Page 63 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

60üRON ION EFACT 2.15

particular ion is thus derived from the compound already in solution and also
from the added reagent, hence the name 'common ion'. If the original compound
is a weak electrolyte, the Law of Mass Action will be applicable. The result is
that there is a higher concentration of this ion in solution than that derived
from the original compound alone, and new equilibrium conditions will be
produced. Examples of the calculation of the common ion effect are given below.
In general, it may be stated that if the total concentration of the common ion
is only slightly greater than that which the original compound alone would
furnish, the effect is small; if, however, the concentration of the common ion is
very much increased (e.g. by the addition of a completely dissociated Salt), the
effect is very great, and may be of considerable practical importance. Indeed,
the common ion effect provides a valuable method for controlling the concentration
of the ions furnished by a weak electrolyte.
Example 7. Calculate the sulphide ion concentration in a 0.25M hydrochloric
acid solution saturated with hydrogen sulphide.
This concentration has been chosen since it is that at which the sulphides of
certain heavy metals are precipitated. The total concentration of hydrogen
sulphide may be assumed to be approximately the same as in aqueous solution,
i.e. 0.1 M; the [H+] will be equal to that of the completely dissociated HC1,
i.e. 0.25M, but the [SZ-] will be reduced below 1 x 10-l4 (see Example 6).
Substituting in equations (d) and (e) (Example 6), we find:

K, [HS-1 (1 10-14) x (4 10-8)
[Sz-] = - - = 1.6 x 10-" mol L-'
CH+] 0.25
Thus by changing the acidity from 1 .O x 10 -4 M (that present in saturated HzS
water) to 0.25M, the sulphide ion concentration is reduced from 1 x 10-l4 to
1.6 x 10-".

Example 8. What effect has the addition of 0.1 mol of anhydrous sodium acetate
to 1 L of 0.1 M acetic acid upon the degree of dissociation of the acid?
The dissociation constant of acetic acid at 25 OC is 1.75 x IO-' mol L-' and
the degree of ionisation cc in 0.1 M solution may be computed by solving the
quadratic equation:
CH+] x [CH,COO-] cc2c
-- = 1.75 x IO-'
-
[CH ,COOH] (1 -a)
For Our purpose it is sufficiently accurate to neglect cc in (1 - cc) since cc is small:

Hence in 0.1 M acetic acid,

and [CH, COOH] = 0.0987 mol L - '
The concentrations of sodium and acetate ions produced by the addition of the