Hanna Instruments HI 98713 - Manual (Page 9)

16 17
• Open the instrument lid, slightly rotate the cuvet and take a new reading.
• Repeat the last step for the second cuvet until the reading is within 0.01 FNU of the value
obtained for the first cuvet.
• Alternatively, keep READ pressed and, after the first value is displayed, open the lid and start
rotating the cuvet until the displayed value matches the first cuvet.
• Mark this position on the second cuvet with a water resistant pencil.
• Follow the same procedure for all the necessary cuvets.
Note: If the cuvet is indexed, use the index to position it in the instrument.
APPLICATION OF VACUUM
Vacuum works by decreasing the atmospheric pressure. In this way the bubbles from the solution
came out to the surface.
Application of vacuum is a very simple procedure and can be applied with any vacuum source at hand.
The simplest equipment at hand is a syringe and a rubber stopper for vacuum degassing.
Notes: • Pay attention that the vacuum equipment be clean and oil-free.
• It is not recommended to apply vacuum to a viscous sample that contains volatile
components. In such cases the vacuum can determine the volatile component of the
viscous sample to increase the bubbles from the sample.
ADDITION OF SURFACTANT
Surfactant addition works by changing the surface tension of the water. In this way bubbles are
released from the sample. This method is effective in samples that are supersaturated with air.
The procedure consists in the addition of a drop of surfactant in the cuvet before adding the sample
to be analyzed.
A convenient surfactant to use for degassing is Triton X-100.
Warning: Pay attention that changing the surface tension will cause a rapid settling of particles that
cause turbidity. To avoid this problem, analyze as soon as possible the sample.
Do not shake vigorously the sample because the surfactant may foam. If you are using the same
cuvet, rinse it before adding a new sample in order to avoid surfactant accumulation.
Surfactant contribution to the turbidity readings is negligible.
Note: Surfactant addition should be used for degassing only when other methods are ineffective.
USE OF AN ULTRASONIC BATH
The ultrasonic waves are very effective in removing air bubbles from samples. However, ultrasonic
waves should be used with care because they can alter sample turbidity characteristics, by modifying
the shape and size of particles which cause turbidity. The ultrasonic waves can also break the existing
air bubbles, leading to a complication of the degassing process.
In order to avoid excess application of the ultrasonic waves you can apply ultrasound until all visible
air bubbles are removed, and then measure the sample turbidity. This is the most used procedure for
degassing.
If you are not sure that all air bubbles were removed, apply ultrasonic waves again for a short period
of time and then measure the turbidity. Repeat this procedure until the turbidity is increasing instead
of decreasing, sign that turbidity of the sample was altered.
In order to degas a sample fill a clean cuvet with sample and immerse it (1/2 to 2/3 immersed) in
an ultrasonic bath. Follow the degassing procedure described above. Only after the degassing
procedure is finished, the cuvet can be capped.
SAMPLING TECHNIQUE
When taking turbidity measurements it is very important to select a representative sample. For
consistent results, follow the next tips when sampling:
• Gently mix the water before taking the sample.
• If the sample is taken from a pipe, discard the first few liters.
• If measuring a non uniform source, collect samples from different places and mix them.
When measuring the collected sample, keep in mind the following:
• Samples should be analyzed immediately after collection because the turbidity can change in time.
• To avoid dilution of the sample it is better to rinse the cuvet with a quantity of sample and then
discard. Only after this you can fill the cuvet with sample.
• Pay attention that cold samples do not condense on the sample cell.
REMOVING AIR BUBBLES
Any air bubbles present in the sample will cause high turbidity readings. To obtain accurate
measurements, remove the air bubbles using one of these methods:
• Application of a partial vacuum;
• Addition of a surfactant, such as Triton X-100;
• Use of an ultrasonic bath;
• Heating the sample.
Sometimes it is necessary to combine two or more methods for efficient air bubble removal.
Note: Each method can alter the sample turbidity, if misused, so they have to be used with caution.