Construction and Working of the Daniell Cell

The Daniell cell, named after the English chemist John Frederic Daniell, is built from two half-cells: a zinc rod dipped in a $1\ \text{M}\ \text{ZnSO}_4$ solution, and a copper rod dipped in a $1\ \text{M}\ \text{CuSO}_4$ solution. The two rods are connected externally through a wire (with a voltmeter or external load), and the two solutions are connected internally by a salt bridge — commonly a U-shaped tube filled with an agar gel soaked in an inert electrolyte such as $\text{KCl}$ or $\text{KNO}_3$, chosen so that neither of its own ions interferes with the cell reaction.

At the zinc electrode, zinc metal is spontaneously oxidized: $\text{Zn}(s) \to \text{Zn}^{2+}(aq) + 2e^{-}$. This is the anode. The two electrons released travel through the external wire — not through the solution — to the copper electrode, where they reduce copper(II) ions from the solution: $\text{Cu}^{2+}(aq) + 2e^{-} \to \text{Cu}(s)$. This is the cathode. Adding the two half-reactions gives the overall spontaneous cell reaction, $\text{Zn}(s) + \text{Cu}^{2+}(aq) \to \text{Zn}^{2+}(aq) + \text{Cu}(s)$, exactly the same net chemical change as dropping zinc directly into copper sulfate solution — but now the electron transfer is forced through the external wire, where it can do measurable electrical work. …