Electrode potential is simply the potential of a metal to gain electrons and undergo reduction and is measured in volts with the help of a voltmeter. Thus, it can also be called reduction potential.The proper definition is given below:
''It is the potential of an electrode measured with a voltmeter when the electrode in question is connected with a standard hydrogen electrode or SHE through a salt bridge.''
The equation given above is the standard way of writing electrode reactions i.e. in the form of reduction equations. It however, should be noted that the reaction can proceed in either direction depending on the environment as shown by the sign of equilibrium.
Standard Electrode Potential:
Standard electrode potential can simply be taken as electrode potential measured under standard conditions. Well of course, you will need a better definition:
"Potential of a cell when an electrode is connected to a standard hydrogen electrode under standard conditions.''
OR
'' It is the voltage of a half-cell measured under standard conditions with a standard hydrogen electrode as the other half-cell."
Similarly, Standard Cell Potential can be defined as the difference of the electrode potential values two electrodes connected through a salt bridge under standard conditions, which forms the voltage of that cell.
Standard Hydrogen Electrode forms the basis of thermodynamic scale of oxidation of oxidation-reduction potentials. Its standard potential has been declared to be zero at all temperatures to form the basis for comparison with all other electrode reactions. Platinum is used as an electrode because it provides an inert connection to the H2(g)/H+(aq) system and adsorbs hydrogen gas at its surface to facilitate the formation of equilibrium quickly.
Note: The standard conditions are necessary to maintain in the measurement of standard electrode potential because electrode potential varies with conditions such as temperature. The standard conditions are:
1. Concentration of 1 mol/dm^3.
2. Temperature of 25 degree centigrade of 298 K.
3. Pressure of 1 atmospheric pressure.
"Potential of a cell when an electrode is connected to a standard hydrogen electrode under standard conditions.''
OR
'' It is the voltage of a half-cell measured under standard conditions with a standard hydrogen electrode as the other half-cell."
Similarly, Standard Cell Potential can be defined as the difference of the electrode potential values two electrodes connected through a salt bridge under standard conditions, which forms the voltage of that cell.
Note: The standard conditions are necessary to maintain in the measurement of standard electrode potential because electrode potential varies with conditions such as temperature. The standard conditions are:
1. Concentration of 1 mol/dm^3.
2. Temperature of 25 degree centigrade of 298 K.
3. Pressure of 1 atmospheric pressure.
To measure Standard Electrode Potential connect the electrode in question to an SHE under standard conditions as shown by the diagram above. If the system in question concerns ions of same element in different oxidation states like that of Fe2+/Fe3+ then a platinum electrode is used in equilibrium with the aqueous solution of the ions. The reading is then taken from the voltmeter. Here it should be noted that the electrode potential of the hydrogen electrode is taken to be 0.0V for comparison with other electrodes. Therefore, the value of voltmeter here is the standard electrode potential of the electrode in question.
If the voltmeter gives negative value then that indicates that SHE is behaving as a cathode and if the voltmeter gives positive value then that means SHE is behaving as an anode. Remember that oxidation occurs at anode and reduction at cathode. You can remember this with the help of this short phrase, AN OX and RED CAT.
However, if we know the electrode potentials of the two electrodes connected in a cell we can calculate the Eo value of the cell with the help of the formula given above in the diagram. Since, the electrode potential values are the measure of reduction potentials the electrode with higher electrode potential undergoes reduction and the one with lower electrode potential value is the anode. Cell potential can then be found by subtracting electrode potential of the anode from that of cathode.
The direction of electron flow is always from where the electrons are being produced towards where they are consumed i.e. from anode to cathode.
Salt Bridge?
It consists of a U-tube filled with concentrated solution of potassium chloride along with gelatin or agar. It is used to electrically connect the two half cells and to provide electrical neutrality by allowing migration of ions. However, the salt used in salt bridge must not react with chemicals in the half cells.
-Psycho Killer
