Hanna Instruments Marine Instruments Turbidity and Free/Total Chlorine Meter User Manual

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19
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.

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