For a standard hydrogen electrode, which statement about its electrode potentials is correct?

Introduction / Context:
This question concerns the standard hydrogen electrode, often abbreviated as SHE, which is the reference electrode used to define standard electrode potentials. In electrochemistry, all standard reduction potentials are measured relative to this electrode. It is assigned a conventional value, but its true absolute potential in volts is different from that conventional assignment. The question tests whether the learner can distinguish between absolute and conventional standard potentials.

Given Data / Assumptions:

• Standard hydrogen electrode is considered under standard conditions.
• Four statements about its potentials are given.
• Standard electrode potential of SHE is conventionally taken as zero volt.
• Absolute potential is the potential of the electrode relative to a vacuum reference, which is more difficult to measure.

Concept / Approach:
By international convention, the standard electrode potential of the hydrogen half cell, written as 2H+ + 2e− → H2, is assigned as exactly 0.00 volt at all temperatures where standard conditions apply. This is not because the true potential is physically zero but because we need a reference point. In reality, when compared to a theoretical vacuum level, the hydrogen electrode has a non zero absolute potential, often quoted around 4.4 volts. Therefore, any statement claiming that the absolute electrode potential is zero is incorrect, while the statement that only the absolute potential is non zero is correct.

Step-by-Step Solution:
Step 1: Recall the convention. The standard hydrogen electrode is assigned a standard electrode potential E standard = 0.00 volt by definition. Step 2: Understand absolute potential. Absolute electrode potential is measured relative to a vacuum and is not set by arbitrary convention. Experimental and theoretical studies show that this absolute value is around 4.4 volts, not zero. Step 3: Evaluate each statement. Statement A: Absolute electrode potential is not zero. This agrees with the idea that the true physical potential has a non zero value. Statement B: Absolute electrode potential is zero. This contradicts the above understanding. Statement C: Both absolute and standard electrode potentials are zero. This cannot be correct because only the standard value is defined as zero. Statement D: Electrode potential is zero only at 25 degree Celsius. In fact the standard value is taken as zero at all temperatures under standard conditions, so this is also wrong.

Verification / Alternative Check:
Electrochemistry tables explicitly state that the standard hydrogen electrode has been assigned a standard reduction potential of zero volt and that all other electrode potentials are measured relative to this. Specialist sources on absolute electrode potentials mention approximate values around 4.4 volts. Since the question contrasts absolute and standard potentials, we can confidently say that the absolute one is not zero. This cross check confirms statement A as the correct one.

Why Other Options Are Wrong:
Option B: Saying that the absolute electrode potential is zero ignores the physical meaning of absolute potential and is inconsistent with theoretical and experimental estimates. Option C: Claiming both absolute and standard potentials are zero combines the errors of option B and contradicts known data. Option D: The standard hydrogen electrode is defined so that its standard potential remains zero by convention at all temperatures, not only at 25 degree Celsius, so limiting the statement to a single temperature is incorrect.

Common Pitfalls:
Students sometimes treat the assigned standard value and true physical value as the same quantity. It is important to remember that standard electrode potentials are relative numbers based on a chosen reference. Confusing absolute potential with standard potential can lead to wrong answers in conceptual questions like this. Keeping the distinction clear between a conventional zero point and a measured quantity avoids this confusion.

Final Answer:
For a standard hydrogen electrode, the correct statement is that absolute electrode potential is not zero.