Standard Hydrogen Electrode: Definition, Construction

Standard Hydrogen Electrode is one of the most significant topics of the chapter on electrochemistry. It is also known as SHE, in short. In thermodynamics, a Standard hydrogen electrode is a redox electrode, that is, it performs both oxidation as well as reduction reactions. So, SHE acts as a cathode and anode respectively in an electrochemical reaction. The topic is extremely important if you want to pursue a career in chemical engineering.

Key Takeaways: Standard hydrogen electrode, Electrode, Electrochemistry, Chemical engineering, cathode, anode, electrochemical reaction, Reduction reaction, oxidation-reduction potential

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What is Standard Hydrogen Electrode?

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• Abbreviated as SHE, the Standard Hydrogen Electrodes, a redox electrode, is the foundational basis of the scale of the oxidation-reduction potential. Moreover, it is the main electrode where the electrochemical reaction takes place.
• A Redox reaction is a reaction in which both oxidation (gaining of an electron of an atom) and reduction (losing of an electron of an atom) takes place simultaneously. 
• In electrochemistry, the absolute potential of the Standard Hydrogen Electrode is approximately 4.44 ± 0.02 V at 25 °C. 
• To compare hydrogen’s potential with other electrochemical reactions, it is said to have a zero volts potential (E°) at a certain temperature (298 K).
• At a given temperature, the potential of any electrode is compared to the potential of a hydrogen electrode. It is referred to as the standard hydrogen electrode because it acts as a reference electrode.

Let’s learn about standard hydrogen electrodes in detail 

• Combinations of different half-cells can be created in several ways on a Daniel cell pattern. The metallic electrode of the half-cell is dipped into an electrolyte.
• Through a metallic wire, a voltmeter and a switch are connected externally to the two half-cells.
• A salt bridge connects the two half-cells of the electrolytes internally. In a few cases, we don't use the salt bridge because both the electrodes are dipped into the same electrolytic solution.
• The metal ions deposit on the metal electrode at the interface of each electrode-electrolyte interface. Thus, the ions try to develop positive charge.
• On the contrary, metal atoms of the electrode leave behind the electrons at the electrode. Thus, the atoms tend to be negatively charged.
• The charges separate at equilibrium. However, the separation of charges depends on the opposing tendency of the reactions.
• A difference between the electrode potential and that of the electrolyte develops when they come into contact.
• One half-cell has an electrode potential of 0 when all the charges are unified in it. A standard electrode potential is known as a standard electrode potential according to IUPAC convention.
• Earlier, the standard hydrogen electrode was called reduction potential.
• The following reaction takes place at the Redox half-cell of standard hydrogen electrodes.

2H⁺ (aq) + 2e^– → H₂ (g)

• The above reaction takes place on a platinum electrode.

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Use of Platinum in the Standard Hydrogen Electrode

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Platinum is used in the standard hydrogen electrode for the following reasons.

• Platinum is an inert catalyst. So, it doesn’t corrode during the electrochemical and speeds up the rate of the reaction.
• Platinum is a less reactive metal. So, it doesn’t easily react with other metals. As a result, it provides the surface for the redox reaction.
• Being a good absorber of hydrogen, platinum improves the chemical kinetics of the electrochemical reaction at the interface.
• Unlike other metals such as gold, silver, copper, mercury; platinum doesn’t poison the electrode of another half-cell.
• Owing to its catalytic properties, platinum promotes the proton reduction reaction. 

Note: The physical technique of covering the surface of platinum with black platinum (powder) is called platinizing of the platinum. The electrode at which platinizing is done is called the platinized platinum electrode.

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Construction of Standard Hydrogen Electrode

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The following equipment is required for the construction of the standard hydrogen electrode.

• Hydrogen Gas/Blow
• Platinized platinum electrode
• Hydro-seal (to prevent oxygen interference)
• Acid solution with molarity equal to 1 mol dm⁻³ (activity H⁺ = 1 mol dm⁻³)
• Reservoir to attach the second half-element of the galvanic cell (create an ionically conductive path)

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Working of a Standard Hydrogen Electrode

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• The half-cell where the process of oxidation in a Galvanic cell occurs is called anode. Anodes have a negative potential relative to the solution.
• The other half-cell in a Galvanic cell where reduction takes place is called cathode. Cathode has a positive potential.
• A potential difference between these two electrodes, i.e., cathode and anode is created.
• When the switch is in the on position, the electrons start flowing from one end (negative electrode) to another end (positive electrode) in the cell.
• In the Galvanic cell, the current flows from the opposite direction to that of the electron flow.
• Cell potential is the potential difference between the two electrodes (and anode) of a galvanic cell. The unit of cell potential is volts.
• When no current is drawn through the cell, the force applied is called the cell electromotive force.
• As per IUPAC convention, the anode is on the left and the cathode is on the right in the galvanic cell.
• Cells are considered positive if their emf is positive. The formula for emf is given by the potential of the half-cell (right hand side) subtracted with the potential of the half-cell (left hand side).

E (cell) = E (right) – E (left)

• The cell reaction can be understood with the help of the following example:

Cu(s) + 2Ag⁺(aq) → Cu²⁺(aq) + 2 Ag(s)

• Half-cell reactions is as follows:

1. Cathode (reduction):

2Ag⁺(aq) + 2e^– → 2Ag(s)

2. Anode (oxidation):

Cu(s) → Cu²⁺(aq) + 2e^–

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Advantages of Standard Hydrogen Electrode

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• Due to minute potential, the potential of a standard hydrogen electrode is taken as zero, therefore, a reference point for other metals.
• Standard Hydrogen Electrode performs the dual function as a cathode half-cell as well as anode half-cell.
• It can be used to determine the unknown potential of the half cell.

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Disadvantages of Standard Hydrogen Electrode

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• Difficulty in transportation, construction, and maintenance.
• Difficulty in maintaining the pressure of hydrogen gas.
• Difficulty in levelling the concentration of the acid solution.
• Difficulty in obtaining pure hydrogen gas.
• Difficulty in the construction of an ideal platinum electrode.
• Impurities reduce the life cycle of platinum electrode which in turn increases the costing.

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Things To Remember

• In an ideal gas solution, the potential of a platinum electrode is called the Standard Hydrogen Electrode. To expand, the theoretical idea solution has the current standard for zero potential for all temperatures. 
• Oxidation is the process of giving or donating electrons from a compound to form a new salt.
• Reduction is the process of bargaining or accepting electrons from a compound to form a new salt.
• The chemical reaction where both oxidation and reduction take place simultaneously is called a redox reaction. However, it is also known as oxidation-reduction reaction.
• Despite other metals, only platinum is opted as a choice of metal in the standard hydrogen electrode.
• Platinum has special properties such as non-corrosive, inert,
• The difference between the position end (anode), and negative end (cathode) is called the cell potential.
• As per CBSE, Standard hydrogen electrode is included in the syllabus of Chemistry class-12. CBSE devotes 3-4 questions from this topic so as to follow the new marking scheme of the academic session 2021-22. The overall weightage of thermodynamics is 10-12 marks which is pretty good for a single topic. Check previous year questions and solved solutions to get a fair idea about the type of questions asked in the board examinations.

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