Potentiometric Titration

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What is Potentiometric Titration?

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 a reference electrode, salt bridge, analyte and an indicator electrode. Generally, the 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.

Potentiometric Titration principle of Potentiometry Principle

When the pair of electrodes are placed in the sample solution or analyte, it shows the potential difference between two electrodes by the addition of the titrant or by the change in the concentration of ions.

The two electrodes are named reference electrodes and indicator electrodes. The reference electrode is the electrode that maintains its potential and remains stable when dipped into a sample solution. An indicator electrode is an electrode that responds to variation in the potential of analyte solution. A salt bridge is used to prevent interference of the analyte with the reference electrode.

The electromotive force or overall potential difference can be calculated by using the following formula –

E_cell = E_ind – E_ref + E_j

Where

E_cell  = electromotive force of the complete cell

E_ind = electromotive force of the indicator electrode 

E_ref = electromotive force of the reference electrode

E_j = electromotive force at the junction across the salt bridge

(Image will be uploaded soon)

The electric potential of the cell is dependent on the concentration of ions in contact with the indicator electrode, as shown in the above graph. As a result, the Ecell is measured after each titrant addition.

Potentiometric Titration Procedure

Potentiometric titration involves the 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 endpoint, we start adding titrants in very small quantities. The most straightforward and most used method of endpoint 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 endpoint. As it is shown below in a sample graph –

Types of Potentiometric Titration

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. 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, the precipitate is formed, as the name suggests. When the addition of the titrant no longer forms a precipitate, that point is noted as the endpoint.

Applications of potentiometric titrations

• It is used in clinical chemistry for the analysis of metals. 

• It is used for the analysis of cyanide, ammonia etc., in water or wastewater. 

• It is used in agriculture for the detection of different elements in soils, fertilizers etc. 

• It is used in detergent manufacturing, food processing etc.

Let us look at the application of potentiometric titrations across various industries in detail.

Clinical chemistry

They are useful sensors because analytes in complexmatrices are selective for ion-selective electrodes in clinical chemistry. Analytes including sodium, potassium, calcium, hydrogen, and chlorine, as well as dissolved gases like CO₂, are commonly used.

Environmental chemistry

Environmental chemistry is used to quantify CN^–, NO₃, F₃, and NH₃ in water and wastewater.

Potentiometric titrations

To calculate the equivalence point, a potentiometric acid-base titration is used. Chemical reactions such as precipitation, acids, bases, redox processes, complexations, and also including the other chemical reactions may be titrated in both aqueous and non-aqueous solvents. 

Agriculture

NO₃, NH₄, I, Ca, K, and CN are all elements found in soils, plant materials, feed, and fertilisers.

Detergent production

Studying the effects of calcium, barium, and fluorine in detergent manufacturing.

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