Engineering Chemistry S Datta

ELECTROCHEMISTRY 213
dilution more and more of the electrolyte ionises. After a certain dilution there remains no
more electrolyte molecule to ionise and then there is no change i.e., no increase of λ v
or λ m
with
dilution. The maximum value of equivalent conductance λ v
or molar conductivity λ m
is known
as equivalent conductance at infinite dilution or molar conductivity at infinite dilution and
abbreviated as λ ∞
or µ ∞
.
Normality
Table 9.3
Specific conductivity (Κ) Equivalent conductivity (λ v )
KCl CH 3
COONa CH 3
COOH KCl CO 3
COONa CO 3
COOH
1.0 0.0982 0.0412 0.00132 98.2 41.2 1.82
0.5 0.05115 0.0247 0.001005 102.3 49.4 2.01
0.1 0.01119 0.00611 0.00024 111.9 61.1 2.30
0.01 0.001225 0.000702 0.000143 122.5 70.2 14.3
0.001 0.0001276 0.0000752 0.000041 127.6 75.2 41.0
0.0001 0.0000129 0.00000768 0.0000107 129.5 76.8 107.0
Classification of Electrolytes
(i) Strong electrolytes: Strong acid HCl, strong alkali NaOH have high conductances
even at low dilutions and there is no sharp increase in their equivalent conductances on dilution.
Such electrolytes are called strong electrolytes.
(ii) Weak electrolytes: Weak acid like acetic acid has a low value of λ v
at higher
concentration and there is rapid increase of λ v
with dilution. Such electrolytes whose λ v
values
increase with dilution are known as weak electrolytes (see Fig. 9.7).
Molar conductivity
Strong electrolyte
Weak
electrolyte
Concentration
Fig. 9.7 Change of equivalent conductivity with dilution.
Experimental Determination of Specific Conductance (K)
The arrangement of apparatus adopted for the determination is modified Wheatstone’s
meterbridge arrangement (see Fig. 9.8). The modification is necessary as the passage of
current will electrolyse the electrolyte. The products of electrolysis will accumulate on the
electrodes which will change the effective resistance. The use of A.C. is necessary to avoid this
difficulty.