Determination of Electrode Potential by SHE

Structure of Standard Hydrogen Electrode (SHE):

It consists of a specially prepared platinum rod, immerse in 1M acid solution. Temperature is kept at 298 K (25°C). Hydrogen gas is bubbled at 1 atm.

Both oxidation and reduction is possible

H₂ →  2H⁺   + 2e^- [oxidation]

2H⁺   + 2e^- →  H₂ [reduction]

Figure 5.1: Schematic representation of SHE

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Working of SHE:

i. SHE is coupled with an electrode of unknown potential.

ii. A voltaic cell is formed and Spontaneous run will take place.

iii. Cell potential is measured with voltmeter. And we know that Ecell= E_R-E_O.

iv. If reduction takes place at SHE, then SHE is cathode and anode is unknown.

E⁰cell= E⁰_R-E⁰_O

E⁰cell = E⁰_H - E⁰_unknown As E⁰_H = 0.00V So,

E⁰cell = - E⁰_unknown

v. If oxidation takes place at SHE, then SHE is anode and cathode is unknown.

E⁰cell= E⁰_R-E⁰_O

E⁰cell = E⁰_unknown -E⁰_H As E⁰_H = 0.00V So,

E⁰cell =  E⁰_unknown

 vi. In case of SHE:

· Reduction shows increase in pressure.

· Oxidation shows decrease in pressure.

SHE as cathode	SHE as anode
2H+   + 2e- →  H2 M  →  M+2 + 2e-	H2 →  2H+   + 2e- M+2 + 2e- → M
Final eq: M + 2H+  → M+2 + H2	M+2 + H2 → M + 2H+

Measurement of electrode potential by SHE.

Suppose a Zn electrode. A cell is formed by coupling SHE with Zn electrode. This cell consists of SHE in one beaker and Zn rod in 1M ZnCl₂ or Zn ions in another beaker. When circuit is complete, (electrodes are connected) then following observations can be noticed:

· Potential of a cell = 0.763.

· Concentration of Zn ions increases (Zn  → Zn⁺²).

· Concentration if H+ ions in SHE compartment decreases(2H⁺ →  H₂).

· Zn electrode is acting as anode.

· SHE is acting as cathode.

Reaction occurred as:

Zn  → Zn+2  + 2e 2H+   + 2e- →  H2	E⁰cell = 0.763V E⁰cell = 0.00V
Zn  +2H+  → Zn+2  + H2	E⁰cell = 0.763V

Cell potential of Zn

E⁰cell = E⁰_R - E⁰_O

0.763V = 0.00V - E⁰_O

E⁰_O = -0.763 [Cell potential of Zinc]

Or E⁰_Zn = -0.763

Now suppose a Cu + SHE Voltaic cell. This cell will consist of SHE in one beaker and Cu in Cu⁺² 1M in in another beaker. When circuit is complete following observations can be noticed:

· Ecell = 0.337V

· concentration of H⁺ increases (H₂ →  2H⁺)   

· concentration of Cu⁺² decreases (Cu⁺² → Cu)

· SHE is acting as anode  

· Cu is acting as cathode

Reaction occurred as:

H2 →  2H+   + 2e- Cu+2 + 2e → Cu	E⁰cell = 0.00V E⁰cell = 0.337V
Cu+2 + H2 → 2H+ +Cu	E⁰cell = 0.337V

Cell potential of Cu

E⁰cell = E⁰_R - E⁰_O

0.37V = E⁰_R – 0.00v

E⁰_R = 0.337 [Cell potential of copper]

Or E⁰_Cu = 0.337

Potential of a cell having copper and zinc electrode

Zn  → Zn+2  + 2e Cu+2 + 2e → Cu	E⁰cell = 0.76V E⁰cell = 0.337V
Zn + Cu+2  → Zn+2  +Cu	E⁰cell = 1.1V

Cell potential:

E⁰cell = E⁰_R - E⁰_O

E⁰cell = 0.337 – (-0.76)

E⁰cell = 1.1V 

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