"Symbolic representation of cells "
The cell is represented by keeping in view the following points:
1- It is customary to write the anode of the cell on the left and the
cathode to the right.
2- In a cell, there are two electrodes. Each of the electrodes in contact
with its ions is called a single electrode or half cell. In the Daniell
cell, the half cells are Zn/Zn++ and Cu++/Cu.
3- A salt bridge is indicated by two vertical lines separating the two
half cells.
4- if two electrolytes are in contact and no salt bridge is used a single
vertical line is used to separate their symbols.
5- Subscripts may be used to indicate to the nature of the electrolyte
such as
(
)
(
)
Daniell cell
A Daniell cell is a zinc-copper cell that can be represented by
Zn/ZnSo4 // CuSo4/Cu
In a Daniell cell, a zinc rod partially immersed in a ZnSo 4
solution and a copper rod in a CuSo4 solution are the two electrodes. The
two solutions are separated by a porous barrier that allows the ions to
pass through but prevents mass diffusion. A current begins to flow
through the outer circuit when the two electrodes are joined with the help
of copper wire.
At the anode: the zinc – electrode were zinc give up its electrons, that is
oxidation occurs, is the anode or negative electrode of the cell.
At the Cathode: The electrons that were released at the anode move
along the external circuit and are used up at the Cathode or the positive
electrode (Copper electrode), where copper ions take up electrons to give
copper metal. Reduction takes place at the cathode.
Cu++ +
Because the electrons flow from the zinc rod toward the copper rod, the
electrical energy is produced.
The cell reaction is Zn + Cu++
.
fig(1):Daniell cell
Thus the Daniell cell can be represented as
(
)
(
)
/C
Concentrations may be mentioned in brackets.
Reference electrodes and determination of pH:
The reference electrode is an electrode of standard potential, with
which we can compare the potentials of other electrodes.
Reference electrodes are classified into two types:
Primary Reference Electrode: standard hydrogen electrode is the
primary reference electrode. The emf of such a cell has arbitrarily been
fixed at zero.
A hydrogen electrode may be employed to find the pH value of an
unknown solution. The solution, whose pH is to be determined, is taken
in a vessel, and a platinum electrode is half dipped in it. A slow current
of hydrogen gas at 1 atmosphere is bubbled through the solution. The
platinum catalyzes the reaction.
(
)
( )
This reaction develops a definite potential at the electrode,
depending on the H+ ion concentration of the solution under test.
[
]
[ (
[
)]
]
[ (
[
(
)]
(
)]
Fig. (2): Normal hydrogen electrode
)
The preceding half-cell so formed is connected to a standard or normal
hydrogen electrode (i.e., having a solution of 1N HCl). The two solutions
are separated by a salt bridge to eliminate the liquid junction potential.
The emf of a full cell is then determined by using a potentiometer. With
the emf of the reference electrode being zero, the observed emf directly
supplies the emf of the half-cell containing the solution that is being
tested (Fig. 3).