Potentiometric titration is a method for determining the end point of titration by measuring the change in potential during the titration. Compared with the direct potentiometric method, the potentiometric titration method does not need to accurately measure the electrode potential value. Therefore, the influence of temperature and liquid junction potential is not important. The accuracy is better than the direct potential method. The ordinary titration method relies on the color change of the indicator to indicate the end point of the titration. If the solution to be tested has color or turbidity, the indication of the end point is more difficult, or no suitable indicator can be found at all. . Potentiometric titration is based on a sudden jump in the electrode potential to indicate the end point of the titration. Before and after the titration to the end of the operation of the potentiometric titrator, the concentration of the ions to be measured in the drip is continuously changed by n orders of magnitude, causing a jump in the potential, and the content of the measured component is still calculated by consuming the amount of the titrant.
Potentiometric titration can be used for acid-base titration, redox titration, titration and precipitation titration using different indicator electrodes. The acid-base titration uses a PH glass electrode as an indicator electrode, and in a redox titration, a platinum electrode can be used as an indicator electrode. In the titration, if EDTA is used as the titrant, a mercury electrode can be used as the indicator electrode. In the precipitation titration, if the halide ion is titrated with silver nitrate, a silver electrode can be used as the indicator electrode. During the titration process, as the titrant is continuously added, the electrode potential E changes continuously, and when the electrode potential suddenly jumps, the titration reaches the end point. It is easier to determine the end point of the titration with a differential curve than a normal titration curve.
If an automatic potentiometric titrator is used, the titration curve can be automatically drawn during the titration process, the titration end point is automatically found, the volume is automatically given, and the titration is quick and convenient.
Potentiometric titration is performed, a reference electrode is inserted into the solution to be tested, and an indicator electrode constitutes a working battery. With the addition of the titrant, the concentration of the measured ions changes continuously due to the chemical reaction, and the potential of the indicating electrode also changes accordingly. A sudden jump in potential occurs near the equivalence point. Therefore, the change in the electromotive force of the working battery can be measured to determine the end point of the titration.
The basic instrumentation for potentiometric titration includes a burette, a titration cell, an indicator electrode, a reference electrode, a stirrer, and an instrument for measuring the electromotive force.
The potentiometric titration curve is a graph in which the electrode potential value (battery electromotive force) E is plotted against the volume V of the standard solution as the titration progresses.
According to different methods of drawing, there are three types of potentiometric titration curves, E-V curve, ordinary potential titration curve, and the inflection point e is the equivalent point.
Determination of the inflection point: Make two 45° oblique lines tangent to the titration curve, and the intersection of the bisector and the curve is the inflection point.