2.4.3 Standard Hydrogen Electrode

To measure the electrode potential of any half-cell, we need a reference point—a standard against which all other potentials can be compared. The Standard Hydrogen Electrode (SHE) serves this exact purpose. It is the universal reference electrode in electrochemistry, and its potential is arbitrarily defined as exactly 0.00 volts at all temperatures.

Main Idea

The main idea is that the absolute potential of a single electrode cannot be measured. We can only measure the difference in potential between two electrodes. By choosing one electrode as a standard (the SHE) and assigning it a potential of 0.00 V, we can measure the relative potentials of all other half-cells by connecting them to the SHE.

Core Concept

The Standard Hydrogen Electrode is based on the reversible reduction of hydrogen ions to hydrogen gas: 2H^+_{(aq)} + 2e^- ⇌ H_{2(g)} By convention, the potential for this half-reaction is defined as exactly 0.00 V. When any other half-cell is connected to the SHE, the measured voltage of the resulting cell is equal to the electrode potential of that half-cell.

Detailed Explanation

Construction of the Standard Hydrogen Electrode

The SHE consists of the following components:

• Platinum Electrode: A platinum foil coated with finely divided platinum (platinum black) is used. Platinum is a noble metal that is chemically inert, so it does not participate in the reaction. It simply acts as a surface for the electron transfer.
• Hydrogen Gas: Hydrogen gas at a pressure of 1 atm is bubbled around the platinum electrode.
• Acidic Solution: The electrode is immersed in a solution of hydrochloric acid with a concentration of 1 M (1 mol dm⁻³) for H^+ ions.
• Temperature: The entire setup is maintained at 298 K (25°C).

The platinum electrode provides a surface for the reaction, and the hydrogen gas and H^+ ions establish the equilibrium: 2H^+_{(aq)} + 2e^- ⇌ H_{2(g)}

The SHE as a Reference Electrode

• The standard electrode potential of the SHE is arbitrarily chosen as zero at all temperatures.
• By convention, the half-cell potential for reduction of H^+ to H_2 gas, or the potential for the oxidation of H_2 to H^+, in the standard hydrogen half-cell is defined as exactly 0.00 V.
• The symbol E⁰ designates a standard potential, meaning the potential measured under standard conditions (1 M concentration, 1 atm pressure, and 25°C).

Important: The SHE can act as a cathode or an anode depending on the nature of the electrode with which it is connected.

• If the other electrode has a more negative reduction potential, the SHE acts as the cathode (reduction occurs at the SHE).
• If the other electrode has a more positive reduction potential, the SHE acts as the anode (oxidation occurs at the SHE).

How the SHE is Used to Measure Electrode Potentials

The SHE is the primary reference electrode used to measure the standard electrode potentials of other half-cells, such as copper and zinc. It has a standard electrode potential of 0 V by definition.

Determination of Standard Electrode Potential of Zn^{2+}/Zn Electrode:

• An electrochemical cell is set up with a zinc rod dipped in 1 M zinc sulphate solution. This half-cell is combined with a standard hydrogen electrode through a salt bridge.
• The deflection of the voltmeter indicates that current is flowing from the zinc electrode to the hydrogen electrode, meaning electrons are moving from the zinc rod to the hydrogen electrode. Hence, the zinc electrode acts as the anode, and the SHE acts as the cathode.
• The reading (E⁰) by the voltmeter is 0.76 V.
• The cell can be represented as:

Calculation: The E⁰_cell is 0.76 V. E⁰_cell = E⁰_cathode - E⁰_anode 0.76 = 0.00 - E⁰_anode E⁰_anode = -0.76 V Hence, the oxidation potential of zinc is +0.76 V, and the reduction potential of zinc is -0.76 V.

Why is the SHE Necessary?

A single half-cell potential or electrode potential cannot be measured directly. This is because one half-cell reaction cannot occur without a simultaneous reaction in another half-cell. The SHE provides a known, stable reference point (0.00 V) against which any other half-cell can be compared.

