9
4.1 Output signal
The electrochemical process in the thin electrolyte layer upstream
from the glass membrane proceeds as per the following equation:
Hydrogen ions are taken out of the electrolyte, moving the pH value
up to higher values.
The correlation between ammonia concentration [NH
3
] and voltage
U, is given by the Nernst equation:
In the ideal case, the value of slope S is 58.16 mV/decade (at 20°C).
U
0
is the cell zero point.
The dependence levels off as the ammonia concentration falls, that
is, there is a lower detection limit. The following diagram shows a
typical characteristic line.
NH + H O NH
3
2
4
+
-
+ OH
U = U - S * log [NH ]
0
3
(1)