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< 7. Calibration >
7-6
IM 12E01B30-02EN 2nd Edition : June 09 , 2021-00
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Preparing a Formazin Standard Stock Solution
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Reagents Required
• Hydrazine sulfate, (NH
2
)
2
·H
2
SO
4
• Hexamethylene tetramine, (CH
2
)6·N
4
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Equipment Required
• Measuring flask, 2 x 100 ml
• Measuring flask, 1000 ml
• Volumetric pipette, 50 ml
• Analytical balance
Note: Class A volumetric flasks and pipets are required.
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Procedure
1. Weigh out 1.000 g ±0.001 g of hydrazine sulfate with a balance, place it in a 100-ml
measuring flask, and add deionized water to make 100 ml (solution A).
2. Weigh out 10.00 g ±0.01 g of hexamethylene tetramine with a balance, place it in another
100-ml measuring flask, and add deionized water to make 100 ml (solution B).
3. Make sure that both reagents have been completely dissolved in solutions A and B,
respectively. Pipette 50 ml each of solutions A and B into a 1000-ml measuring flask and mix
well.
4. Allow the mixed solution to stand for 24 hours at 25 ±3°C.
5. Bring the total volume to 1000 ml with deionized water.
This is a 400 NTU formazin standard stock solution and its storage life is one month. To make a
standard solution of the desired turbidity, dilute this stock solution with deionized water.
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Preparing a Formazin Calibration Standard Solution
To make a formazin calibration standard solution, dilute the 400 NTU formazin standard stock
solution.
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Precautions
• Use clean, high quality laboratory glassware and measure the volume needed accurately.
• To dilute the formazin standard stock solution, use deionized water or water filtered through
a 0.2 µm zero turbidity filter.
• Before dilution, mix the stock solution thoroughly.
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Procedure
To make a 2000 ml formazin calibration standard solution of the desired turbidity, take the needed
volume of the 400 NTU formazin standard stock solution and dilute to 2000 ml.
Turbidity Aliquot of Stock Solution Total Volume
20 NTU 100 ml 2000 ml
10 NTU 50 ml 2000 ml
5 NTU 25 ml 2000 ml
2 NTU 10 ml 2000 ml
1 NTU 5 ml 2000 ml