Thermo Fisher Scientific Ion Selective Electrodes Ammonia - Manual (Page 16)

The ammonia that diffuses through the membrane dissolves in the internal filling solution, reacting
reversibly with water in the filling solution, to a small extent:
NH
3
+ H
2
O
⇒ NH
4
+ + OH-
The equilibrium equation gives rise to the equilibrium constant in the following equation:
[NH
4
+] [OH-]
constant =
⎯⎯⎯⎯⎯⎯⎯
[NH
3
]
The ammonium ion concentration can be considered fixed, since the internal filling solution
contains ammonium chloride at a sufficiently high level. As a result:
[OH-] = [NH
3
] x constant
The electrode sensing element's potential, with respect to the internal reference element, varies in a
Nernstian manner with changes in the hydroxide level:
E = Eo - S log [OH-]
where S is the slope of the electrode.
Because the hydroxide concentration is proportional to the ammonia concentration, electrode
response to ammonia is also Nernstian:
E = Eo1 - S log [NH
3
]
Eo1 represents the reference potential and is partly determined by the internal reference element
which responds to the fixed level of chloride in the internal filling solution.
Ammonia Chemistry
As mentioned earlier, ammonia dissolves in water to form the ammonium ion. Ammonia also
reacts with hydrogen (hydronium) ions to form ammonium ions:
NH
3
+ H
3
O+
⇒ NH
4
+ + H
2
O
The relative amounts of the ammonium ion and of ammonia is determined by the solution's pH.
Virtually, all the ammonia is converted to ammonium ion where hydrogen ion is readily available,
as in acid solution. Half the ammonia will be in the form of ammonium ion at a pH of about 9.3.
(See Figure 3).