Page 51 - Soil and water contamination, 2nd edition

38 Soil and Water Contamination

Solution
-1
The remaining concentration NH (in mmol l ) after the above dissociation reaction can
3
be expressed as
-3
-
[NH ] = 0.1·10 – [OH ]
3
+
-
Because the dissociation of NH produces equal amounts of NH and OH , we may write
3

for basicity constant K
b
[NH ][OH ] [OH ] 2 . 4 75 5
K 4 10 . 1 78 10
b 4
[NH ] 0 . 1 10 [OH ]
3
[OH ] 2 . 1 78 10 5 0 . 1 ( 10 4 [OH ])
2
[OH ] . 1 78 10 5 [OH ] . 1 78 10 9 0
-5
-9
Subsequently, use the quadratic formula with a = 1; b = 1.78·10 ; and c = -1.78·10 to
-
solve this equation and calculate the OH concentration:
b b 2 4ac
[OH ]
2a
2
. 1 78 10 5 . 1 ( 78 10 5 ) 4 . 1 78 10 5
5
. 5 20 10 mol l -1
2
+
Finally, calculate the H concentration and the pH using Equation (2.45b):
K w [H ][OH ] 10 14
10 14 10 14
[H ] . 1 92 10 10
[OH ] . 5 20 10 5
So, the pH is
-10
+
pH = – log[H ] = – log[1.92·10 ] = 9.72
2.9.4 Buffering
Buffered solutions are solutions that are barely sensitive to changes in their pH as a result
of the addition of moderate quantities of strong acids or strong bases. The pH of such
solutions is controlled by reversible equilibria involving weak acids (HA) and their conjugate
salts (MA, where M is a cation). Because HA is a weak acid , it only partly dissociates at
-
equilibrium . The solution also contains A from the dissolved salt, which suppresses the
already slight dissociation of HA by shifting the equilibrium in Equation (2.50) to the left.
+
The H concentration of the buffer solution follows from Equation (2.51):
[HA ]

[H ] K (2.55)
a
[A ]
So,
[HA ]
pH pK a log (2.56)
[A ]

10/1/2013 6:44:16 PM
Soil and Water.indd 50
Soil and Water.indd 50 10/1/2013 6:44:16 PM