Shifts in the pKa value of acid–base indicators caused by immobilization on solid substrates via water-soluble polycationic polymers: a case study of Congo Red

Objectives. Herein, the effects of cationic polyelectrolytes on the properties of solid substrate immobilized acid–base indicators are investigated to predict shifts in their spectral patterns and characteristics.

Methods. The properties of the silica gel immobilized indicator dye in a solution of the cationic polyelectrolyte were studied using automatic photometric titration in the visible region and spectrophotometry using a specialized computerized setup.

Results. The measured pKa value of the immobilized dye, which had shifted by three units to the acidic region, was very similar to the pKa value observed for the indicator in the modifying polymer solution. The observed change in pKa of the immobilized dye and the influence of the solution’s ionic strength were attributed to the local electric potential of the polymer globule. In contrast to the processes associated with covalent immobilization, the effect exerted by the solution’s ionic strength on the indicator reaction diminishes, which, in turn, affects the measured values obtained.

Conclusions. The creation of a sensor for continuous visualization of pH levels based on Congo Red immobilized on silica gel was described. Here, a color transition was noted between pH 1 and 4. These materials can be used to monitor metal extraction processes from industrial effluents or to optimize the extraction of valuable actinides. The approach demonstrated in this work can be applied to immobilize other indicators for pH level monitoring purposes or the production of sensors for other analytes.