What is Potentiometric Titration?

Potentiometric titration is one of the important titration methods and is mainly carried out to characterize acid solutions. It follows a method similar to that of the normal titration technique, but the main difference is that there is no indicator used in the potentiometric titrations. Instead of using an indicator, electrodes are employed for monitoring the whole titration process. A scientist named Robert Behrend did the potentiometric titration for the first time, and it was done using the mercurous solution and potassium chloride.

What is the Principle of Potentiometric Titrations?

Potentiometric titration measures the given analyte concentration. In potentiometric titrations, two electrodes are generally used, one is the reference electrode and the other is the indicator electrode. In the potentiometric titrations, the potential variations that exist between these two electrodes are generally measured. The indicator electrode will respond to the potential variations in the cells. The reference electrode will have a fixed potential, and it is by keeping this potential as a reference, the potential difference across both the electrodes is being determined.

The mathematical relation used to evaluate the overall electric potential can be written as shown below.

Ecell=E_indicator-E_reference+E_solution

Where,

• E_indicator is the indicator electrode’s electrode potential.
• E_reference is the reference electrode’s electrode potential.
• E_solution corresponds to the potential drop in the analyte solution across the two electrodes.

The total potential difference across the cell, that is, Ecell is measured at each interval whenever the titrant is added. The ions in the electrolyte solution are in contact with the indicator electrode. The value of the total potential difference Ecell is mainly dependent on the concentration of these ions. A change in the concentration of these ions will cause subsequent changes in the value of the overall potential difference. Potentiometric titrations will measure this value of the total potential difference, which is obtained through titrant addition.

A potentiometer connected to the electrodes systematically measures the variations in the potential difference. A graph can be constructed using these measured values and the added volume of the titrant. The graph plotted using this potential difference and the titrant volume give a curve of an “S” shape as depicted in the graph given below.

Apparatus for Potentiometric Titrations

A typical potentiometric apparatus comprises the following parts.

Analyte Solution

The sample solution which is being analyzed in the potentiometric titrations is named analyte solution. An electrolyte solution is used as this analyte solution mainly.

Reference Electrode

The reference electrode stays stable when dipped in the analyte solution. It is the reference electrode that maintains the potential in the solution. Robert Behrend used a mercury electrode with mercurous nitrate for reference. The most used and preferred reference electrodes are saturated calomel electrodes (shown below) and silver chloride electrodes. Sometimes hydrogen electrodes are also used. The reference electrode forms one of the half-cells of the apparatus. This electrode is called a reference electrode since it possesses a fixed potential.

Indicator Electrode

It is defined as the electrode in the electrochemical half cell wherein the ions are in the analyte solution. This electrode react differently with the potential variations in the electrolyte solution. The most used indicator electrode is a glass electrode. Sometimes metal ion indicator electrode is also used.

Burette

As always, the purpose of the burette is to transfer the titrant solution to the analyte solution.

Potentiometer

A potentiometer is a device that records the potential difference between the electrodes, that is, the reference and indicator electrodes. It records the potential value in volts or millivolts soon after titrant addition to the analyte solution.

Magnetic Stirrer

The purpose of the magnetic stirrer is to ensure the proper mixing of the titrant solution with the analyte solution.

All the above-mentioned parts in a potentiometric apparatus are illustrated in the schematic diagram below.

The Procedure of Potentiometric Titration

As mentioned earlier, potentiometric titrations involve using two electrodes, the reference and indicator electrodes. Both these electrodes are connected using a salt bridge. The main objective of this titration method is the measurement of the indicator electrode’s unknown potential by the known potential of the reference electrode and thereby to determine the total electric potential of the cell.

The procedure of this method involves the measurement of electric cell potential upon the titrant addition of each volume. Normally, the analyte solution used is an acid and the titrant added is a base (acid-base titrations). Every value obtained from the potentiometer is noted, and it is plotted against the corresponding added volume of the titrant. A plot with an “S” shaped curve will be obtained, as illustrated earlier. From the steep curve, the midpoint is identified. It gives the value of the endpoint of the titration.

Classification of Potentiometric Titrations

Potentiometric Titrations are generally categorized into four classes, and they are acid-base titration, redox titration, complexometric titration, and precipitation titration. Each one of them is briefly discussed below.

Acid-Base Potentiometric Titrations

The acid-base potentiometric titration is the most common potentiometric titration conducted. The corresponding concentrations of a particular acid or a base can be evaluated using acid-base potentiometric titrations. The estimation of unknown concentration is done by neutralizing the acid solution or base solution using a standard base or acid solution, respectively. This standard solution has a known concentration. An example of such a kind of titration method is the HCl and NaOH titration.

Redox Potentiometric Titration

In redox potentiometric titration, the respective analyte and titrant undergo a redox reaction. Titration between halide ions and ${\mathrm{KMnO}}_4$ using suitable electrodes is an example of redox potentiometric titrations.

Complexometric Titrations

To identify metals ions present in a particular analyte solution, complexometric titrations are used. Sometimes, complexometric titrations are also referred to as chelometry. The formation of a colored complex understands the endpoint of these titrations.

Precipitation Titration

In precipitation titration, a precipitate formation occurs due to the analyte’s reaction with the titrant. The point at which no precipitate formation occurs corresponds to its end point.

Uses of Potentiometric Titrations

The important applications or uses of potentiometric titrations are listed below.

• Used in the agriculture industry for the identification of various elements present in the soil.
• Used for food processing.
• Used for analysis of metal ions.
• Used for detection of ammonia in water.

Common Mistakes

Students may get confused with acid-base potentiometric titrations and general acid-base titrations. They may think that both are identical, which is incorrect. Both are different in the case of indicators.

• In acid-base titrations, respective chemical indicators are used, whereas in acid-base potentiometric titrations, there is no use of such indicators, instead, electrodes are used to detect the end points.
• In acid-base titrations, the indicator’s color change is taken as the endpoint, while in acid-base potentiometric titrations, it is evaluated from the values of potential difference.
• The plot in acid-base titrations is constructed between volume and pH, while in acid-base potentiometric titrations, it is plotted between volume and potential difference.

Context and Applications

This topic is helpful in the professional exams for undergraduate, and graduate courses, Especially for

Bachelors in Chemistry

Bachelors in Industrial Chemistry

Masters in Industrial Chemistry

Masters in Pure and Applied Chemistry

Bachelor in Technology in Chemical Engineering

Related Concepts

Acid-Base Titrations

Redox Titrations

Indicators

Electrochemical Cell

Chemical Laboratory Techniques

Potentiometry

Practice Problems

Q1: Who performed the first potentiometric titration?

(a) Robert Behrend

(b) Robert Remez

(c) Ronald Behrend

(d) Robert Boyle

Correct option: (a)

Q2: How many categories are there of potentiometric titration?

(a) Six

(b) Three

(c) Four

(d) Five

Correct option: (c)

Q3: Which metal mixture is present in a calomel electrode?

(a) Silver

(b) Mercury

(c) Nickel

(d) Potassium

Correct option: (b)

Q4: Which variable is placed on the x-axis of a potentiometric titration curve?

(a) Volume

(b) Potential

(c) Time

(d) Concentration

Correct option: (a)

Q5: Which unit of volume is used in a potentiometric titration curve?

(a) Deciliters

(b) Kiloliters

(c) Liters

(d) Milliliters

Correct option: (d)