56
The ammonia NH
3
that has been converted to gas permeates through the
membrane, and disassociates into ammonium ions NH
4
+
and hydroxide ions
OH
-
in the internal solution.
The expression shown below can be obtained if the disassociation constant is
assumed to be K.
Here, the [Molecular formula] and the [Ionic formula] respectively indicate the
concentration of molecules and ions. Moreover, [H
2
O] is constant.
When the log of both sides is taken and the ammonia concentration pNH
3
= -log
[NH
3
] and pK’ = -logK’ is assumed, the expression shown below can be
obtained.
On the other hand, the disassociation constant Kw of water can be obtained by
the expression shown below.
When the log of both sides is taken and pH = -log [H
+
] and pKw = - log Kw is
assumed, the expression shown below can be obtained.
Thus, the following expression for determining the ammonia concentration
pNH
3
can be obtained from the expressions (3) and (4).
If ammonium chloride NH
4
Cl is used as the internal solution, a large number of
ammonium ions NH
4
+
will exist in the internal solution, and therefore, the log
[NH
4
+
] of the second term on the right of the expression (5) is considered to be
almost constant.
Therefore, the ammonia concentration pNH
3
becomes proportionate to the pH
value, and if the pH value is measured, the ammonia concentration pNH
3
can
be determined.
Thus, the ammonium ion concentration is determined by the measured
ammonia concentration because almost of the ammonium ion is converted to
the ammonia.
NH
3
(Gas) + H
2
O
NH
4
+
+ OH
-
… (2)
[NH
4
+
] [OH
-
]
= K [H
2
O] = K’
[NH
3
]
pNH
3
= -pK’ - log [NH
4
+
] - log [OH
-
]
… (3)
Kw = [H
+
] [OH
-
]
log [OH
-
] = pH - pKw
… (4)
pNH
3
= -pH - log [NH
4
+
] + pK’’
where pK’’ = -(pKw + pK’)
… (5)
Содержание 5002S-10C
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