Daniell Cell

A Daniell cell is a type of electrochemical cell that consists of a copper pot that is filled with a solution of copper(II) sulphate. An unglazed earthenware is immersed in this solution containing sulphuric acid and a zinc electrode. The Daniell cell was invented by a British chemist and meteorologist, John Frederic Daniell in 1836.  The Daniell cell was invented, while the chemist was seeking a way to eliminate the hydrogen bubble issue found in the voltaic pile. The Daniell cell was seen as good progress in battery development, if we consider the technology of those times.  

Definition of Daniell cell

The Daniell cell can be defined as a primary voltaic cell that has sulphuric acid separated by a porous barrier from a copper electrode in sulphate solution. 

The Daniell cell is an improved version of the voltaic cell. The polarisation issue faced in a voltaic cell is addressed in a Daniell cell. 

Construction of the Daniell cell

The Daniell cell is basically a container made of copper that contains a concentrated solution of copper sulphate. There is one more container inside this copper container. This container is porous and contains dilute sulphuric acid. A zinc amalgam rod is immersed in this sulphuric acid within the porous pot. 

Working of Daniell cell

There are H+ ions and SO4- ions in dilute sulphuric acid. When the two metal electrodes are connected with help of an external circuit, the copper electrode attracts the electrons leftover in the zinc as the metal oxidises in zinc sulphate. The oxidation process of zinc causes it to corrode, and it is indicated by the darkening of the zinc.

The electrons of zinc that the copper receives through the circuit combine with the positively charged copper ions from the copper sulphate solution and forms deposits of solid copper on the copper electrode. 

The sulphates play a crucial role in maintaining the circuit. For every pair of electrons of zinc that goes to the copper electrode, one sulphate ion passes through the solution for compensation from the copper side through the porous earthenware immersed in the dilute sulphuric acid. This allows the flow of electrons through the circuit. The zinc metals break up into positive ions that combine with the sulphate ions to release more electrons. 

Chemical reactions

At the anode, zinc is oxidised according to this reaction:

Zn(s)→ Zn+2 + 2e-

At the cathode, the copper is reduced according to this reaction:

Cu+2 + 2e- → Cu(s)

One must notice that the positively charged copper ions always move towards the anode that is positively charged. This is due to reduction in Chemical energy.

The whole reaction can be written as:

Zn(s) + Cu+2 → Zn+2(aq) + Cu(s)

These processes lead to the corrosion of the zinc electrode into the solution as zinc ions and the accumulation of solid copper at the positive electrode. 

The Daniell cell produces approximately 213 kJ per mole of zinc. This is due to the weaker bonding in zinc compared to copper metal.

Usually, for demonstration in classrooms, a form of Daniell cell also called the two-half cells is used to simplify the explanation.

Applications of Daniell cell

 Daniell cells are used mostly in electrometallurgical processes. 

Bird’s cell

A Daniell cell variant was invented in 1837 by a physician named Golding Bird. To make up for the porous barrier, he used plaster of Paris to separate the two solutions. This experiment was crucial in the progress of electrometallurgy as this discipline was quite new at those times. It was observed in Bird’s experiment that solid copper deposited on the plaster of Paris that too without any contact with the metal itself. This was quite shocking as it was usually believed during those times that metal was deposited only on metal surfaces.

Electrotyping

For the very first time, Daniell cell’s features were used commercially for copper plating by an instrument maker from Liverpool known as John Dancer. In a process called electrotyping, one could mould objects to any desired shapes by using the porous barrier and John Dancer made good use of these features. However, many others had made the same discovery at that time therefore, John Dancer couldn’t take the credit for the invention of electrotyping instead it goes to a Russian named Moritz von Jacobi.

Conclusion

A Daniell cell is a type of electrochemical cell that consists of a copper pot filled with copper sulphate solution. This copper sulphate solution has a porous earthenware filled with dilute sulphuric acid and a zinc electrode.  The Daniell cell was invented in the year 1836 by a British chemist, John Frederic Daniell. One can find its uses in electrometallurgy and electrotyping.