Hach-Lange ULTRATURB sc basic_plus_seawater - Manual (Page 24)

24
Maintenance
5.5 Monitoring test equipment
As part of a quality assurance system for test equipment monitoring, it is
necessary to check the instrument calibration; this can be performed using a
formazine standard series in accordance with DIN EN ISO 7027.
You can either make up the formazine solution yourself as per the instructions that
follow
5.5.1 Preparing formazine solution in accordance with DIN EN ISO 7027 or
procure it from the manufacturer (LCW 813 or Stabl Cal).
1. To check the instrument calibration, first thoroughly rinse out the measuring
chamber using distilled water.
2. Prepare a turbidity series using formazine reference solutions as per DIN EN
ISO 7027; the measuring series should correspond to the measuring range
selected. For example, in the measuring range FNU 0–5.0 chose reference
solutions with FNU 0.5–1.0–2.0–3.0–4.0.
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.
5.5.1 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
CAUTION
Hydrazine sulphate is toxic and possibly carcinogenic
• 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) (77 °F (± 5.4 °F)) top up this solution with
water to 100 mL.