bulk reactive carboxyl groups. Coulometric and colorimetric determination and analytical applications for heterogeneous microtitration

Copolymeric acrylate microspheres were prepared from hexyl acrylate using different amounts of methacrylic acid, resulting in a series of microspheres of gradually changing properties. The distribution of carboxyl groups – between surface and bulk of microspheres was evaluated. Bulk reactive carboxyl groups were determined using reverse coulometric titration with H+ ions, following hydroxide ions have been generated and allowed to react with microspheres in the first step. It was found that the number of reactive carboxyl groups available in copolymeric microspheres is lower compared to number of methacrylic acid units used for polymerization process. Moreover, there is correlation between the number of groups introduced and found to be reactive in microspheres. On the other hand, the number of surface reactive groups was proportional to the number of groups introduced in course of polymerization. Thus, the surface reactive groups can be used as reagent, in novel heterogeneous microtitration procedure, in which a constant number of microspheres of different carboxyl groups contents is introduced to the sample to react with the analyte. The applicability of novel proposed method was tested on the example of Ni2+ determination.