Standard Hydrogen Electrode (SHE) and Standard Electrode Potential
Chemistry · Electrochemistry · NEET
We cannot measure a single electrode's potential alone, so we pick one electrode as the "zero mark". That reference is the Standard Hydrogen Electrode (SHE), and its potential is fixed at exactly 0 V. Every other electrode's standard electrode potential (E°) is just its reading compared to SHE. Memory hook: SHE = the "sea level" of chemistry. Just like we measure mountain heights from sea level (0 m), we measure all E° values from SHE (0 V).
The SHE: a platinum-black plate in 1 M H+ solution with H2 gas at 1 bar (298 K). This half-cell is fixed at E° = 0 V and is the reference for measuring every other standard electrode potential.
Your doubts, answered
Why is the potential of the standard hydrogen electrode taken as zero?
We cannot measure the potential of a single electrode by itself. A voltmeter always needs two points to give a reading. So chemists agreed to fix one electrode as the zero point. They chose the hydrogen electrode under standard conditions and gave it E° = 0 V at all temperatures, by convention. It is not that hydrogen truly has zero potential in nature. It is a chosen reference, like calling sea level 0 metres. Once SHE = 0 V, every other electrode can be measured against it.
What is the exact setup of a standard hydrogen electrode?
It is a platinum (Pt) plate coated with platinum black (finely divided Pt), dipped in an acidic solution where the H+ concentration is 1 mol per litre (1 M). Pure hydrogen gas is bubbled over the plate at 1 bar pressure. The temperature is usually 298 K. Its cell notation is Pt(s) | H2(g, 1 bar) | H+(aq, 1 M). The Pt does not react; it only gives a surface for the reaction 2H+ + 2e- ⇌ H2 to happen and carries electrons.
What is the difference between electrode potential and standard electrode potential?
Electrode potential is the tendency of an electrode to gain or lose electrons in any solution. Standard electrode potential (E°) is this tendency measured under fixed standard conditions: all ions at 1 M, gases at 1 bar, and 298 K, against the SHE. In short, E° is the electrode potential measured under standard conditions. NEET numerical problems almost always use E° values, so read the question for the word 'standard'.
Why do we always use reduction potential and not oxidation potential now?
By IUPAC convention, standard electrode potential is always written as a standard reduction potential (the tendency to gain electrons). So a table value like E°(Zn2+/Zn) = -0.76 V is a reduction value. A more negative E° means the species is harder to reduce, so it prefers to lose electrons (get oxidised) and acts as a strong reducing agent. NEET tables give reduction potentials, so never flip the sign unless you truly reverse a reaction.
Does the SHE always act as the anode?
No. SHE can be either anode or cathode depending on the other electrode. If the other electrode has a positive E° (like Cu2+/Cu, +0.34 V), that electrode pulls electrons, so SHE gives electrons and becomes the anode. If the other electrode has a negative E° (like Zn2+/Zn, -0.76 V), that electrode loses electrons, so SHE gains them and becomes the cathode. The sign of E° tells you the direction.
⚠️ The NEET trap ✗ Zinc is coated on iron because zinc has a lower (less) negative electrode potential than iron, so it protects iron. ✓ Zinc has a HIGHER (more) negative electrode potential (E°(Zn2+/Zn) = -0.76 V) than iron (E°(Fe2+/Fe) = -0.44 V). A more negative E° means zinc is oxidised more easily, so zinc corrodes first and protects the iron. 🧠 More NEGATIVE E° = stronger reducing agent = gets oxidised first. Students flip 'higher' and 'lower' because -0.76 is a bigger number in size but a lower value. Read the number line: -0.76 is to the LEFT of -0.44, so it is lower and more negative.
Real NEET questions
NEET 2016 Phase 2
Zinc can be coated on iron to produce galvanized iron but the reverse is not possible. It is because
A · Zinc is lighter than iron
B · Zinc has lower melting point than iron
C · Zinc has lower negative electrode potential than iron
D · Zinc has higher negative electrode potential than iron ✓
Solution: Compare standard reduction potentials: E°(Zn2+/Zn) = -0.76 V and E°(Fe2+/Fe) = -0.44 V. Since -0.76 V is more negative than -0.44 V, zinc is oxidised more easily. So zinc acts as the anode and is corroded first (sacrificial protection), shielding the iron underneath. Coating iron on zinc would leave zinc exposed, so it is not done. This directly uses standard electrode potential values measured against SHE.
NEET 2019 Odisha
The standard electrode potential (E°) values of Al3+/Al, Ag+/Ag, K+/K and Cr3+/Cr are -1.66 V, 0.80 V, -2.93 V and -0.74 V respectively. The correct decreasing order of the reducing power of the metals is:
A · Ag > Cr > Al > K
B · K > Al > Cr > Ag ✓
C · K > Al > Ag > Cr
D · Al > K > Ag > Cr
Solution: Reducing power increases as E° becomes more negative (lower). Order of E° from lowest to highest: K+/K (-2.93) < Al3+/Al (-1.66) < Cr3+/Cr (-0.74) < Ag+/Ag (+0.80). So decreasing reducing power is K > Al > Cr > Ag. All these E° values are reduction potentials measured against the SHE (0 V).
NEET 2016 Phase 1
The pressure of H2 required to make the potential of the H2 electrode zero in pure water at 298 K is:
A · 10^-14 atm ✓
B · 10^-12 atm
C · 10^-10 atm
D · 10^-4 atm
Solution: For the hydrogen electrode 2H+ + 2e- → H2, E° = 0. Using the Nernst equation E = E° - (0.059/2) log (P(H2)/[H+]^2). In pure water [H+] = 10^-7 M. Set E = 0 and E° = 0: this gives P(H2)/[H+]^2 = 1, so P(H2) = [H+]^2 = (10^-7)^2 = 10^-14 atm. This tests that SHE's zero potential is tied to the standard conditions of 1 M H+ and 1 bar H2.
Solved Electrochemistry NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Exactly 0 volts (0 V), by convention, at all temperatures. It is the fixed reference point for all other E° values.
What are the standard conditions for a standard hydrogen electrode?
H+ ion concentration = 1 mol per litre (1 M), hydrogen gas pressure = 1 bar, and temperature usually 298 K, with a platinised platinum electrode.
Why is platinum used in the SHE?
Platinum is inert, so it does not take part in the reaction. It only provides a surface for the H2/H+ reaction and conducts electrons. Platinum black (rough Pt coating) increases surface area for faster reaction.
Is the standard hydrogen electrode a reduction or oxidation electrode?
It can act as either. By IUPAC, its E° is written as a reduction potential (0 V), but in a real cell it becomes the anode or cathode depending on the other electrode's E°.
Why is SHE important for NEET?
Almost every E° value in your data booklet is measured against SHE. Reducing power order, feasibility of reactions, and EMF calculations all depend on these values, so understanding the zero reference is essential.
