54
Maintenance
Note: If there are any variations in the measurements, it is recommendable to measure a
new standard preparation prior to changing the instrument calibration! Experience has
shown that the variation in the formazine preparation can be greater than the possible
drift in the ULTRATURB instrument.
When checking the instruments as per EN 450901 or GLP we therefore
recommend recording the date in an instrument log and checking for any long-
term drift. Checks on the operation of the ULTRATURB can in practice also be
made by means of parallel measurements with turbidity photometers designed in
accordance with DIN and ISO.
6.5.1 Preparing formazine solution in accordance with DIN EN ISO 7027
Chemicals
Chemicals of "analysis" purity are used. The chemicals must be stored in hard
glass bottles.
Water
•
Place a membrane filter, pore size 0.1 µm (for bacteriological
experiments), for 1 hour in 100 ml of distilled water
•
Filter 250 ml water through this filter and discard the water
•
Then filter 500 ml of distilled water twice through the same filter and use
this water for making up the standard solution
Formazine (C
2
H
4
N
2
) parent solution
•
Dissolve 10.0 g of hexamethylentetramine (C
6
H
12
N
4
) in water and top up
to 100 ml (solution A)
•
Dissolve 1 g of hydrazine sulphate (N
2
H
6
SO
4
) in water and top up to 100
ml (solution B)
•
Mix 5 ml of solution A with 5 ml of solution B
•
After 24 hours at 25° C (±3 °C) top up this solution with water to 100 ml
Warning: Hydrazine sulphate is toxic and possibly carcinogenic
The turbidity of this parent solution is 400, stated in Formazine Attenuation Units
(FAU) or Formazine Nephelometric Units (FNU). This solution can be stored in a
dark place at 25 °C (±3 °C) for around 4 weeks.
Formazine reference solution
Dilute the parent solution with the aid of pipettes and measuring flasks to produce
reference solutions in the required range. These solutions can only be stored for a
very limited period.
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