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8 MAINTENANCE
8.1 Chemistry
The reagent is phosphate based and is added to the sample at
the flowcell in order to provide pH and ionic strength adjustment.
EDTA is included to prevent precipitation of hardness salts.
The reagent and standard solutions described below are required
to maintain the monitor in operation. Where possible, they should
be freshly made and stored in plastic (e.g. polythene) bottles.
Typical reagent solution consumption for a continuously
operating monitor is approximately 10 litres per month. The
monitor uses 50 to 80 ml of each standard solution each
calibration cycle; consumption of the standard solution, also
depends on the frequency with which this cycle is carried out.
8.1.1 Reagent Solution
Dissolve 215 (±5) g of analytical reagent grade potassium di–
hydrogen phosphate in approximately 5 litres of high purity water
(distilled or de–ionised water containing less than 0.01 mg/l
nitrate nitrogen). Add to this solution 160 (±5) g laboratory reagent
grade EDTA (di–sodium salt).
Make the solution up to 10 litres with more high purity water and
add 50 (±5) ml of 10% 'Brij 35' solution. 'Brij 35' (polyoxyethylene
lauryl ether, a non–ionic surfactant) is available as a solid or as a
30% solution from Merck Ltd.
8.1.2 Standard Solutions
Two standard solutions of known nitrate concentration
appropriate to the measuring range are required. To prepare a
stock solution of 1000 mg l
–1
Nitrate as NO
–
3
a) Dissolve 1.371 (±0.001) g analytical reagent grade sodium
nitrate in high purity water and make up to one litre with more
high purity water.
Nitrate as N –
b) Dissolve 6.070 (±0.001) g analytical reagent grade sodium
nitrate in high purity water and make up to one litre with more
high purity water.
c) Dilute the stock solution appropriately with more high purity
water to make up the two standard solutions for the
measuring range of the monitor. Store in plastic bottles.
Low value nitrate standard solutions should be avoided. The
lowest practical standard is 1 mg l
0.2 mg l
–1
N).
Note. The mass relationship of nitrate (NO
is 62/14).
30
follow instructions below:
–1
NO
–
(approximately
3
–
) to nitrogen (N)
3
8.1.3 Reference Salt Bridge Solution
The Reference Electrode contains a salt bridge solution of 1 M
sodium sulphate. Prepare as follows:
a) Dissolve 14.2 (±0.1) g of analytical reagent grade, anhydrous
sodium sulphate (Na
SO
2
purity water.
b) Dilute the solution to 100 ml with more high purity water.
c) Store the solution in a tightly stoppered plastic bottle.
Note. Bottles of solution are available from ABB – see
Section 10.
8.2 Scheduled Servicing
The following servicing schedule has been produced as a general
guide only. Because the monitor has been designed for a very
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.
8.2.1 Regular Visual Checks
It is recommended that the monitor and the sampling system is
visually inspected on a regular basis to assure correct functioning
of the system and to check the integrity of the readings.
a) Check for leaks, particularly around the sample and drain
pipework connections.
b) Confirm sample flow by checking delivery to the constant
head unit and effluent from the drain.
c) Check liquid flow through sensor flowcell.
d) Check for excessive buildup of air in the flowcell.
e) Check liquid levels in the reagent and standard solution
containers.
f) Check for malfunction indications on the instrument display.
Note. The mains and pump/heater switches are situated on
the right hand side of the User Junction Box.
8.2.2 Four Weekly
a) Inspect all tubing and flowcell for leaks and deterioration.
b) Examine all tubing and flowcell for signs of accumulation of
solid deposits. These tend to accumulate in the reference
electrode compartment of flowcells and may be cleaned to
drain by squirting high purity water from a 'squeezy' bottle into
the de-bubbler vent on top of the flowcell, and the use of small
bottle brush.
c) Clean monitor pipework if there are signs of algae visible.
d) Check the salt bridge solution in the reference electrode and
top-up if necessary.
e) Discard old reagent and standard solutions. Clean containers
thoroughly before refilling with fresh solutions – see Section
8.1.
) in approximately 80 ml of high
4
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