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Measuring Electrode Potential

Chemistry

definition

Standard Hydrogen Electrode

A Standard Hydrogen Electrode (SHE) is an electrode that scientists use as a reference on all half-cell potential reactions. SHE is the most commonly used reference electrode since its potential is exactly equal to Zero at all temperatures. Since the potential of SHE is zero, it forms the perfect basis to calculate cell potentials using different electrodes or different concentrations. 

definition

Standard Calomel Elecrode

The Saturated calomel electrode (SCE) is a reference electrode based on the reaction between elemental mercury and mercury(I) chloride. The electrode consists of a broad glass tube with a side tube. The broad tube consists of pure at the bottom, covered with a saturated paste of and . The tube is then covered with saturated . Electrical contact with is made by a platinum wire sealed in the glass tube. The side tube is immersed in the desired solution. Electrode potential depends upon concentration of solution.
Reactions that can occur at Calomel electrode are:
- Oxidation
- Reduction
Representation of this electrode is given by:

definition

Standard electrode potential

The electrode and cell potentials depends on the concentration of solutions, partial pressure of gases and the temperature.Therefore , these potentials are measured under standard conditions defined as standard electrode potential.

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Applications and difficulties of standard electrode potential

It is used in calculation of strength of oxidizing and reducing agents is done by its standard electrode potential.
Difficulties of standard electrode potential  : although the overall potential of a cell can be measured, there is no simple way to accurately measure the electrode potential in isolation. The electric potential also varies with temperature, concentration and pressure.

definition

Working of S.H.E

S.H.E. consists of a platinum wire which is connected to a square-shaped platinum electrode which is covered with finely divided platinum black. This covering of particles of platinum black helps in increasing the surface area of the electrode, which helps in the efficient conduction of electricity. The whole apparatus is then kept in a glass tube and is placed in a beaker containing 1M HCl solution. Hydrogen gas is then passed through the tube and into the solution causing the reaction

During the working of S.H.E. either of the following reactions can take place at the surface of the platinum:
- Reduction 
- Oxidation
Depending upon the tendency of other electrodes, S.H.E. can either act as a Cathode (Reduction reaction will take place) or as an Anode (Oxidation reaction will take place). 
If the natural tendency of the other electrode is to undergo Oxidation, S.H.E. will act as a Cathode.
If the natural tendency of the other electrode is to undergo Reduction, S.H.E. will act as an Anode.
S.H.E. is represented as:

definition

Reference Electrode

A reference electrode is an electrode which has a stable electrode potential and it's value is known. Such stable potential is achieved by maintaining the concentrations of constituent species at a fixed value. 
Reference electrodes are used at various places, but the most important of all is in the electrochemical cell. This is where it's used as a half cell in the electrochemical cell to allow for the determination of the other half's cell potential.
Common examples include Standard hydrogen electrode, Saturated calomel electrode, Silver chloride electrode etc.

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Advantages and Disadvantages of S.H.E.

Advantages of Standard Hydrogen Electrode (S.H.E.):
1. During electrolysis, only small potential is developed on the hydrogen electrode. Hence assumption of Zero potential of S.H.E. holds true for almost all the cases.
2. In determining the single electrode potential, using S.H.E. as a reference, the potential of the unknown potential will be equal to the e.m.f. of the cell as electrode potential of S.H.E. is Zero. Hence it becomes very handy to use S.H.E. as a reference electrode for determining the potential of unknown electrode. This way, we can determine the absolute value of the electrode potential.
Disadvantages of Standard Hydrogen Electrode (S.H.E.):
1. It is not convenient to assemble the apparatus.
2. It is difficult to maintain the pressure of hydrogen gas at 1atm and concentration of HCl at 1M all the time.
3. It is difficult to get pure, dry hydrogen gas and prepare ideal platinised platinum plate.
4. Its very costly and is difficult to transport due to its bigger size.

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Applications and Advantages of S.C.E.

Applications of Standard Calomel Electrode (S.C.E.):
1. The S.C.E is used in pH measurement, cyclic voltammetry and general aqueous electrochemistry. 
2. It serves the same purpose as the S.H.E. It helps in determining the absolute value of the electrode potential of an unknown electrode.
Advantages of S.C.E.:
1. It is very handy, compact and easy to transport.
2. Its potential can remain constant and it can easily be reproduced.
3. It is easy to construct and maintain.
It is for these reasons, it is more convenient to go for S.C.E. instead of S.H.E.

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Determination of Standard Electrode Potential of Zn electrode

To determine the Standard Electrode Potential of Zn electrode, a zinc rod is dipped in 1 M zinc sulphate solution. This half-cell is combined with a standard hydrogen electrode through a salt bridge. Both the electrodes are connected with a voltmeter. The deflection of the voltmeter indicates that current is flowing from hydrogen electrode to metal electrode or the electrons are moving from zinc rod to hydrogen electrode. The zinc electrode acts as an anode and the hydrogen electrode as cathode.
Cell can be represented as:
Oxidation half-cell reaction will be:
Reduction half-cell reaction will be:
EMF of the cell is 0.76V.

i.e.
i.e.
As the reaction on the anode is oxidation, i.e.,,
Hence standard reduction potential of Zn electrode will be -0.76V.

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Determination of Standard Electrode Potential of Cu electrode

To determine the Standard Electrode Potential of Cu electrode, a copper rod is dipped in 1 M solution of . It is combined with hydrogen electrode through a salt bridge. Both the electrodes joined through a voltmeter. The deflection of the voltmeter indicates that current is flowing from copper electrode towards hydrogen electrode, i.e., the electrons are moving from hydrogen to copper electrode. The hydrogen electrode acts as an anode and the copper electrode as a cathode. The cell can be represented as: Cell can be represented as:
Oxidation half-cell reaction will be:
Reduction half-cell reaction will be:
EMF of the cell is 0.34V.

i.e. i.e.
As the reaction on the cathode is reduction, i.e.,,
Hence standard reduction potential of Zn electrode will be 0.34V.