half reactions. A redox couple is defined as having together the oxidised and reduced forms of a substance taking part in an oxidation or reduction half reaction. This is represented by separating the oxidised form from the reduced form by a vertical line or a slash representing an interface (e.g. solid/solution).
For example in this experiment the two redox couples are represented as Zn + /Zn and Cu + /Cu. In both cases, oxidised form is put before the reduced form. Now we put the beaker containing copper sulphate solution and the beaker containing zinc sulphate solution side by side (Fig. .
). We connect solutions in two beakers by a salt bridge (a U-tube containing a solution of potassium chloride or ammonium nitrate usually solidified by boiling with agar agar and later cooling to a jelly like substance). This provides an electric contact between the two solutions without allowing them to mix with each other. The zinc and copper rods are connected by a metallic wire with a provision for an ammeter and a switch.
The set-up as shown in Fig. . is known as Daniell cell. When the switch is in the off position, no reaction takes place in either of the beakers and no current flows through the metallic wire.
As soon as the switch is in the on position, we make the following observations: . The transfer of electrons now does not take place directly from Zn to Cu + but through the metallic wire connecting the two rods as is apparent from the arrow which indicates the flow of current. . The electricity from solution in one beaker to solution in the other beaker flows by the migration of ions through the salt bridge.
We know that the flow of current is possible only if there is a potential difference between the copper and zinc rods known as electrodes here. The potential associated with each electrode is known as electrode potential . If the concentration of each species taking part in the electrode reaction is unity (if any gas appears in the electrode reaction, it is confined