14
The probe responds to the partial pressure of
ammonia in the sample solution. When the
probe is immersed in a sample, ammonia is
transferred across the gas-permeable
membrane until the partial pressure of the gas in
the thin film of solution between the glass
electrode and the membrane is equal to the
partial pressure in the sample. The internal filling
solution contains ammonium ions. The ratio of
the amount of ammonia to the activity of these
ions in the thin film gives rise to a characteristic
pH which is measured by the glass electrode/
reference electrode system. The value of this pH
is a function of the chemical reactions in the
solution. As a result of the equilibria the probe
exhibits a potential in ammonia solutions which
may be represented by:
[NH
3
]
log
10
2.3RT
F
–
E = E
∞
Ammonia obeys Henry’s Law in dilute solutions,
so the partial pressure of ammonia in the
solutions is directly proportional to its
concentration, i.e.
PNH
3
= H[NH3]aq
where PNH
3
is the partial pressure of ammonia,
H is the Henry’s Law constant and [NH3] aq the
concentration of ammonia in the aqueous
solution. The constant, H, is as previously
discussed, a function of temperature but also of
the total concentration of dissolved species in
the solution. Thus the temperature and total
concentration of dissolved species in samples
should be as closely similar as possible to those
of the calibration standards.
The effect of the osmotic pressure of samples on
the probe response is discussed in Reference 1.
Concentration Range
0.1 mgl
–1
to 1000mgl
–1
NH
3
(5 x 10
–6
M to 5 x 10
–2
M)
pH Range:
7 to 14pH
Temperature Range:
5 to 40
°
C
Reproducibility:
Better than
±
2% of concentration
Minimum Sample Size
5ml in a 50ml beaker
Storage
Probe should be stored in 0.1M ammonium
chloride solution.
Caution.
Do not store dry.
Life
Under normal operating conditions the probe life
is at least 60 days without renewal of membrane
Dimensions
Total Length 152mm
Length below cap
105mm
Stem diameter
17.5mm
4
THEORY
5
SPECIFICATION
