Standard Hydrogen Electrode

Introduction

The Standard Hydrogen Electrode (SHE) was established in the early 20th century as a critical component of electrochemical measurement. William Cruickshank in 1801 initial work on the hydrogen electrode began with William Cruickshank, who used hydrogen gas and platinum in electrochemical experiments. However, his work did not lead to the development of a standard reference electrode. And then Walther Nernst played a key role in developing the theoretical foundation for the SHE. Nernst’s work on electrode potentials and the Nernst equation laid the groundwork for defining standard electrode potentials.

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The concept of a standard reference electrode was further refined and standardized in the 1920s. The International Union of Pure and Applied Chemistry (IUPAC) established the SHE as the standard reference electrode for electrochemical measurements. SHE was needed because. Before the SHE, there was no universal reference electrode. The SHE provided a consistent and reproducible reference point (0.00 V) for measuring and comparing the electrode potentials of other half-cells. The SHE allows for the determination of standard electrode potentials, making it possible to compare the relative strengths of various redox reactions.

The SHE’s standardized conditions (1 M H⁺, 1 atm H₂, 25°C) ensure accurate and consistent measurements of electrode potentials, which are essential for reliable electrochemical experiments and industrial processes. The SHE is used as a reference electrode in pH meters, providing a stable reference for measuring the voltage difference and determining the pH of solutions. The SHE is integral to research in electrochemistry, helping scientists understand redox reactions, develop new materials, and explore various electrochemical phenomena.

Standard Hydrogen Electrode

A hydrogen electrode in which the pressure of hydrogen gas is maintained at 1 atm and the concentration of H⁺ ions in the solution is 1M, is called a standard hydrogen electrode (SHE).
SHE half reaction Electrode potential H₂→2H⁺+2e⁻0.0 V (Anode)

2H⁺+2e⁻→H₂0.0 V (Cathode)

• The emf of a standard hydrogen electrode is taken as 0.00 V at all temperatures.
• It is a reversible electrode.
• It is used as a primary reference electrode.
• The potential of other species can be calculated by conducting a cell with SHE as one of the electrodes and then calculating the potential difference by various methods.

• A hydrogen electrode in which the pressure of hydrogen gas is maintained at 1 atm and the concentration of H⁺ ions in the solution is 1M, is called a standard hydrogen electrode (SHE).
  SHE half reaction Electrode potential H₂→2H⁺+2e⁻0.0 V (Anode)

• 2H⁺+2e−→H₂0.0 V (Cathode)

• A standard hydrogen electrode emf is taken as 0.00 V at all temperatures.
• It is a reversible electrode.
• It is used as a primary reference electrode.
• The potential of other species can be calculated by conducting a cell with SHE as one of the electrodes and then calculating the potential difference by various methods.

• This equation gives the relationship between electrode potential and the concentration of ions in the solution. In other words, it shows the dependency of electrode potential on the concentration of the ions with which the electrode is reversible.

  For a single electrode involving the reduction process,

  Mn⁺+ne⁻→M(s)

  The reaction quotient Q is defined as aM[M

  ⇒E=E^∘−2.303RTnFlog⁡Q

  This is the Nernst equation which helps us to calculate the non-standard EMF of any Half cell. It can be extended to full of any half cell. It can be extended to full cell which we will be learning later.

  Now, at 25∘C or 298 KNow, at 25∘C or 298 K

  E=E^∘−2.303×8.314×298n×96500log10⁡[M][Mⁿ⁺]

  E=E^∘−0.059nlog10⁡[M]Mⁿ⁺] Here R= Gas constant T= Absolute temperature E^∘= Standard Emf of the cell E= Electrode potential of cell F= Faraday number n= number of electrons transferrec

• If the electrode is solid its activity mass is taken as one.
• For an electrochemical cell having a net reaction:
  xA+yB→ne−mC+nD
  The emf can be calculated asEcell =E^ocell −0.059nlog⁡[C]^m[D]ⁿ/[A]^x[ B]^y

• In using the above equation, the following facts should be kept in mind.

Application Of Standard Hydrogen Electrode

The Standard Hydrogen Electrode (SHE) is a reference electrode used in electrochemical measurements. Its applications and significance are.

1. Reference Electrode for Measuring Electrode Potential

- Standardization: The SHE is used as a reference point to measure the electrode potentials of other half-cells. By definition, the SHE has a potential of 0.00 V under standard conditions (1 M H⁺, 1 atm H₂, 25°C).
- Comparison*: Electrochemical cells are set up with the SHE as one half-cell and another electrode as the other half-cell. The potential difference between them is measured to determine the potential of the second electrode.

2. Electrochemical Series

- Determining Standard Electrode Potentials: The SHE is central in defining the electrochemical series, which ranks electrodes based on their standard electrode potentials relative to the SHE.

- Predicting Reactions: The electrochemical series helps predict the direction of redox reactions, their feasibility, and the voltage generated in electrochemical cells.

3. pH Measurement

- pH Reference: In pH meters, the SHE can serve as a reference electrode. Its potential is used to measure the voltage difference with a pH-sensitive electrode, determining the pH of a solution.