Intermediate Level

The Standard Hydrogen Electrode is a fundamental reference point in electrochemistry. It is constructed using a platinized platinum electrode immersed in a 1 M H^+ solution, with hydrogen gas at 1 atm bubbled through it at 25°C. The half-reaction is 2H^+ + 2e^- ⇌ H_2, and its potential is defined as 0.00 V by convention. When measuring the potential of another half-cell, it is connected to the SHE via a salt bridge. The measured cell potential is equal to the standard reduction potential of the unknown half-cell (because E⁰_cell = E⁰_cathode - 0.00). This allows a consistent scale of electrode potentials to be established.

Important Points

• - SHE: The Standard Hydrogen Electrode, used as a reference electrode.
• - Potential: Defined as exactly 0.00 V at all temperatures.
• - Construction: Platinized platinum electrode, 1 M H^+ solution, hydrogen gas at 1 atm, 25°C.
• - Half-Reaction: 2H^+ + 2e^- ⇌ H_2.
• - Role: It can act as either an anode or a cathode depending on the other half-cell.
• - Measurement: Other electrode potentials are measured relative to the SHE.

Remember

• - SHE is the reference electrode.
• - **Potential = 0.00 V at all temperatures.**
• - Construction: Platinum, 1 M H^+, 1 atm H_2, 25°C.
• - Half-reaction: 2H^+ + 2e^- ⇌ H_2.
• - Acts as cathode if connected to a more negative electrode.
• - Acts as anode if connected to a more positive electrode.
• - All standard reduction potentials are relative to the SHE.

Common Mistakes

• - Thinking the SHE has a measurable potential. Its potential is defined as 0.00 V, not measured.
• - Forgetting the standard conditions. The SHE operates at 1 M H^+, 1 atm H_2, and 25°C.
• - Confusing the SHE with a regular hydrogen electrode. The SHE is a very specific, standardized setup with platinized platinum and exact conditions.
• - Assuming the SHE is always the cathode. It can be the anode if the other half-cell has a more positive reduction potential (e.g., copper).
• - Thinking platinum participates in the reaction. Platinum is inert; it only provides a surface for electron transfer.

Quick Revision

The Standard Hydrogen Electrode (SHE) is the reference electrode used to measure the standard electrode potentials of all other half-cells. It consists of a platinized platinum electrode immersed in a 1 M solution of H^+ ions, with hydrogen gas at 1 atm bubbled through it at 25°C. Its potential is arbitrarily assigned a value of 0.00 V. The half-reaction is the reduction of hydrogen ions to hydrogen gas. The SHE can act as either an anode or a cathode depending on the electrode it is connected to. It provides a common reference point for all electrochemical measurements.

60-Second Revision

The Standard Hydrogen Electrode (SHE) is the reference point for all electrode potentials, with an assigned potential of 0.00 V. It consists of a platinized platinum electrode in a 1 M H^+ solution, with 1 atm of H_2 gas at 25°C. Its half-reaction is 2H^+ + 2e^- ⇌ H_2. It is used to measure the relative electrode potential of other half-cells. It can act as either the anode or cathode, depending on the other half-cell.

Frequently Asked Questions (FAQs)

Q1. Describe the role of the standard hydrogen electrode in electrochemistry. Answer: The Standard Hydrogen Electrode (SHE) serves as the universal reference electrode in electrochemistry. It is used to measure the standard electrode potentials of all other half-cells. Since the potential of a single half-cell cannot be measured directly, the SHE provides a known reference point (0.00 V) against which other half-cells can be compared.

Q2. What are the conditions and components of the standard hydrogen electrode? Answer: The SHE consists of a platinum foil coated with finely divided platinum, surrounded by hydrogen gas at 1 atm pressure in contact with a 1 M HCl solution at 298 K (25°C). The platinum electrode is inert and provides a surface for electron transfer.

Q3. Why is the potential of the standard hydrogen electrode set to zero? Answer: The potential of the SHE is arbitrarily set to zero by convention to provide a common reference point. This allows all other electrode potentials to be measured and compared relative to a single standard. It is not because the electrode has no potential; rather, it is a defined starting point.

Q4. Can the standard hydrogen electrode act as an anode? Answer: Yes, the SHE can act as either a cathode or an anode depending on the nature of the electrode with which it is connected. It acts as an anode when connected to a half-cell with a more positive reduction potential (e.g., copper), and as a cathode when connected to a half-cell with a more negative reduction potential (e.g., zinc).

