Measuring EMF

EMF Definition

EMF is the abbreviation for electromotive force. Although the name has force in it, it is actually not a force, but a form of energy. Let us look into the definition of EMF from the perspective of chemistry.

What is electrode potential?

To understand what it is, let us first recapitulate the processes that happen at each electrode.

Fig. 1: Daniel cell.

The above picture represents a Daniel cell, an example of voltaic cell. You already know from the article- Electrochemical Cells, what happens and how electricity is generated. Let us cover the main points to understand the electrode potential.

Zinc, dipped in zinc sulfate is the anode half-cell. The zinc metal atoms lose electrons that pass through the wire and reach the cathode where the copper ion in the electrolyte accepts those electrons. This is the net ionic reaction.

Thus, Zinc, on losing electrons becomes oxidised changing into its cation- \(Zn^{+2}\) . This is oxidation-half-reaction. (These zinc cations jump into the electrolyte.)

As the zinc oxidises to zinc cations, there a separation of charge develops between the electrode and electrolyte. This separation of charge leads to a potential difference of that electrode which is called single electrode potential. Thus, the tendency of an electrode to generate a potential difference between itself and the electrolyte is called the single electrode potential of the half-cell( \(Zn\) in \(ZnSO_4\) ).

Similarly, Copper ions (+2) get reduced to copper atoms(0) which is reduction half-reaction. (The reduced copper now gets deposited on the copper electrode.) This will result in a potential difference in this cathode half cell between the electrode and electrolyte.

Simply put,

This standard electrode potential is the emf of that half-cell. The difference in the electrode potentials of the half-cells is the total emf of the cell, also called cell potential, E or \( \epsilon \)

EMF, the driver of electron flow

For the electrons to flow, there must be some energy that propels them forward. The difference in electrode potentials - the Potential difference, or the total emf of the cell, makes the electrons flow.

How emf makes the electrons flow?

Electrons have potential energy in them. As there is an increase in the number of electrons, the overall potential energy increases, thus making the anode half-cell as a region with high potential energy (PE).^[2] Also, accumulation of electrons leads to repulsions and electrons just want to get away from there. While on the other hand, the region of cathode is short of electrons, hence short of potential energy-thus, cathode is a region with low potential energy.^[2]

Now, this potential difference (High vs low) causes the electrons to move from anode (high PE) to cathode (low PE) just like in a water fall where the water from high-pressure flows down to the region of low pressure. This flow of electrons generates electricity as discussed in the introduction.

This potential difference is the driving factor for the movement of electrons and generation of electricity. EMF is the difference in the individual potential differences (electrode potentials/reduction potentials) of the two electrodes. Just like the difference in masses is again a mass and measured in grams , the difference in individual potential differences is again a potential difference measured in Volts.

Hence, \(E_{Cathode}- E_{Anode\} \) gives us the emf of that cell.

The journey of electrons in terms of reduction potential

This concept of potential energy is not to be confused with electrode potentials. Anode has low electrode potential-meaning low reduction potential-and this is the reason the metal at anode does not want to get reduced, but wants to oxidise. Hence, wants to kick the electrons. On the other hand, Cathode has high electrode potential-meaning high reduction potential, so it grabs the incoming electrons.

Fig. 2: Representation of an anode and a cathode.

To put the journey of electrons in terms of reduction potential, we can say that in a galvanic cell, electrons travel from a region of low potential (Anode) to high potential (Cathode)- [from a region where the tendency to get reduced is low to a region where the tendency to get reduced is high.]

EMF units of measurement

EMF is measured in Volts (the SI unit). In physics, emf is also described as the work done-measured in Joules to move a unit charge- in Coulomb from one point to the other.

Thus, Volt, \(V = \frac{W} {Q} \)

Where:

• W = Work done = J
• Q = Unit charge = Coulomb

$$V=\frac{J}{C}$$

Volt is also the unit for standard reduction potential.

EMF Measuring Devices

Two devices are used to measure the emf- the Potentiometer and the Voltmeter.

Usually, the potentiometer is preferred over the voltmeter because the potentiometer does not draw any current from the circuit to measure the emf, thus does not disturb the circuit. Voltmeter, on the other hand, will draw some current from the circuit to measure the emf. Therefore, the potentiometer is more reliable and accurate as compared to the voltmeter,but, at the same time, more expensive than the voltmeter which is a disadvantage.

Fig. 3: Photo of Voltmeter | Wikimedia Commons

Back EMF measurement

The name itself gives us some information regarding this type of EMF. Back EMF or rear EMF is that potential difference that works against the applied voltage/actual emf, which can be defined as:

The best example to explain back emf is an electrolytic cell containing platinum electrodes dipped in sulfuric acid solution. We know what happens in electrolysis of acidified water. Hydrogen gas evolves at cathode, while oxygen gas evolves at anode. Some external voltage(EMF through battery) is applied to drive this reaction as this is non-spontaneous.

After a while, the electrolysis comes to a halt because of the adsorption of gases on the surface of the electrodes. This causes back emf ,which when surpasses the applied external voltage, causes the electrolysis to stop. To overcome this counter attack, we must slowly increase the external voltage. When this external emf again exceeds the back emf, the electrolysis proceeds smoothly.

Applications of EMF Measurement

There are several applications of measuring emf. Here, we give you a list of a few significant applications:

• The Equilibrium constant for a half-cell reaction can be determined by measuring the emf.
• The gibb's free enegy.
• The solubility product of a sparingly soluble salt can be determined.
• The valency of the number of electrons transferred in a half cell reaction can be determined.
• Free energy.
• \(p^H\) of a solution can be evaluated.
• Spontaneity of a electrochemical reaction can be determined. If the cell potential is positive, it is a spontaneous reaction (of the galvanic cell). If the cell potential is negative, it is a non-spontaneous reaction (of the electrolytic cell).

That concludes the topic of measuring emf. Through this article, we have explained what emf is and how it is responsible for the flow of electrons and in turn for the generation of electricity. Now you might have understood the concept behind the batteries you are using in your day-to-day life.