Navigator 600 Silica
Single-stream
7 Calibration & Maintenance
44
IM/NAV6S/SS–EN Rev. E
7.1 Zero Calibration
During the zero calibration the sample is diverted from its normal flowpath by the Auto Zero valve into the second mixing stage. The
pH value at this point is too low to allow the silica-molybdate reaction to take place, consequently a solution equivalent to a zero silica
concentration is produced. During the zero calibration the system continues to allow the first acid and molybdate solutions (reagents
1 and 2) to flow, allowing the silica content of these reagents to react and therefore be compensated for in the final measured silica
value.
Each time the reagents are replenished, a BASELINE
CALIBRATION is initiated. During the Zero calibration section,
the background offset is compensated for and the Zero Offset
parameter is set to 0.0ppb. In a Baseline Calibration the Zero
section is always followed by a Secondary Calibration.
During continuous running the analyzer exhibits a certain
amount of drift (that usually affects the zero calibration) due to:
changes in the reagent solutions
'dirtying' of the cuvette windows.
If this is not compensated for, by carrying out regular
calibrations, significant inaccuracies can be introduced.
The degree of zero drift is indicated by the Zero Offset – see
Fig. 7.4.
Fig. 7.3 Chemical Schematic During a Zero
1.4 to 1.8 pH providing conditions for the
production of yellow ß-molybdosilicic acid
First Acid
0.8 to 1.0 pH providing reducing conditions
to stop formation of yellow complex
Reacted sample to optical unit
Sample
2 Minutes
2 Minutes
1 Minute
Reduction Solution
Second Acid
Molybdate
Production of blue
molybdosilicic acid complex
Auto-zero valve
Temperature controlled at 45 ° C (167F)
Small quantity of
SiO2 in reagents
Fig. 7.4 Zero Calibration
Theoretical Response
Calibration Factor 1.0
Di
sp
la
y an
d
Re
adi
n
g
pp
b Si
O
2
Zero Offset 2 ppb
Absorbance
Baseline
