The Influence of pH on Nonlinear Refractive Index of Bromophenol Blue

Problem statement: Bromophenol blue is a pH indicator, a kind of chemical that changes colour when pH goes up or down over a narrow linear dynamic range of 3.0-4.6 pH units. Therefore a good approach must be taken to extend the linear response of the indicator in order to have simple calibration and precise measurements of PH sensors. The purpose of this experiment is to measure the nonlinear refractive index of Bromophenol blue in a PH range of 2-9. Approach: Samples of Bromophenol blue solutions at an optimized final concentration of 5.25 µg mL^-1 were prepared each having one unit pH intervals from a pH of 2 using 10 mM Tris-Cl as a buffering agent. The nonlinear refractive index response of the samples were investigated at CW laser at 514 nm using z-scan technique. Results: The closed aperture z-scan of the samples displayed a prefocal peak followed by a postfocal valley (self defocusing). A variation of the nonlinear refractive index within a pH range over 2-9 is observed. This reflects the interaction of the Bromophenol molecules with the environment. The nonlinear refractive values measured range was from 1.0×10^-12-5.5×10^-12 m²/w for pH varying from 2-9 pH units. The correlation between the peak of the absorption and measured nonlinear refractive index is reported. Conclusion/Recommendations: The z-scan showed that Bromophenol blue exhibits a nonlinear effect of thermal origin. The experiment illustrated that the nonlinear refractive index measurements can be considered as a good indicator of pH values.