Potentiometry is the method to find the concentration of solute in a given solution by measuring the potential between two electrodes. As the name suggests potentiometric titration involves the measurement of the potential of indicator electrode and reference electrode. It gives more accurate and precise results than other titrations in which different reagents are used as indicators. That’s why potentiometric titration is preferred over manual titrations. 1st potentiometric titration was carried out by Robert Behrend in 1893.
Potentiometric titration is a technique similar to direct titration but in this no indicator reagent is used instead an electrode is used as an indicator. In potentiometric titrations a cell is used with reference electrode, salt bridge, analyte and an indicator electrode. Generally, electrolyte solution is used as an analyte. Hydrogen electrodes, silver chloride electrodes and calomel electrodes are generally used as reference electrodes. Indicator electrode is generally glass electrode and metal ion electrode.
When the pair of electrodes are placed in the sample solution or analyte it shows the potential difference between two electrodes by addition of the titrant or by the change in the concentration of ions.
The two electrodes are named as reference electrode and indicator electrode. The reference electrode is the electrode which maintains its potential and remains stable when dipped into a sample solution. Indicator electrode is the electrode which responds to variation in the potential of analyte solution. A salt bridge is used to prevent interference of analyte with reference electrode.
The electromotive force or over all potential difference can be calculated by using following formula –
Ecell = Eind – Eref + Ej
Where Ecell = electromotive force of the complete cell
Eind = electromotive force of the indicator electrode
Eref = electromotive force of the reference electrode
Ej = electromotive force at the junction across the salt bridge
Potentiometric titration involves measurement of the potential of an indicator electrode with respect to a reference electrode as a function of titrant volume. In this titration we measure and record the cell potential (in millivolts or pH) after adding titrant each time. As we approach the end point, we start adding titrants in very small quantities. The most straight forward and mostly used method of end point detection in potentiometric titration is plotting a graph between cell potential and volume of titrant. The midpoint of the steeply rising portion of the graph or curve is estimated visually and taken as an end point. As it is shown below in a sample graph –
Following four types of titrations can be performed by potentiometric titration –
Acid - base titration – Titration of HCl with NaOH can be done by potentiometric titration. In this concentration of a given acid/base is determined by using a standard solution.
Redox Titration – Potentiometric titration was first used for redox titration by Crotogino. He titrated halide ions with KMnO4 using a platinum electrode and calomel electrode.
Complexometric titration – In this type of potentiometric titration concentration of metal ions are determined in the analyte. In this membrane electrodes are used.
Precipitation Titration – In this type of titration precipitate is formed as the name suggests. When addition of the titrant no longer forms a precipitate, that point is noted as the endpoint.
It is used in clinical chemistry for analysis of metals.
It is used for analysis of cyanide, ammonia etc. in water or wastewater.
It is used in agriculture for detection of different elements in soils, fertilizers etc.
It is used in detergent manufacturing, food processing etc.