Liquid Handling Section; Principle Of Operation - ABB 8232 Instruction Manual

4 LIQUID HANDLING SECTION

4.1 Principle of Operation – Fig. 4.1
The monitor uses an ABB Ammonia Probe. This contains a
glass pH electrode, whose pH-sensitive glass membrane forms
a slightly convex tip, and a robust, long-life reference electrode.
The two electrodes are combined into a single assembly, and
are connected as a pH measuring pair through an internal
reservoir of filling solution containing ammonium ions.
The filling solution is 0.1 M ammonium chloride saturated with
silver chloride and is separated from the sample by a gas-
permeable hydrophobic membrane fitted in the tip of the
probe. Sample is caused to flow past the probe membrane,
whereupon the partial pressures of ammonia gas in the two
solutions on either side of the membrane equilibrate,
transferring gas across the membrane. At equilibrium, the
concentration of ammonia in the thin film of filling solution
between the probe membrane and the glass electrode
membrane equals that in the sample. The resultant change in
pH value of the thin film is measured by the pH electrode pair
which thus develops an output potential related to the
ammonia concentration in the sample. Like most ion-selective
electrodes, the Ammonia Probe produces an output which is
logarithmic with respect to concentration.
Range of measurement can be set to any two consecutive
decades of concentration between 0.05 to 2000 mg l
+
NH , or NH .
3 4
Under typical circumstances, with appropriate standard
solutions and calibration frequencies, accuracies better than
±5% of reading or 0.1 mg l –1
achieved.
Out of Sample
Constant Head Unit
Float Switch
From
Flowcell
Contaminated Overflow Sample
Drain Drain Inlet
–1 as N,
whichever is the greater, can be
Sample / Calibrate
Solenoid Valves
SV1 SV2
Standard Standard
Solution 1 Solution 2 Reagent
(Low) (High) Solution
Orange
Channel
Fig. 4.2 Flow Schematic
Filling
Solution
Aperture
pH
Electrode
Reference
Element
Filling
Solution
Washer
Membrane
Fig. 4.1 Ammonia Probe
Three Channel
Peristaltic Pump
Heater
Reaction
Coils
Orange
Heater Coil
Channel
Red
Channel
EDTA NaOH
Reagent
Note. In applications in the power
Solution
industry where hardness is very low, the
Red
first reagent (EDTA) is not required.
Channel
To Electronics Section
Ammonia
Heat Exchanger
Probe
Block
Mat
Sample
Temperature
Drain via
Constant
Head Unit
Body
Body
Seal
Washer
Membrane
End Cap
Sensor
7