Daniell cell with salt bridge

Figure 3.2 Schematic of a Daniell cell with a salt bridge.

Ans. No. If the Daniell cell were to be recharged, the Cu2t ions would get into the zinc half-cell through the salt bridge. There, they would react directly with the zinc electrode, and the cell would be destroyed. 

The two half-reactions can also be spatially separated from each other by dividing them into the two half-cells of a galvanic cell where they are connected to each other by an exterior circuit. For example, the so-called Daniell cell (Fig. 23.5) is composed of a Zn and a Cu electrode that are immersed in corresponding Zn " or Cu " solutions whereby these electrolyte solutions are in contact with each other through a diaphragm. To avoid diffusion voltages, a salt bridge can be used instead. 

F" Let us take the example of the Daniell cell presented In figure 1.4. It Includes two compartments containing respectively a ZnS04 aqueous solution In contact with zinc metal and a CUSO4 aqueous solution In contact with copper metal. These two compartments are electrically connected by a third aqueous solution, e.g., a concentrated KNO3 solution, which Is called a salt bridge. 

The second example deals with a Daniell cell, and shows to what extent the solution that is contained within the salt bridge has an impact on the overall ionic junction voltage. Here the voltage is the algebraic sum of two liquid Junction voltages, illustrated by the following electrochemical chain ... 

So our next experimental setup. Fig. 9.7 - it does look a little complicated but we already know its main parts - is made of a Cu(s) CuS04(aq) half-cell and a Zn(s) I ZnS04 (aq) half-celL The two half-cells are connected in two ways by wire (or electron conduit) which allows flow of electrons and by a salt bridge which allows ion transfer. The electrons released in the zinc block flow through the wire toward the copper block and reduce ions which get deposited. The surplus of negative S04 anions from the copper half-cell travels toward the zinc chamber to make up for the charge balance with the surplus of Zn ions there. Everything works the way it should and we have a fully operational cell - a Daniell cell. 

A simple voltaic cell, known as the Daniell cell (Figure 9.39), can be constructed by placing a zinc electrode in a solution of zinc sulfate and a copper electrode in a solution of copper(ii) sulfate. The two electrodes are connected via wires and a high-resistance voltmeter. This is known as the external circuit and allows electrons to flow. This is a spontaneous process and no external energy source is required. The circuit is completed by a salt bridge which allows ions to flow in order to maintain electrical neutrality (Chapter 19). A simple salt bridge consists of a filter paper soaked in saturated potassium nitrate. Potassium and nitrate ions are chosen because they will not react with the other ions in solution or with the electrodes. 

The Daniell cell is one example of a simple voltaic cell. Similar voltaic cells can be made from two different metals in contact with an aqueous solution of their ions and connected by a salt bridge and external circuit. In each case the more reactive metal forms the anode which supplies electrons to the cathode. 

In the Daniell cell, although both electrodes are reversible, diffusion of ions will occur at the liquid junction and, as a result, the e.m.f. will change continuously with time. The liquid junction, and the corresponding liquid junction potential (q.v.), can be almost eliminated by interposing a salt bridge between the two electrolyte solutions. 

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