Page 52 - Soil and water contamination, 2nd edition

Basic environmental chemistry 39

If a strong acid is added to the buffer solution, the reaction (2.50) is driven to the left; if a
strong base is added, the reaction is driven to the right. Thus, since the solution can shift in
response to either addition of acid or base, the change in pH will be small. The range of pH
over which the buffering effects are effective depends on the concentration and nature of the
weak acid and its conjugate salt. In general, a weak acid is an effective buffer at pH = pK ± 1
a
pH unit. Most natural waters are buffered to some extent by carbonate equilibria, which will
be dealt with further in Section 5.10.

Example 2.13 Buffer solution
-1
-1
A buffer solution contains 100 mmol l acetic acid (CH COOH) and 100 mmol l
3
-1
sodium acetate (NaCH COO). Two (2.0) ml of 10 mol l HCl is added to this buffer
3
solution. What will be the pH change? The pKa for acetic acid is 4.75.
Solution
First, calculate the pH of the buffer solution before addition of the strong acid . Use
Equation (2.51):
[CH 3 COOH ]
pH pK log
a
[CH 3 COO ]
The acetate ion is derived from both the sodium acetate and the acetic acid . The salt
(sodium acetate) completely dissociates:
+
NaCH COO → Na + CH COO -
3 3
In contrast, the weak acid (acetic acid) is only slightly ionised:
+
CH COOH ↔ H + CH COO -
3 3
The high concentration of the acetate ion provided by the sodium acetate shifts the
+
weak acid equilibrium to the left, decreasing the concentration of H . Thus, solutions
that contain a weak acid plus a salt of the weak acid are always less acidic than solutions
that contain the same concentration of the weak acid alone. The dissociation of the acetic
acid is so small that we may approximate the CH COOH concentration by the initial
3
-
-1
concentration of the acetic acid (= 0.100 mol l ) and the CH COO concentration
3
-1
by the initial concentration of the NaCH COO (= 0.100 mol l ). Because both
3
concentrations are equal, the log term in Equation (2.50) becomes zero, so the pH equals
the pK value. Thus, the pH is 4.75.
a
Second, calculate the pH after addition of the HCl solution. Since the added volume
(2 ml) is negligible compared to the total volume (1 ml), we neglect the change in
volume. The amount of HCl added is
-1
0.0020 l × 10 mol l = 0.020 mol HCl
+
The H is nearly completely consumed by the acetate ion to form acetic acid . By
approximation,
[CH COOH] = 0.100 + 0.020 = 0.120 mol l -1
3
-
[CH COO ] = 0.100 – 0.020 = 0.080 mol l -1
3
Use Equation (2.50) to calculate the new pH
[CH COOH ] . 0 120
pH pK a log 3 . 4 75 log . 4 57
[CH COO ] . 0 080
3

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