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Making 4000-NTU formazin stock solution
W A R N I N G
Chemical exposure hazard. Obey laboratory safety procedures and wear all of
the personal protective equipment appropriate to the chemicals that are handled.
Refer to the current material safety data sheets (MSDS) for safety protocols.
Note: Making formazin stock solution from raw materials is not recommended.
Preparation of formazin stock solution is temperature and technique sensitive. Use
Hach formazin stock solution to get the best instrument performance and analytical
standard accuracy.
1. Dissolve 5.000 grams of reagent grade hydrazine sulfate
((NH
)
–H
H
SO
) in about 400 mL of demineralized water.
2
2
4
2
4
2. Dissolve 50.000 grams of reagent grade hexamethylenetetramine in
about 400 mL of demineralized water.
3. Quantitatively, put the two solutions in a 1-liter volumetric flask, and
dilute to volume with demineralized water. Mix fully.
4. Let the solution stand for 48 hours at 25 ± 1 °C (77 ± 1 °F).
Calibrate the turbidimeter with user-selected formazin
standards
The instrument may be calibrated using user-selected values of formazin
standards.
Calibration with user-selected values of formazin standards is done
using the same method that is used to calibrate the instrument with
recommended formazin standards with two differences:
• The prepared formazin standards used are user-selected standards
and not the recommended standards. Refer to
standards – user selected
on page 46.
• The calibration points that are shown on the display must be changed
as they occur so they agree with the turbidity of the user-defined
standards. Refer to
Change the calibration points
Note: Unknown performance may occur if standards other than the recommended
calibration points are used. The recommended calibration points (< 0.1, 20, 200,
1000, 4000 and 7500 NTU) provide the best calibration accuracy. Refer to
Application note 128, Calibration Methods for Low-Level Turbidity Measurement.
46 English
Prepare formazin standards – user selected
User-selected values of formazin standards are prepared using the
same method that is used to prepare the recommended formazin
standards. Refer to
Prepare user-selected values of formazin standards to span the entire
range of the instrument. Four standards are necessary. Suggested
standards are in the range of:
• 10–30 NTU
• 180–220 NTU
• 900–1000 NTU
• 4000 NTU
Formazin standards greater than 80 NTU must have a difference of at
least 60 NTU.
Change the calibration points
When using user-selected values of formazin standards during
calibration, change the calibration points that are shown on the display
as they occur. Change the calibration points so that they agree with the
turbidity of the user-defined values.
For example: A 25-NTU standard is put in the sample cell holder
instead of the recommended 20-NTU standard during calibration.
Change the "20.000" on the display to "25.000" before pushing ENTER
to start the measurement.
To change the value on the display during calibration:
1. Push the right arrow key. The decimal point flashes.
2. Push the right arrow key to move the cursor to the next position.
Prepare formazin
3. Push ENTER to accept the new cursor position.
4. Use the up and down arrow keys to change the number on the
on page 46.
5. Do steps 2–4 again if necessary to change the other digit.
6. Push ENTER to save the change and start the measurement.
Prepare formazin standards
display.
on page 43.
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