• Q5. What is the half-reaction for the standard hydrogen electrode?
• Answer: The half-reaction is the reversible reduction of hydrogen ions to hydrogen gas:
• 2H^+_{(aq)} + 2e^- ⇌ H_{2(g)}
• The standard electrode potential for this reaction is defined as 0.00 V.

Q6. Explain with the help of the standard hydrogen electrode, how the oxidation potential of Zn is +0.76 V and its reduction potential is -0.76 V. Answer: When a zinc half-cell is connected to the SHE, zinc acts as the anode and the SHE as the cathode. The voltmeter reads 0.76 V. Using the formula E⁰_cell = E⁰_cathode - E⁰_anode: 0.76 = 0.00 - E⁰_anode Therefore, E⁰_anode = -0.76 V. This is the standard reduction potential of zinc. The oxidation potential is the reverse of the reduction potential, so it is +0.76 V.

Q7. Why is platinum used in the standard hydrogen electrode? Answer: Platinum is used because it is an inert (noble) metal that does not participate in the chemical reaction. It serves as a surface for the electron transfer between the hydrogen gas and the hydrogen ions in solution.

Q8. What is the significance of the standard conditions (1 M, 1 atm, 25°C) for the SHE? Answer: The standard conditions ensure that the measurement is reproducible and comparable across different experiments. The potential of a half-cell depends on the concentration of ions and the pressure of gases, so the standard conditions provide a fixed baseline for all measurements.

Multiple Choice Questions (MCQs)

• Q1. What is the standard electrode potential of the hydrogen electrode defined as?
• A. 0 V
• B. 1 V
• C. -1 V
• D. 0.5 V
• Correct Answer: A
• Explanation: The standard hydrogen electrode (SHE) is arbitrarily assigned a potential of exactly 0.00 V at all temperatures.

Q2. What is the role of the standard hydrogen electrode in electrochemistry? A. It acts as a catalyst in redox reactions. B. It serves as a reference electrode for measuring other electrode potentials. C. It provides a source of hydrogen gas for the reaction. D. It increases the cell potential. Correct Answer: B Explanation: The SHE is the primary reference electrode used to measure the standard electrode potentials of other half-cells. It has a standard electrode potential of 0 V by definition.

Q3. Which of the following is the correct half-reaction for the SHE? A. H_2 \rightarrow 2H^+ + 2e^- B. 2H^+ + 2e^- → H_2 C. H_2 + 2e^- \rightarrow 2H^+ D. 2H^+ → H_2 + 2e^- Correct Answer: B Explanation: The SHE can act as a cathode or anode, but by convention, the standard reduction potential is defined for the reduction half-reaction: 2H^+ + 2e^- → H_2.

• Q4. The standard hydrogen electrode consists of a platinum electrode immersed in:
• A. 1 M HCl solution
• B. 1 M CuSO_4 solution
• C. 1 M ZnSO_4 solution
• D. 1 M NaOH solution
• Correct Answer: A
• Explanation: The SHE uses a 1 M HCl solution to provide a 1 M concentration of H^+ ions.

• Q5. In the standard hydrogen electrode, hydrogen gas is bubbled at a pressure of:
• A. 0.5 atm
• B. 1 atm
• C. 2 atm
• D. 3 atm
• Correct Answer: B
• Explanation: The SHE operates under standard conditions, which require the pressure of hydrogen gas to be 1 atm.

Q6. When a zinc electrode is connected to the SHE, the measured cell potential is 0.76 V. What is the reduction potential of zinc? A. +0.76 V B. -0.76 V C. +1.52 V D. -1.52 V Correct Answer: B Explanation: E⁰_cell = E⁰_cathode - E⁰_anode. The SHE is the cathode (0.00 V), and the zinc is the anode. 0.76 = 0.00 - E⁰_anode, so E⁰_anode = -0.76 V.

Q7. The SHE can act as an anode when connected to a half-cell with: A. A more negative reduction potential B. A more positive reduction potential C. A potential of 0.00 V D. A potential equal to its own Correct Answer: B Explanation: The electrode with the more positive reduction potential acts as the cathode. If the SHE is connected to an electrode with a more positive reduction potential, the SHE will be the anode and will be oxidized.

