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8 MAINTENANCE
8.1 Chemical Solutions
The reagent and standard solutions listed below are
necessary to maintain the monitor in operation.
Caution. Great care should be taken to prevent
contamination of these solutions with phosphates. Reagent
and standard solution containers must be emptied and then
rinsed with high purity water, not simply topped up. The
performance of the monitor relies heavily on the integrity of
these solutions, so it is very important that they are prepared,
stored and handled with great care.
If solutions are purchased from a proprietary chemical
supplier, care should be taken in storing the containers. They
should be date stamped, used in strict rotation, and not used
after their expiry date.
8.1.1 Reagent Solution
The following single, combined reagent will maintain the
monitor in operation for a period of approximately five weeks.
The container and associated tubing are color coded for ease
of identification.
Warning.
• Concentrated nitric acid must be handled with great
care; ensure that when diluting concentrated acid, it is
added to the water, not water to the acid. Wear
appropriate protective clothing, i.e. rubber gloves and
full face protection, and work under a fume hood.
• Care should be taken when mopping up spillages; wear
eye protection and rubber gloves and dilute with plenty
of water.
Proceed as follows:
a) Place approximately 4 liters high purity water in a plastic
beaker and add carefully 2500 ml (10 ml) analytical
reagent grade concentrated nitric acid, HNO
Stir the solution continuously during the addition and allow
it to cool to room temperature before proceeding.
For phosphate concentrations up to 10 mg l
–1
–3
30 mg l
as PO
4
b1) Transfer
10.0 g (0.1 g)
ammonium metavanadate, NH
acid, stirring the solution continuously to dissolve the solid.
(Use a little high purity water to help transfer the solid.)
c1) Dissolve 200 g (1 g) analytical reagent grade ammonium
molybdate, (NH
)
Mo
O
4
6
7
high purity water.
Go to step d).
OR
For phosphate concentrations up to 20 mg l
–1
–3
60 mg l
as PO
4
b2) Transfer
12.5 g (0.1 g)
ammonium metavanadate, NH
acid, stirring the solution continuously to dissolve the solid.
(Use a little high purity water to help transfer the solid.)
(1.42 s.g.).
3
–1
analytical
reagent
VO
, slowly to the nitric
4
3
.4H
O, in approximately 2 liters of
24
2
–1
analytical
reagent
VO
, slowly to the nitric
4
3
c2) Dissolve 250 g (1 g) analytical reagent grade ammonium
molybdate, (NH
high purity water.
For either concentration
d) Carefully add the acid/metavanadate solution to the
molybdate, stirring well to dissolve any precipitate and then
make up to 10 liters with more high purity water to give a
pale yellow solution.
This reagent has a shelf life of several months if kept in a
tightly-stoppered plastic container.
8.1.2 Standard Solutions
The following instructions are for the preparation of stock
solutions of 1000 mg l
a) Phosphate as P
Dissolve 4.393 g (0.001 g) of analytical reagent grade
potassium
approximately 500 ml of high purity water.
Phosphate as PO
Dissolve 1.433 g (0.001 g) of analytical reagent grade
potassium
approximately 500 ml of high purity water.
b) Transfer the appropriate solution to a one-liter volumetric
flask and make to volume with more high purity water.
Information. The accuracy of the monitor over its
total range is governed by the choice of the secondary
standard solution value. A monitor calibrated at 2 mg l
for example, would not exhibit the best accuracy at
–1
20 mg l
.
In the case of a multi-stream the concentration of the
solution should be chosen to coincide with the point of
greatest required accuracy. This could also apply to a
single stream version. But in the absence of this, the value
should correspond to 80% of the current output range.
as P,
c) Dilute the appropriate stock solution with high purity water
to make the secondary calibration solution. The dilution
grade
should be carried out using polyethylene apparatus
wherever possible.
Information. High purity water, i.e. de-ionized, is
used for the ZERO solution.
The stock solutions are stable for at least 3 months.
Calibration solutions should be replaced at weekly intervals.
8.1.3 Rinse Solution for Internal Pipework
It is important that the internal pipework of the monitor is
as P,
cleaned every five weeks as part of the routine maintenance.
This prevents the gradual build-up of molybdate precipitation
grade
in the pipework which can introduce errors. Problems such as
noise can also be caused by contaminated pipework. The
solution can also be used to clean the chemical panel and
neutralize any acid spillages.
)
Mo
O
.4H
O, in approximately 2 liters of
4
6
7
24
2
–1
3–
either as P or PO
.
4
dihydrogen
phosphate,
3–
4
dihydrogen
phosphate,
KH
PO
,
in
2
4
KH
PO
,
in
2
4
–1
,
25
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