Principle Of Operation; Chemical Principle - ABB Navigator 600 Silica User Manual

Navigator 600 Silica
Single-stream

3 Principle of Operation

3.1 Chemical Principle

The chemical method used by the analyzer is based on the
reaction of ammonium molybdate with silicate species, under
acidic conditions, to form a yellow molybdosilicic 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.
The sequence of events in the analyzer is as follows:
Referring to Fig. 3.2:
1. The sample first flows into a preheater coil (
maintained at a temperature of 75 °C (167 °F).
2. The sample then passes through the primary debubbler
4
( ) to remove any air bubbles formed during the
preheating stage.
Referring to Fig. 3.1:
1. The sample now flows into a reaction coil that is
maintained at a temperature of 45 °C (113 °F).
2. The first acid reagent is added to the sample to reduce the
pH to a value between 1.4 and 1.8.
3. Ammonium molybdate is added to the acidified sample.
4. The first reaction coil provides a 2-minute delay where the
yellow molybdosilicic acid is developed.
5. The second acid reagent is then added to reduce the pH
value further to between 0.8 and 1.0. This is the value
required to stop the formation of the yellow complex. The
solution enters the second delay coil that provides a
further 2 minutes for the reaction to take place.
Note. In applications where the sample contains
phosphate, the second acid reagent has a different
formulation to destroy any molybdophosphoric acid
complexes that would form an interfering blue
complex in the next stage of the reaction.
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6. The reduction solution is added to reduce the yellow
molybdosilicic
optically-absorbent blue form. The solution enters the third
reaction coil that provides a 1 minute delay.
7. The amount of blue color formed is directly proportional to
the concentration of silica in the sample. The fully
developed solution passes to the measuring cuvette
where the intensity of the color is measured.
8. During a zero calibration, the analyzer generates a zero
3
) that is
solution by diverting the sample to the point where the
second acid is added. The pH at this point is too low for
any silica in the sample to react with the molybdate, so a
zero solution is produced (refer also to Section 7.1,
page 44.
During a zero calibration, the system enables any silica
present in the first acid and molybdate solutions
(Reagents 1 and 2) to form the yellow molybdosilicic acid
complex. This is then converted to the blue form when it
reacts with the reduction solution.
3 Principle of Operation
acid complex to the
IM/NAV6S/SS–EN Rev. E
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