Aztec 600 ISE ammonia and fluoride
Single-stream ion-selective analyzers
Appendix D – Principle of Operation – Fluoride Analyzers
OI/AXM630–EN Rev. I
83
Appendix D – Principle of Operation – Fluoride Analyzers
D.1 General Operation
The analyzer uses an ion-selective electrode comprising a
fluoride sensing tip and a reference electrode. The fluoride
sensing tip contains a single crystal of Lanthanum Fluoride, an
ionic conductor in which fluoride ions are mobile. When the
electrode is immersed in a solution containing fluoride ions, the
difference in fluoride ion activity across the membrane causes
an electrical potential be generated. Like most ion-selective
electrodes, the fluoride probe produces an output that is
logarithmic with respect to the measured concentration.
The analyzer's sequence of operation is as follows:
1. Sample is supplied to a side sample pot constant head
unit by a sample pump. Excess sample overflows to drain.
2. From the constant head unit, the sample is drawn through
the normally open ports of solenoid valves SV1 and SV2
by one channel of a peristaltic pump.
3. Reagent is drawn through another channel of the
peristaltic pump and mixed with the sample.
4. The conditioned sample is pumped first through a static
mixer, then into a flowcell containing the fluoride probe.
The probe is enclosed in a heated block housing a
heat-exchanger, ensuring that the body of the probe
remains at a constant temperature and also at a
temperature in agreement with that of the sample and
standard solutions.
5. When exposed to the reacted sample, the probe
produces an electrical potential that changes in proportion
to the changes in activity of the measured ion.
6. The probe is connected to the electronics section where,
after digital conversion, the signal is processed by
microprocessor.
7. After measurement, the sample flows to waste via a
contaminated drain connection.
During calibration, Low and High standard solutions are
introduced sequentially in place of the sample by means of
solenoid valves SV1 and SV2. The resultant millivolt Low and
millivolt High readings from the probe are stored as the
calibrated readings. The analyzer can be configured to perform
automatic calibrations from every 6 hours to once per week. A
calibration can also be initiated manually if required.
Fig. D.1 Flow Schematic
Waste
Standard
solution (low)
Standard
solution (high)
Reagent
Sample from
side sample pot
Reagent
pump
Fluoride flowcell
assembly (blue)
Solenoid valve and manifold assemblies
Conditioned
sample
Static mixer
Note.
The constant head unit is fitted with a float
switch that signals an 'Out-of-sample' condition by
triggering an 'Out-of-Sample' alarm.
Note.
The sample and reagent tube diameters are
sized to obtain the correct ratio of sample and
reagent.
