Navigator 600 Phosphate
Multi-stream
3 Principle of Operation
8
IM/NAV6P/MS–EN Rev. F
3 Principle of Operation
3.1 Chemical Principle
The chemical method used by the analyzer is based on the
reaction of ammonium molybdate with dissolved
orthophosphate species to form a yellow phosphomolybdic acid
complex. To improve the sensitivity of the method the yellow
complex is reduced to the blue form. The absorbance of the
blue complex is measured spectroscopically in the optical
system.
Referring to Fig. 3.1, the sequence of events in the analyzer is as
follows:
1. Sample flows into a reaction coil that is maintained at a
temperature of 45 °C (113 °F).
2. The first reagent is added to the sample which contains
ammonium molybdate and reduces the pH to a value less
than 1.0.
3. The first reaction coil provides a 2-minute delay where the
yellow phosphomolybdic acid is developed.
4. The reduction solution is added to reduce the yellow
phosphomolybdic acid complex to the more
optically-absorbent blue form. The solution enters the
second reaction coil that provides a 3-minute delay.
5. The amount of blue color formed is directly proportional to
the concentration of phosphate in the sample. The fully
developed solution passes to the measuring cuvette
where the intensity of the color is measured.
3.2 Liquid Handling
Referring to Fig. 3.2:
The sample enters the analyzer via the constant head unit
1
situated at the bottom of the unit. Multi-stream analyzers can
have 2, 4 or 6 constant head units fitted, depending on the
version. Each constant head unit is fitted with an 'Out of
Sample' switch that determines if sample is present. This is
used by the microprocessor to measure only streams with
sample present. If all streams are out of sample, the analyzer
instigates a Shutdown procedure (see Section 3.5, page 11).
Once a sample stream returns the analyzer re-starts
automatically.
Each constant head unit is fitted with a solenoid valve. The
valves are used to select each stream to be sampled.
The sample is drawn up from the constant head unit by the
peristaltic pump
2
via a sample filter that protects the liquid
handling section from blockages caused by particulates in the
sample.
The sample passes through the clean, secondary and zero
calibration valves before entering a pre-heater coil that heats the
sample to encourage dissolved air in the sample to form
bubbles. These bubbles are removed by the primary debubbler
3
and pumped to drain by the de-gassing channel on the
peristaltic pump. This is necessary to reduce the effect of
bubbles in the sample causing variable reagent mixing, resulting
in noisy readings.
The sample then passes into the heated reaction coil
4
where
it mixes with the reagents to form a blue complex (described in
detail in Section 3.1, page 8).
The sample exits the reaction coil and passes into the
secondary debubbler assembly
5
. The debubbler removes
any bubbles that may have originated from the reagents or by
further degassing of the sample.
The solution then passes into the cuvette
6
where the
absorbance of the solution is measured.
Fig. 3.1 Chemical Schematic
Reacted Sample
to Optical Unit
Production of Blue
Phosphomolybdic
Acid Complex
pH <1.0 Providing
Conditions for the
Production of Yellow
Phosphomolybdic
Acid
Reduction
Solution (Brown)
Acidified
Molybdate
Solution (Violet)
45 °C
3 Minutes
2 Minutes
Sample
Note.
If particulates exceed 60 microns in size, it is
essential to fit external filters to the sample lines prior to
entering the analyzer.
