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6 MAINTENANCE
6.1 Standard Solutions
Because UV absorption is a non-specific, aggregate
measurement of organic carbon concentration, true standards
are not available. In this situation it is convenient to use a
standard solution made from a single known organic
compound. The monitor then produces readings in units of
mg/l of carbon defined against the calibration standard.
Potassium hydrogen phthalate is the recommended standard
but other suitably pure, soluble and stable organic compounds
could be used instead. If, as is to be expected, an alternative
standard solution absorbs more or less strongly at 254 nm
than the recommended solution, then different monitor
readings will be obtained on a given sample.
The monitor does not require frequent calibration but, when
calibrating, particular care must be taken to avoid
contamination of standard solutions, particularly the zero
standard, with organic material which may be present inside
the flowcell.
6.1.1 Zero Standard
Calibration is performed using carbon-free de-ionized water.
The practicality of obtaining water which is truly carbon-free is
questionable so, as zero is the more important calibration
point, it is necessary to use the best quality water available
and to ensure that it is not subsequently contaminated by
organic matter. In some cases distilled water, while less
chemically pure, may contain less organic carbon than de-
ionized water.
The zero standard solution should be as fresh as possible but,
if storage is unavoidable, a glass container should be used to
avoid possible contamination due to leaching of chemicals
from a plastic bottle.
6.1.2 Span Standard
Prepared
from
potassium
(KOOC.C
H
.COOH, carbon content = 47.05 %), Analytical
6
4
Reagent grade, and high quality de-ionized or distilled water.
–1
To prepare a 1000 mgl
C carbon stock standard solution:
dissolve 2.125 (± 0.005) g of potassium hydrogen phthalate,
previously dried at 120 °C for 2 hours, in about 500 ml de-
ionized water and make up to 1 litre in a volumetric flask. This
solution may be stored in a glass bottle in a refrigerator,
without freezing, for up to 3 months.
Working standard solutions for monitor calibration should be
freshly prepared from the stock standard when required. Dilute
the stock solution with high quality de-ionized or distilled
water; the standard solution should be discarded after use.
Low Range, 10 mgl
-1
C:
Dilute 10 ml of the stock standard solution to 1 litre in a
volumetric flask.
-1
High Range, 40 mgl
C:
Dilute 40 ml of the stock standard solution to 1 litre in a
volumetric flask.
18
hydrogen
phthalate
6.1.3 Calibration Checks
The system uses an optical system with very stable electronics
which avoids the risk of electronic drift. Therefore, routine
calibration is normally unnecessary. However, it may be
necessary to routinely check the system accuracy (particularly
after cleaning). The should then be considered as a calibration
check and not a calibration adjustment.
The calibration check can be simply carried out by filling the
flowcell with the Zero and Span Standards and observing the
readings on the Operating Display Page.
The solutions are poured in from the top of the flowcell.
High Range Sensors:
Remove filler plug on top of the flowcell and use the funnel
provided.
Low Range Sensor:
Remove the Cleaner Module, fill flowcell and fit Cleaner
Module.
6.2 Scheduled Servicing
Warning. Do NOT open the emitter unit as it uses
high voltages which could cause serious injury or death.
Caution. Both emitter and receiver units contain
no user serviceable parts and are sealed in clean air
conditions at the factory. Opening them could lead to
degraded performance. See also the warning above.
The following servicing schedule has been produced as a
general guide only. Because the systems are designed for a
wide range of applications, where the nature of the sample can
vary considerably, it may be necessary to amend the schedule
to suit the particular installation and sample conditions.
6.2.1 Cleaning the Flowcell
The required automatic cleaning frequency of the flow
chamber and optical windows can only be determined by plant
experience. It is recommended that checks are made are
appropriate intervals.
Routine servicing is limited to manually cleaning out the
flowcell to remove any fouling or sediment which has
accumulated over a lengthy period. In particular, if there is a
need to calibrate the instrument it is important that no
contamination occurs when setting the zero condition. To
clean out the high range sensor the cell it must be 'split'. Four
stainless steel screws hold it together, but two of them provide
a jacking action when unscrewed, thus affording a controlled
splitting operation. See Section 6.2.3, Fig. 6.2, for details.
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