Bromothymol blue is an acidic pH chromoionophore where acid to base change corresponds to its neutral to anionic form, respectively. For this reason it can be electrostatically immobilized on the surface on anion-exchanger resin like Amberlite IRA 401.
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Solutions of different concentration of the indicator were used. The time of the adsorption and temperature of the solutions were maintained in all cases. Calibration curves of the obtained sensors based on bromothymol blue attracted to Amberlite IRA 401 ion-exchanger are shown in figure 5.
Figure 5. Calibration curves of pH sensors based on Amberlite IRA 401 related to immobilized dye concentration.
It was found that the working range of BTB was shifted in comparison to the bulk solution <61>, which can be attributed to change in the dissociation constant of the dye during the immobilization procedure. The linear range of the sensor depends on the concentration of the immobilized dye. The narrowest linear range was obtained for the membrane with the largest amount of the indicator, which was also accompanied by the widest dynamic range of the sensor. When the concentration of the indicator decreased the linear range broadened and was shifted to the base range. The dynamic range was also reduced. The figure 5 shows a possibility of the optimization of the sensor by changing the amount of the immobilized indicator. The other way is to change the nature of indicator applied.
Figure 6 shows the response time characteristic of a pH sensor containing the widest dynamic range membrane.
Figure 6. Response time of the pH sensor based on Amberlite IRA 401 (arrows mark points where acid or base were added).
The response of the sensor was rapid (less than 30 s) because the hydrogen ions interacted directly with the indicator molecules immobilized on the surface of the Amberlite beads. There was no additional membrane, which could cause a delay in the response. Typical differences in response time for changes from basic to acidic solutions and vice-versa were observed.
We have chosen another type of Amberlite resin, polymeric adsorbent XAD 1168, as a support in order to investigate its influence on dissociation constant of the dye. An identical, as previously described, procedure of immobilization was applied and a saturated solution of BTB was used. In this case the dye-support binding forces were mainly based on physical sorption interactions. Figure 7 compares calibration curves of the pH sensors based on the membranes prepared from different Amberlite resins.
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The linear range of the pH sensor was shifted in comparison with the previous one due to the different mechanism of the dye immobilization. Although the indicator was adsorbed from identical solution and under comparable conditions (time, temperature) it was found that the sensor based on XAD 1168 exhibited a wider dynamic range. This can be attributed to higher adsorptive properties of this kind of Amberlite, which leads to a greater amount of immobilized reagent.