Potentiometric titration is a method of chemical analysis. This technqiue relies on the measurement of electromotive force (EMF) of a solution using a set of indicator and reference electrodes. The potential or EMF of a solution depends on the nature and concentration of the ions of the test substance.
The EMF or potential is measured in millivolts (mV) using a potentiometer having an indicator and reference electrode.
In a potentiometric titration, the endpoint is determined by measuring the changes in potential of the solution. This change in potential is caused due to the addition of titrant. During the process, the addition of titrant causes changes in the activity and also the concentration of ions in the solution.
Instead of potential, even pH can be measured to determine the endpoint.
Potentiometric instrument and experiment.
The instrumentation has two electrodes, a beaker, stirrer and a burette. The electrodes include a reference electrode and an indicator electrode. Saturated Calomel electrode is used as a reference electrode while a glass electrode is employed as an indicator electrode.
Titration is done by taking the test solution in a beaker and adding the titrant from the burette. The end point is determined by the indicator method. If not, then a titration curve is drawn. From this titration curve, a graph is plotted to determine the endpoint.
There are three types of titrations curves. In the potentiometric titrations, the rate of change in potential is maximal at the endpoint. This can be determined by three kinds of curves like
- Normal titration curve
- First derivative curve
- Second derivative curve
Advantages of Potentiometric titration:
- Regular titrations are done using colorless solutions. But here, colored and turbid solutions can also be analyzed by titration.
- The entire method can be automated without the need for manual mixing, swirling, etc.
- A mixture of two or more components can also be analyzed.
- The method is highly accurate with minimal chances of errors.
- The technique is also inexpensive.
Application of potentiometry:
This method is used for different types of titrations like
- Acid bases titrations
- Diazotization titrations
- Redox titrations
- Complexometric titrations
- Precipitation titration
- Dead stop endpoint titration.
Acid-base titrations: This potentiometric titration can be used for both aqueous and nonaqueous titrations. The indicator electrode would be a glass electrode and a reference electrode can be a S. calomel electrode.
The following equation can determine the potential of such a system.
E= K – 0.0592 pH. K is ‘asymmetry potential’ constant of the electrode system).
This potentiometric titration can analyze all types of acid-base titration. Further, the mixture of acids, polybasic acids can also be analyzed.
Redox titrations: The redox titrations are done by using platinum foil as an indicator electrode and Sat. Calomel electrode or silver chloride electrode as a reference electrode. The endpoint is denoted by mV.
The potential is obtained by the following equation as
E = E0 + 0.0592/n x Log [OX] / [Red]
Here E0 is standard potential, n is the number of electrons involved in the reaction, cons. [OX] is the concentration of oxidant and cons. [Red] is the concentration of reductant.
Examples: Ferrous ammonium sulfate in dilute. Sulfuric acid against either potassium permanganate or potassium dichromate. Sodium arsenite against Potassium bromide etc.
Diazotization titration. Herecompounds containing amino groups can be titrated against Sodium nitrite. This is done to identify the aromatic primary amino group in an acidic medium. The reaction leads to the formation of diazonium salts.
Here glass electrode is taken as indicator electrode while the saturated cal. electrode is taken as a reference electrode.
Examples: Substances like alkaloids, amines, sulpha molecules are titrated against 0.1N sodium nitrite.
In this method, metallic ions are titrated against disodium edetate (EDTA). Silver or mercury electrodes are used as indicator electrodes. Saturated cal. electrode is used as a reference electrode.
Examples of substances analyzed include divalent ions, trivalent ions etc.
Here precipitating agents are used for quantitative estimation of ions and elements.
Substances like mercury, lead, silver, copper in ionic form are precipitated into insoluble salts.