14
4.1
Sensor Calibration Standard Solutions
UV absorption is a non-specific, aggregate measurement of
organic carbon concentration therefore true standards are not
available. A standard solution, made from potassium hydrogen
phthalate, is used for calibration purposes and the monitor
produces readings in units of mgl
–1
of carbon defined against this
calibration standard.
Note.
Clean the flowcell internally (see Section 8)
before calibration to ensure that the standard solutions,
particularly the zero standard, are not contaminated
with organic material that may be present inside the
flowcell.
4.1.1
Zero Standard Solution
High purity water is used for the zero standard solution and must
be as fresh as possible. If storage is unavoidable, use a glass
container to prevent contamination. Some plastics, for example
polythene and polypropylene, may be acceptable, but
regardless of material, the container must be meticulously clean
and kept solely for the purpose of storing the zero standard
solution.
Note.
The high purity water used for the zero solution
and for diluting the span standard solution must contain
less than 50µgl
–1
TOC. It is recommended that the
water is obtained from purification systems comprising
reverse osmosis and de-ionization units but freshly
distilled water can also be used. De-ionized water is not
recommended as it often contains significant levels of
organics.
4.1.2
Span Standard Solution
The span standard solution is prepared from potassium hydrogen
phthalate (KOOC.C
6
H
4
.COOH, carbon content = 47.05 %),
Analytical Reagent grade and high purity water.
To prepare a 1000mgl
–1
C carbon stock standard solution:
1.
Dry 2.125±0.005g of potassium hydrogen phthalate at
120°C for 2 hours.
2.
Dissolve the dried potassium hydrogen phthalate in
500ml high purity water.
3.
make up to 1 litre in a volumetric flask.
The solution may be stored in a glass bottle in a refrigerator,
without freezing, for up to 12 months.
Working standard solutions for system calibration must be
freshly prepared from the stock standard when required. Dilute
the stock solution with high purity water. Discard the working
standard solution after use.
7320 100 Low Range Sensor – 10mgl
-1
C:
Dilute 10ml of the stock standard solution to 1 litre high purity
water in a volumetric flask.
7320 200 High Range Sensor – 50mgl
-1
C:
Dilute 50ml of the stock standard solution to 1 litre high purity
water in a volumetric flask.
4.1.3
Calibration Checks
The sensor's emitter module contains an optical system with
very stable electronics that eliminate electronic drift, therefore,
routine calibration is normally unnecessary. However, it may be
necessary to check system accuracy, particularly after cleaning
the flowcell.
A calibration check is carried out by filling the flowcell with the
Zero and Span standard solutions and observing the readings in
the
Operating Page
– see Section 2.3.
The solutions are poured in from the top of the flowcell.
7320 100 Low Range Sensor:
Remove the wiper module, fill the flowcell and refit the wiper
module.
7320 200 High Range Sensor:
Remove the filler plug on top of the flowcell and use the
funnel provided.
4
SETUP
