- Daniell Cell Definition: A Daniell Cell is defined as an improved version of the Voltaic Cell that prevents polarization by converting chemical energy into electrical energy.
- Construction of Daniell Cell: The cell consists of a copper container with copper sulfate solution and a porous pot filled with diluted sulfuric acid containing a zinc rod.
- Oxidation and Reduction: Oxidation occurs at the zinc rod (cathode), forming zinc sulfate, while reduction occurs at the copper container (anode), depositing copper.
- Ion Movement: Hydrogen ions move through the porous pot to form sulfuric acid in the copper sulfate solution, facilitating continuous cell reactions.
- Avoiding Polarization: The Daniell Cell prevents hydrogen gas buildup on the anode by converting it into sulfuric acid, ensuring efficient operation.
A Daniell Cell is a zinc-copper galvanic cell developed as a more stable alternative to the early Voltaic Cell. It separates the zinc and copper reactions into different electrolytes, reducing the hydrogen polarisation that affected simple acid cells. A historical Daniell Cell produces about 1.1 V, although its voltage depends on solution activities and operating conditions.
Construction of Daniell Cell
The original Daniell design used a copper electrode in concentrated copper sulfate solution outside a porous earthenware pot. An amalgamated zinc rod stood inside the pot, which could contain dilute sulfuric acid, zinc sulfate or acidified zinc sulfate. Modern diagrams commonly show a zinc electrode in zinc sulfate and a copper electrode in copper sulfate, joined by a porous separator or salt bridge. In either form, the separator permits ionic conduction while slowing bulk mixing of the electrolytes.
When the external circuit is closed, zinc metal is oxidised to Zn²+ and releases two electrons at the zinc electrode. Copper ions gain those electrons and are reduced to copper metal at the copper electrode. Therefore, the zinc is the negative anode and the copper is the positive cathode. Ion migration through the separator maintains charge balance; sulfate does not release the electrons that oxidise zinc.
Working of Daniell Cell
The cell reaction can be followed at the two electrodes and through the separator.
If dilute sulfuric acid is used in the porous pot, it contains H+ and SO42− ions.
The H+ ions remain part of the acid electrolyte, while zinc atoms at the metal surface become Zn2+ and release two electrons. Sulfate SO42− provides ionic charge balance and zinc sulfate ZnSO4 accumulates in the zinc-side electrolyte. This loss of electrons is oxidation, so the zinc rod is the negative anode, not the cathode.
Ions move through the porous wall to maintain electroneutrality. In the historical acid-filled form, H+ and sulfate redistribute as zinc sulfate forms, and the overall chemistry can be represented with H2SO4 on both sides of the combined reactions. Copper ions Cu2+ remain the species reduced at the copper surface.
At the copper electrode, Cu²+ + 2e− → Cu. Copper metal deposits as electrons arrive through the external circuit. Reduction makes the copper container the positive cathode, not the anode. The electrons flow through the load from zinc to copper. The net reaction is Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s).
In a Daniell Cell, copper ions accept electrons at the cathode and deposit as copper. This avoids the insulating hydrogen film associated with the simple acid Voltaic Cell and gives a more stable output. The improvement does not come from hydrogen forming sulfuric acid before reaching a copper anode: copper is the cathode, and the defining cell reaction transfers zinc into solution while copper leaves solution.
