Potentiometric titration

Potentiometric titration is a versatile titrimetric analysis technique. The word potentiometric is derived from potential difference. This implies that in potentiometric titrations, an electrical potential difference is measured to determine the unknown quantity of an analyte in the titrand.

This article will guide you through the working principle, the experimental procedure, and the uses and applications of potentiometric titrations. So, without any further delay, let’s start reading!

What is potentiometric titration- Definition and Working Principle

Potentiometric titration was introduced back in 1893 by Robert Behrend. It is a direct titration method that is most commonly used for characterizing acids. In potentiometric titrations or potentiometry, the change in a potential difference between two electrodes is measured as a function of the amount of titrant added of known concentration.

The two electrodes include the reference electrode and the indicator electrode. One half-cell is formed by the reference electrode which is usually a hydrogen electrode, silver chloride (AgCl) electrode, or a calomel (Hg₂Cl₂) electrode. The other half-cell is composed of the indicator electrode and the analyte ions in the form of an electrolyte. The overall cell potential (E_cell) plotted against titrant volume helps determine the unknown analyte concentration.

How to perform potentiometric titration- Experimental Procedure

Step I: A potentiometric electrochemical cell is set up.

• The potentiometric electrochemical cell consists of three main components i.e., 2 half-cells and a salt bridge.
• One half-cell is made up of the reference electrode such as the standard hydrogen electrode (SHE).
• The other half cell is made up of the indicator electrode such as magnesium (Mg) metal in case if we are performing the potentiometric titrimetric analysis for determining the sample concentration of a 100 mL Mg(OH)₂ solution.
• As per the electrochemical series, Mg has a greater oxidation potential than H₂.
• So, in the example above, the indicator electrode serves as a cathode while the reference electrode serves as the anode.
• Oxidation occurs at the anode while reduction takes place at the cathode in potentiometric titrations.
• The oxidation-reduction half-cells are separated by a salt bridge.
• The salt bridge is a porous membrane packed with an inert electrolyte such as MgCl₂. It allows the free movement of electrolyte ions between the two half-cells. In turn, completing the electrical circuit.

Step II: Oxidation-reduction reactions take place.

• Oxidation takes place at the anodic half-cell as Mg metal loses electrons to change into Mg²⁺ ions. The anode gains a negative charge.

• These electrons then travel through the salt bridge to the other half-cell and an electrical signal is recorded.
•  H⁺ ions present at the cathodic half-cell get reduced to hydrogen (H₂) gas by gaining these electrons. The cathode thus gains an overall positive charge.

Step III: The titrant is added dropwise into the analyte solution.

• Dropwise addition of the titrant such as a 0.1 M standard solution of HCl from the burette into the beaker containing Mg(OH)₂ leads to an acid-base neutralization reaction.

• As the Mg²⁺ ions get neutralized, the pH and consequently the electrical potential of the indicator half-cell changes.
• The pH of the reference electrode stays stable.
• E_ind is measured and recorded after each addition of titrant from the burette.
• The titrant is first added rapidly followed by a slow addition while performing a potentiometric titration.

The overall cell potential (E_cell) or electromotive force (EMF) is calculated by the formula given below.

Where E_ind is the EMF of the indicator electrode, E_ref  is the EMF of the reference cell and E_jun is the EMF at the junction across the salt bridge.

E_cell depends on the concentration of analyte in the titrand that come into contact with the indicator electrode.

Step IV: The potentiometric titration curve is plotted.

E_cell is plotted against titrant volume and the graph obtained is known as the potentiometric titration curve.

A large change in potential difference with a small change in titrant volume makes the titration curve steep. The equivalence point of the titration is marked at half the height of this steep.

The titrant volume at this equivalence point is finally used to determine the unknown analyte concentration, as shown below.

Types of potentiometric titration

Potentiometric titration is a general procedure. All the different types of titrations (acid-base titration, complexometric titration, precipitation titration, and especially redox titration) if performed using an indicator electrode, then it falls under the potentiometric titration category.

Advantages of potentiometric titrations

• Potentiometric titrations are accurate.
• It gives consistent and robust experimental results.
• No chemical reagent-based indicators such as acid-base indicators are required in potentiometric titration so there are fewer chances of error in end-point detection.
• Indicator electrodes are suitable for a variety of titrimetric chemical reactions.

What is potentiometric titration used for- Applications

Potentiometric titrations are used for

• Pharmaceutical analysis.
• The analysis of metals in environmental samples.
• The detection of multiple elements in fertilizer in the agricultural sector.
• In the chemical manufacturing industry.

To learn more about the uses and applications of titrations, consult this article.

What is the difference between potentiometric titration and conductometric titration

Potentiometric titration measures the change in potential difference across the analyte solution. In contrast to that, conductometric titration measures the electrical conductivity of the analyte solution.

However, both techniques are quite similar as no visual indicator is required in each. Conductometric titrations are more suitable for colored analytes as opposed to potentiometric titration which is used for colorless analyte mixtures. You may learn more about the differences between the two types of titrations here.

References

1. Harvey, David. 2019. “Potentiometric Methods.” In.: LibreTexts Chemistry

2. Hulanicki, A., M. Maj-Żurawska, and S. Glab. 2013. ‘TITRIMETRY | Potentiometry.’ in Paul Worsfold, Colin Poole, Alan Townshend and Manuel Miró (eds.), Encyclopedia of Analytical Science (Third Edition) (Academic Press: Oxford).