Abstract
A pH colorimetric sensor was prepared and characterized by combining two acid-base indicators, Tetrabromophenol blue (TBB) and Phenol Red (PR) with molar ratios ranging from 0 to 0.47. Indicators were embedded in Organically Modified Silicate (OrMoSil) spots supported in Polyvinylidene fluoride (PVDF) together with a suitable cationic surfactant. The deposition procedure was very repeatable allowing a single calibration for all membranes built in the same preparation lot. The signal was based on the Hue profile (H) vs. pH. The CIE-xy color space contains an achromatic point that is the key point of the colorimetric sensor behavior as it causes the inversion of the slope of the H calibration curve at a specific inversion point. For this reason, the identification of the effective conditions of the improvement of the PR analytical performances are not trivial, 0.186 < nTBB/nPR < 0.324 with prediction error in the range 0.01–0.16 pH unit. The proposed adjustment allowed the construction of pH colorimetric sensors with prediction error comparable to that of potentiometric measurements. The obtained results indicated the route to widen the availability even to those acid-base indicators lacking of suitable color transitions but having suitable pKa values.
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