English (GB)
18
7. Methods
7.1 Important notes
7.1.1 Correct use of reagents
The reagents must be added in the correct sequence.
Tablet reagents:
The tablet reagents must be added to the water
sample straight from the foil without touching them with the
fingers.
SPADNS liquid reagents:
Add the SPADNS liquid reagents to
the water sample using a 2-ml pipette. Observe the storage
instructions, e.g. cool and dry.
7.1.2 Cleaning of vials and accessories
Vials, caps and stirring rods should be cleaned thoroughly after
each measurement to prevent interferences. Even minor reagent
residues can cause errors in the test result.
Procedure:
Clean vials and accessories after each measurement as soon as
possible.
a) Clean vials and accessories with laboratory detergent,
e.g. Extran
®
MA 02 (neutral, phosphatic), Extran
®
MA 03
(alkaline, phosphate-free) from Merck KGaA.
b) Rinse thoroughly with tap water.
c) If necessary, perform special cleaning as required in the
method, for example rinse with diluted hydrochloric acid
solution.
d) Rinse thoroughly with deionised water.
7.1.3 Method notes
1. Vials, caps and stirring rods should be cleaned thoroughly
after each measurement to prevent interferences. Even minor
reagent residues can cause errors in the test result.
2. The outside of the vial must be clean and dry before starting
the measurement. Fingerprints or other marks on the vials will
lead to incorrect measurements.
3. Zero setting and test must be carried out with the same vial,
as there may be slight differences in optical performance
between vials.
4. The vials must be positioned in the sample chamber for
zeroing and test, with the mark on the vial (white triangle)
aligned with the mark on the photometer. See fig.
5. Always perform zeroing and test with the vial cap tightly
closed. Only use cap with gasket to prevent light from entering
the sample chamber.
Correct position of the vial (
∅
24)
Fig. 4
Correct position of the vial (
∅
24)
6. Bubbles on the inside wall of the vial result in incorrect
measurements. To prevent this, remove the bubbles by
swirling the vial before performing the test.
7. Avoid spillage of water in the sample chamber. If water should
leak into the instrument housing, it can destroy electronic
components and cause corrosion.
8. Contamination of the lens in the sample chamber can result in
errors. Check at regular intervals and, if necessary, clean the
light entry surfaces of the sample chamber using a moist cloth
or cotton buds.
9. Large temperature differences between the photometer and
the environment can lead to errors - e.g. due to the formation
of condensation in the area of the lens or on the vial.
10. To avoid errors caused by stray light, do not use the
photometer in bright sunlight.
Correct filling of the vial:
Fig. 5
Correct filling of the vial
7.1.4 Sample dilution techniques
For accurate dilutions, proceed as follows:
1. Pipette the water sample (see table below) into a 100 ml
volumetric flask and fill up to the 100 ml mark with deionised
water. Swirl to mix the contents.
2. Pipette the required volume of the diluted sample into the vial
and proceed as described in the test methods.
7.1.5 Correction of volume additions
If a larger volume of acid or base is used to pre-adjust the
pH-value, a volume correction of the displayed result is
necessary.
Example:
For adjusting the pH-value of a 100 ml water sample,
5 ml of acid must be added. The corresponding displayed result is
10 mg/l.
T
M
04
16
45
27
10
6
TM
04
16
46
27
10
Water sample [ml]
Multiplication factor
1
100
2
50
5
20
10
10
25
4
50
2
Caution
Dilution decreases accuracy! Do not dilute water
samples for measurement of pH-values. This will
lead to incorrect test results. If "Overrange" is
displayed, use another method, for example a pH
meter.
Total volume
= 100 ml + 5 ml
= 105 ml
Correction factor
= 105 ml/100 ml
= 1.05
Corrected result
= 10 mg/l x 1.05
= 10.5 mg/l
CORRECT
WRONG
Summary of Contents for DIT-M
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