AxisPharm Services

What is potentiometric titration?

Potentiometric titration is a method of determining the titration end point by measuring the potential change during the titration process. Compared with the direct potentiometric method, the potentiometric titration method does not require accurate measurement of the electrode potential value. Therefore, the influence of temperature and liquid junction potential is not important. , its accuracy is better than the direct potentiometric method. The ordinary titration method relies on the color change of the indicator to indicate the titration end point. If the solution to be tested is colored or turbid, the indication of the end point is more difficult, or no suitable indicator can be found at all. . Potentiometric titration is based on the sudden jump of electrode potential to indicate the end point of titration. Before and after the titration reaches the end point, the concentration of the ion to be measured in the droplet often changes continuously by n orders of magnitude, causing a sudden potential jump, and the content of the measured component is still calculated by the consumption of the titrant.

Potentiometric Titration Definition

Using different indicator electrodes, potentiometric titration can perform acid-base titration, redox titration, complexometric titration and precipitation titration. In acid-base titration, a PH glass electrode is used as an indicator electrode, and in redox titration, a platinum electrode can be used as an indicator electrode. In complex titration, if EDTA is used as titrant, mercury electrode can be used as indicator electrode. In precipitation titration, if silver nitrate is used to titrate halogen ions, silver electrode can be used as indicator electrode. During the titration process, with the continuous addition of titrant, the electrode potential E changes continuously, and the titration reaches the end point when the electrode potential jumps abruptly. It is easier to determine the titration end point with a differential curve than with 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 can be automatically found, and the volume can be automatically given, making the titration fast and convenient.
During potentiometric titration, a reference electrode and an indicator electrode are inserted into the tested solution to form a working battery. With the addition of the titrant, due to the chemical reaction, the measured ion concentration changes continuously, and the potential of the indicating electrode changes accordingly. A jump in potential occurs near the equivalence point. Therefore, by measuring the change in the electromotive force of the working cell, the end point of the titration can be determined.

Potentiometric titration apparatus

Including burette, titration cell, indicator electrode, reference electrode.

Potentiometric titration to determine the endpoint

Use the method of plotting the potential to determine the curve.
The potentiometric titration curve is a graph of the electrode potential value (battery electromotive force) E versus the added volume V of the standard solution as the titration proceeds.
According to the different drawing methods, there are three types of potentiometric titration curves, E-V curve, ordinary potentiometric titration curve, and the inflection point e is the equivalence point.
Determination of the inflection point: Make two straight lines that are tangent to the titration curve at a slope of 45°, and the intersection of the bisector and the curve is the inflection point.
Ee is the equivalent point potential.
Ve is the volume of standard solution added to the equivalence point.
The larger the potential jump range and slope, the smaller the analytical error.
Curve, first derivative curve, first derivative curve.
The point e at the peak of the curve is the equivalence point.
Use the two adjacent E, V values ​​to find:
=0 is the equivalence point
In the formula, V1 and V2 are the calculated values ​​of the value.

Technical characteristics of potentiometric titration

Potentiometric titration has many advantages over volumetric analysis with indicators. First, it can be used for the titration of colored or turbid solutions. It is not possible to use indicators; in the absence or lack of indicators, this method is used to solve the problem; It can also be used in the case where the concentration of the test solution is relatively dilute or the titration reaction is not complete; the sensitivity and accuracy are high, and automatic and continuous determination can be realized. Therefore, it has a wide range of uses.

Potentiometric titration applications

Depending on the type of titration reaction, potentiometric titration can be used for neutralization titration (acid-base titration), precipitation titration, complexometric titration, and redox titration.

Acid-base titration

General acid-base titration can use potentiometric titration; it is especially suitable for weak acid (base) titration; very weak acid can be titrated in non-aqueous solution;
Indicating electrode: glass electrode, antimony electrode;
Reference electrode: calomel electrode;
(1) Titrate pyridine with HClO₄ in acetic acid medium;
(2) Titration of triethanolamine with HCl solution in ethanol medium
(3) Titrate aniline and alkaloids with HCl solution in a mixed solution of isopropanol and ethylene glycol;
(4) Phenol can be titrated in dimethylformamide medium;
(5) The mixture of perchloric acid, hydrochloric acid and salicylic acid can be titrated in acetone medium.

Precipitation titration

Reference electrode: double salt bridge calomel electrode; calomel electrode
(1) Indicating electrode: silver electrode
Standard solution: AgNO₃;
Titration objects: Clˉ, Brˉ, Iˉ, CNSˉ, Sˉ², CNˉ, etc.
Continuous titration of Clˉ, Brˉ, Iˉ;
(2) Indicating electrode: mercury electrode
Standard solution: mercury nitrate;
Titration objects: Clˉ, Brˉ, Iˉ, CNSˉ, Sˉ², C₂O₄ˉ² etc.
(3) Indicating electrode: platinum electrode
Standard solution: K₄[Fe(CN)6;
Titration objects: Pd²﹢, Cd²﹢, Zn²﹢, Ba²﹢, etc.

reduction titration

Reference electrode: calomel electrode;
Indicating electrode: platinum electrode;
(1) Standard solution: potassium permanganate;
Titration objects: Iˉ, NO₃ˉ, Fe²﹢, V⁴﹢, Sn²﹢, C₂O₄²ˉ.
(2) Standard solution: K₄[Fe(CN)6;
Titration object: Co²﹢.
(3) Standard solution: K₂Cr₂O7;
Titration objects: Fe²﹢, Sn²﹢, Iˉ, Sb³﹢, etc.

Coordination titration

Reference electrode: calomel electrode;
Standard solution: EDTA
(1) Indicating electrode: mercury electrode;
Titration objects: Cu²+, Zn²+, Ca²+, Mg²+, Al³+.
(2) Indicating electrode: chlorine electrode;
Al³+ was titrated with fluoride.
(3) Indicating electrode: calcium ion selective electrode;
Titration objects: Ca²+ etc.