4. Corrosion Studies
- Corrosion Potential Measurement: The SHE is used to measure the corrosion potential of metals and alloys in various environments, which helps in understanding and mitigating corrosion.

5. Electrolysis and Industrial Processes
- Electrolytic Cells: In electrolysis, the SHE helps design and control processes by providing a stable reference for measuring cell potentials and optimizing conditions.

6. Fundamental Research

- Redox Chemistry: Researchers use the SHE in fundamental studies to understand redox reactions, develop new materials, and explore electrochemical phenomena.

Construction and Operation

- Components: The SHE typically consists of a platinum electrode coated with a layer of platinum black, immersed in a solution of hydrochloric acid (HCl) with a hydrogen gas atmosphere.
- Maintenance: It must be kept at 1 atm pressure of hydrogen gas and a 1 M concentration of H⁺ ions to maintain its standard state.

For a better understanding of the topic and to learn more about Standard Hydrogen Electrodes with video lesson we provide the link to the

YouTube video:

Some Solved Examples

Example.1

1. Consider the following cell with hydrogen electrodes at different pressures p₁ and p₂.

Pt,H₂(p₁) & H⁺(aq)1M & H₂(p₂),Pt
The EMF of the cell is given by:

1)RTFln⁡p₁p₂

2) (correct)RT₂ Fln⁡p₁p₂

3)RTFln⁡p₂p₁

4)RT₂ Fln⁡p₂p₁

Solution

For the given concentration cell, the Nernst equation can be written as

Ecell =0.059[pHa−pHc+1/2log⁡(pH₂)a(pH₂)c]=0.059[0−0+1/2log⁡p1p₂]=0.0592log⁡p₁p₂=RT₂Fln⁡p₁p₂

Hence, the answer is the option (2).

Example.2

2. A hydrogen electrode placed in a solution containing sodium acetate and acetic acid in the ratio of x: y and y : x has an electrode potential value E₁ and E₂ volts respectively at 25^oC. The pK_a value of acetic acid is:

1) (correct)−(E₁+E₂)2×0.059

2)(E₁+E₂)2×0.059

3)(E₂−E₁)2×0.050

4)−(E₁+E₂)0.059

Solution

The reaction occurring in the hydrogen electrode is given below:

H⁺+e⁻⟶1/2H₂

Using the Nernst Equation, the electrode potential can be written as

E=−0.059log⁡1[H⁺]=−0.059pH

In the two given cases, the Hydrogen ions are obtained by buffer solution having the ratio of Salt: Acid as x:yandy:x

The pH of an acidic buffer can be calculated as
pH=pKa+log⁡( Salt Acid )
∴pH₁=pKa+log⁡xy
Similarly,pH₂=pKa+log⁡yx

Thus, the respective electrode potentials can be written as

pH₂=pKa+log⁡y

Now, adding the two potentials gives us

(E₁+E₂)=−0.059(pKa+log⁡xy)−0.059(pKa+log⁡yx)

(E₁+E₂)=−0.059×2×pKa

pKa=−(E₁+E₂)2×0.059
Hence, the answer is the option (1).

Example.3

3. Select the correct statement out of the following.

1) The absolute value of potential is defined.

2) (correct) The Potential of SHE is arbitrarily assigned zero value.

3)In SHE, the pH of the solution is 7

4)In SHE, the pressure of H₂ (g) is 760atm.

Solution

The absolute value of the potential is not defined. We can only measure the potential difference. As a convention, the SHE is assigned a value of zero, and the potential of other species is measured concerning the SHE.

Hence, the answer is the option (2).

Example.4

4. The standard reduction potential of Hydrogen is zero because

1) (correct)It is taken as a standard reference.

2)It is the easiest to oxidize.

3)It has a single electron.

4)It is electronegative

Solution

The absolute electric potential for an electrode cannot be measured without forming a cell. Therefore hydrogen is taken as a standard reference and its potential is taken as zero.

Hence, the answer is the option (1).

Summary

The Standard Hydrogen Electrode was developed to provide a consistent and reliable reference point for measuring electrode potentials, which is essential for standardizing electrochemical measurements, understanding reaction dynamics, and advancing scientific and industrial applications in electrochemistry. The SHE is essential for providing a consistent basis for measuring and comparing electrode potentials, which is fundamental for electrochemical research, industrial applications, and theoretical calculations. In designing and testing batteries, fuel cells, and other electrochemical systems, the SHE Has very good performance and also it is very stable. The different materials of SHE can be used to guide the specific material for their other applications. A standard hydrogen electrode is used to select the material for the application of electrochemical cells. Standard hydrogen electrodes are used to take the standard potential of the electrode, which is important to calculate the Gibbs free energy and the equilibrium constant for electrochemical reactions. And by predicting all these terms mentioned above, we can predict the feasibility and the driving force, not the reactions. Standard hydrogen electrodes also have practical applications where it is used to determine the accurate value of electrode potentials of other electrodes. This helps in constructing electrochemical cells and understanding their behavior. By using standard hydrogen potential all the experiments of the electrochemical cells are standardized which gives the result very accurate with zero error and is also very reproductive. This property of SHE is very important for comparing different reactions and understanding their properties such as thermodynamics properties.