Q8. What is the function of the platinum foil in the SHE? A. It reacts with the hydrogen gas. B. It provides an inert surface for electron transfer. C. It acts as a catalyst for the reaction. D. It prevents the hydrogen gas from escaping. Correct Answer: B Explanation: Platinum is an inert metal that does not participate in the reaction but provides a surface for the electron transfer between hydrogen gas and hydrogen ions.

• Q9. What does the symbol E⁰ represent in electrochemistry?
• A. The cell potential under non-standard conditions.
• B. The standard electrode potential.
• C. The equilibrium constant.
• D. The rate of electron transfer.
• Correct Answer: B
• Explanation: E⁰ designates the standard potential, measured under standard conditions (1 M concentration, 1 atm pressure, and 25°C).

Q10. Which of the following statements about the SHE is correct? A. It is a primary reference electrode. B. It is a secondary reference electrode. C. It is an electrolytic cell. D. It is a type of battery. Correct Answer: A Explanation: The SHE is the primary reference electrode because its potential is defined as 0.00 V, and all other standard electrode potentials are measured relative to it.

Multiple Choice Questions (MCQs)

• Q1. The standard potential of the SHE is exactly:
• A. 0.00 V
• B. 1.00 V
• C. -1.00 V
• D. 0.76 V
• Correct Answer: A
• Explanation: By convention, the standard hydrogen electrode is assigned a potential of exactly 0.00 V.

• Q2. Which metal is used as the electrode in the SHE?
• A. Copper
• B. Zinc
• C. Platinum
• D. Silver
• Correct Answer: C
• Explanation: The SHE uses a platinized platinum electrode because platinum is inert and provides a suitable surface for electron transfer.

• Q3. The concentration of H^+ ions in the SHE is:
• A. 0.1 M
• B. 1 M
• C. 2 M
• D. 0.5 M
• Correct Answer: B
• Explanation: The SHE operates under standard conditions, which include a 1 M concentration of H^+ ions.

• Q4. When a zinc electrode is connected to the SHE, the zinc acts as the:
• A. Cathode
• B. Anode
• C. Salt bridge
• D. Voltmeter
• Correct Answer: B
• Explanation: Because zinc has a more negative reduction potential than the SHE, it is oxidized and acts as the anode.

• Q5. The standard hydrogen electrode is used as a:
• A. Catalyst
• B. Reference electrode
• C. Product
• D. Reactant
• Correct Answer: B
• Explanation: The SHE is the universal reference electrode with a potential of 0.00 V, used to measure other electrode potentials.

• Q6. What is the standard reduction potential of zinc when measured against the SHE?
• A. +0.76 V
• B. -0.76 V
• C. +0.34 V
• D. -0.34 V
• Correct Answer: B
• Explanation: The standard reduction potential of zinc is -0.76 V, meaning it is oxidized when connected to the SHE.

• Q7. In the SHE, the hydrogen gas is at a pressure of:
• A. 0.5 atm
• B. 1 atm
• C. 2 atm
• D. 1.5 atm
• Correct Answer: B
• Explanation: The standard conditions for the SHE require hydrogen gas at 1 atm pressure.

• Q8. The temperature at which the SHE operates is:
• A. 0°C
• B. 25°C
• C. 100°C
• D. 273°C
• Correct Answer: B
• Explanation: Standard conditions include a temperature of 298 K, which is 25°C.

• Q9. Why is the SHE necessary for measuring electrode potentials?
• A. To provide a source of electrons for the reaction.
• B. To serve as a known reference point (0.00 V) for comparison.
• C. To increase the rate of reaction.
• D. To prevent the reaction from occurring.
• Correct Answer: B
• Explanation: The SHE provides a stable reference potential of 0.00 V, allowing the relative potentials of other half-cells to be determined.

Q10. When a copper electrode is connected to the SHE, which acts as the cathode? A. Copper B. SHE C. Both D. Neither Correct Answer: A Explanation: Copper has a more positive reduction potential (+0.34 V) than the SHE (0.00 V), so copper acts as the cathode (reduction occurs there) and the SHE acts as the anode.