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3.2 Reagents

All reagents shall be of recognized analytical grade. All reagents should be stored in hard glass bottles.

3.2.1. Water, for the preparation of standard matching solutions

  • Soak a 0.1 µm membrane filter (of the type used for bacteriological studies)
    for 1 h in 100 ml of distilled water.
  • Filter 250 ml of distilled water through it and discard the water.
  • Then pass a 500 ml volume of distilled water twice through the membrane and reserve this water for the preparation of standard solutions.

3.2.2 Formazine (C2H4N2) solution

Formazine is not available commercially and it shall, therefore, be prepared as follows.

  • Dissolve 10.0 g of hexamethylenetetramine (C6H12N4) in water  (3.2.1) and dilute to 100 ml. (Solution A).
  • Dissolve 1.0 g of hydrazine sulfate (N2H6SO4)   in water (3.2.1) and dilute to 100 ml. (Solution B).
    WARNING: Hydrazine sulfate is poisonous and may be carcinogenic.
  • Mix 5 ml of solution A with 5 ml of solution B.
  • Leave for 24 h at 25±3°C. Then dilute the solution to 100 ml with the water (3.2.1).
    The turbidity of this stock solution in formazine attenuation units (FAU) or formazine nephelometric units (FNU) is 400.
  • This solution is stable for about 4 weeks if stored at a temperature of 25±3°C in the dark.

3.2.3 Formazine, standard matching solutions

Dilute the stock solution (3.2.2) with water (3.2.1) using pipettes and volumetric flasks to obtain standard matching solutions of turbidities in the range of interest.

These solutions are only stable for 1 week.

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