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Potentiometry is a chemical titration technique that does not use a colour indicator. Potentiometric titration is another name for it. The method can identify an acid without using a colour indicator; this fact underlines the importance of potentiometry.
This technique measures the potential of the analyte with the help of electrodes. The analyte is nothing but the electrolyte solution.
It uses two rods, an indicator electrode and another reference electrode. The indicator electrode is a metal ion indicator electrode, and the reference electrodes can be H electrodes or AgCl electrodes. This technique has wide applications in the pharmaceutical industry.
What is Potentiometric Titration?
Potentiometric titration is a chemical method used for analysing whether chemical substances are acids or bases. Here, the amount of analyte is determined by adding titrants of known concentration till the endpoint.
The technique uses two electrodes, one being the indicator electrode and the other the reference electrode. The indicator electrode records the change in potential as a function of the volume of titrant added with a known concentration.
The potential difference is calculated between the two electrodes under thermodynamic equilibrium when current flows through the electrodes.
History
Robert Behrend performed the first potentiometric titration experiment in 1893 at Ostwald’s Institute. He titrated mercurous solutions with KCl, KBr, and KI solutions.
These chemicals were titrated with a Hg electrode as an indicator electrode and mercurous nitrate as the reference electrode. He noticed that a cell that contained mercurous nitrate had an initial voltage of 0. Adding potassium chloride to mercurous nitrate yielded mercury (I) chloride precipitate.
This precipitate was responsible for decreasing the osmotic pressure of mercury (I) ions and creating a difference in the potential. This potential difference rose with an increase in the concentration of potassium chloride in the solution.
Wilhelm Bottger, during his work at Ostwald’s Institute, used potentiometry to observe the differences between weak acids and strong acids. He pioneered the use of potentiometry to identify acids and bases for substances that one could not assay using a colour indicator.
Crotogino was the first to use potentiometric titrations for redox reactions between the analyte and the titrant, involving oxidation and reduction. He titrated the halide ions with potassium permanganate with platinum and calomel electrodes.
According to him, adding an oxidising agent to a reducing solution changes the equilibrium of the reaction. In such a case, the equilibrium shifts towards the product side (the left side).
It causes a slow change in the potential until and unless the amount of reducing substance is negligible. A significant increase in the potential difference marks the reaction’s endpoint.
Potentiometry Principles
The potential difference between the two electrodes used forms the basis of the potentiometry principle. The addition of a titrant leads to a change in the ionic concentration, causing changes in the potential difference.
The indicator electrode measures this potential difference. The reference electrode has a potential value and remains stable when dipped into the sample solution.
The salt bridge is a divide used during potentiometric titration to avoid the intervention of the analyte solution with the reference solution. Analyte solution is the solution whose potential we must determine.
We can calculate the total electric potential or the potential difference as:
Ecell = Eind – Eref + Ej
Here,
Ecell = potential of the whole cell
Eind = potential of the indicator
Eref = potential or electromotive force of the reference electrode
Ej = potential at the junction across the salt bridge
The measured electrical potential depends on the concentration of the ions in contact with the indicator electrode.
Importance of Potentiometry
The importance of potentiometry in analytical chemistry is to determine the concentration of a solute in a solution. The pharmaceutical industry uses it as a weight percentage assay for small molecule active pharmaceutical ingredients, intermediates, and raw materials.
We can also use it to determine the potential of the sample solution depending on the concentration of the ions.
Conclusion
Potentiometry or potentiometric titration is a technique in Chemistry. We use it to determine the concentration of a solution. Robert Behrend was the first to carry out a potentiometric titration in 1893 at Ostwald’s Institute.
Instead of indicators, we use electrodes to determine whether a solute is an acid or a base. The electrodes used can be hydrogen, calomel, or metal ion indicator.
This technique is used widely in the Chemistry and pharmaceutical industries. This technique helps calculate the analyte potential and characterise the substance as an acid or base.
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