reparation 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.
(b) Solution II
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 4000 NTU. 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.
NTU of sample =
A(B + C)
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