Hanna Instruments HI 4101, Instruction Manual

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4
III. III.
III. III. III. Theory of Operation
Theory of Operation
Theory of Operation Theory of Operation Theory of Operation:::::
The ammonia electrode is a complete potentiometric cell
that contains both a silver/silver chloride (Ag/AgCl) refer-
ence and a pH measurement element. These elements are
housed within a thermoplastic body in a chloride ion-con-
taining electrolyte, and are isolated from the sample by a
gas permeable membrane made of polytetrafluoroethylene
(PTFE).
Dissolved gas in the sample solution diffuses into the mem-
brane and changes the pH in the thin film of electrolyte on
the surface of the pH glass. Diffusion continues until the
partial pressures of the gas in the sample and thin film are
equal. The change in pH is proportional to the concentra-
tion of dissolved gas in the sample solution.
The Nernst expression for an ammonia sensor is expressed
in the equation below. Note that the potential is a function
of the ammonia gas, which in turn is related to the hydroxyl
ion concentration. The glass internal, Ag/AgCl reference,
and Henry
’
s law constant are rolled into the E
’
and E
o
constants. The Nernst equation for the sensor becomes the
equation noted below:
E = E
’–
2.3RT/nF log [A]= E
o
–
0.059 log [OH
-
]
E = observed potential
E
’
= Reference and fixed internal voltages
R = gas constant (8.314 J/K Mol)
n= Charge on ion (equivalents/mol)
A
ion
= ion activity in sample
T = absolute temperature in K
F = Faraday constant (9.648 x 10
4
C/equivalent)
The mV should decrease in a Nernstian manner as the
ammonia partial pressure increases in the sample.