Which of the following is not characteristic of a Daniel cell?

Negative ions diffuse through the salt bridge toward the copper electrode.

The correct answer is option (4) Negative ions diffuse through the salt bridge toward the copper electrode.

In a Daniel cell, the anode is the zinc electrode, and the cathode is the copper electrode. The oxidation reaction at the anode is:

\(Zn \rightarrow Zn ^{2+} + 2e^-\)

This means that zinc atoms lose electrons, which are then transferred to the cathode through the external circuit. The reduction reaction at the cathode is:

\(Cu^{2+} + 2e^- \rightarrow Cu\)

This means that copper ions gain electrons, which are then transferred from the anode through the external circuit.

The salt bridge is a device that allows ions to flow between the two half-cells, preventing the build-up of charge. The salt bridge contains negatively charged ions, which will diffuse towards the copper electrode. However, the negative ions do not actually participate in the cell reaction.

Therefore, the only characteristic that is not correct for a Daniel cell is 4. Negative ions diffuse through the salt bridge toward the copper electrode.

The other options are all correct.

(1) When one of the reactants of the cell Zn or Cu²⁺ is exhausted, the voltage reduces to zero. This is because the cell reaction will no longer be able to proceed.

(2) Valence electrons flow from zinc atoms into the conducting wire. This is how the electrons are transferred from the anode to the cathode.

(3) Zn²⁺ ions are formed which then diffuse away from the zinc electrode into the solution. This is because the zinc atoms are oxidized, and the zinc ions are released into the solution.

Therefore, the answer is 4. Negative ions diffuse through the salt bridge toward the copper electrode.