The readings shown by turbidimeters are not scaled in the measured light intensity, but as a concentration of a reference suspension. Because the accuracy of this calibration solution determines the reliability of subsequent turbidity measurements, it is of crucial importance.
The internationally established turbidity standard for calibration is formazine. This standard can be reproduced at any time with the recipe taken from standard ISO 7027 (Water quality - Determination of turbidity):
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 10.0g of hexamethylenetetramine (C6H12N4) in Water (3.2.1) and dilute to 100 ml. (Solution A).
- Dissolve 1.0g of hydrazine sulphate (N2H6SO4) in water (3.2.1) and dilute to 100 ml. (Solution B).
WARNING: Hydrazine sulphate 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.
It is extremely important to observe the prescribed preparation temperature, because it affects the particle size of the formazine particles perceptibly. The following Figure illustrates this. Errors caused by temperature variations are on the order of 1..2% K-1. Consequently, SIGRIST keeps the preparation temperature constant to within ± 1°C..
Fig. 19: Particle size distributions of formazine at various preparation temperatures.