To determine the turbidity of the given sample using Nephelometer in NTU.
The method presented below is based on a comparison of the intensity of light scattered by the sample in specific
conditions with the intensity of light scattered by standard reference suspension under the same condition. The
higher the intensity of scattered lights, higher the turbidity. Formazine polymer, which has gained acceptance as the
turbidity standard reference suspension is used as a reference turbidity standard suspension for water. It is easy to
prepare and is more reproducible in its lights scattering properties than the clay or turbid natural water standards
previously used. The turbidity of a given concentration of formazine has an approximate turbidity of 100 NTU,
when measured on candle turbidity meter. Nephelometric turbidity units based on formazine preparation will have
approximate units derived from Jackson candle turbidimeter but will not be identical to them.
Nephelometer with accessories
(b) Solution II
|Reagents (» click to check the preparation of reagents)
(i) Turbidity free distilled water (for setting zero).
(ii) Formazine turbidity concentrate (hydrazine sulphate + hexamine).
(iii) Formazine standard (for setting 100 of the instrument).
Preparation of Turbidity Free Distilled Water
Pass distilled water through a membrane filter having a precision pore size of less than 10 microns (Whatman filter
No. 42). Rinse collecting flask atleast twice with such filtered water and discard the next 200 mL. Use this filtered
water for setting zero of the instrument.
Preparation of Formazine Turbidity Concentrate
(a) Solution I
Weigh accurately 5 g of ‘Anal–R’ quality hydrazine sulphate (NH2)2H2SO4 into a 500 mL volumetric
flask and add distilled water to make up to the mark. Leave the mixture to stand for 4 hours.
Weigh accurately 50g of 'Anal-R' quality hexamethylene tetramine (CH2)6N4 (hexamine) into a 500 mL
volumetric flask and add distilled water to make up to the mark.
Mix equal volume of solution I and II to form formazine turbidity concentrate. Allow it to stand in a
closed container at 25°C to 30°C for 48 hours to produce insoluble white turbidity corresponding to
Note: Once prepared, formazine turbidity concentrate (which corresponds to 10000 ppm SiO2
) is stable for 2 to 3 months.
Preparation of Formazine Standard
Dilute 25mL of the formazine turbidity concentrate to 1 litre with turbidity free distilled water to obtain 250 ppm or
100 NTU for setting '100' of the instrument.
Note: Formazine standard 100 NTU should be prepared weekly.
(1) Switch the instrument on.
(2) Open the lid of the sample compartment.
(3) Insert a test tube filled with distilled water into the sample compartment. Close the lid.
(4) Adjust 'SET 0' control to get '0' displayed on the read out.
(5) Open the lid. Replace the test tube filled with distilled water with a test tube filled with formazine standard.
Close the lid.
(6) Adjust the 'SET 100' control to get '100' displayed on the read out.
(7) Repeat the above operation to get consistent values of 0 to 100 within 1% to 2%.
Measurement of turbidity less than 100 NTU
1. Thoroughly shake the sample.
2. Wait until air bubbles disappear and pour the sample into the nephelometer tube.
3. Read the turbidity directly from the instrument.
Measurement of turbidity above 100 NTU
Dilute the sample with one or more volume of turbidity free distilled water until the turbidity fall below 100 NTU.