Hach 2100P Instrument And Procedure Manual page 30

SECTION 2,
2.3.5.3 Using an ultrasonic bath
Note: The time necessary to expel bubbles may vary from a few seconds to a
minute or more. To avoid excessive application of ultrasound, a simple
procedure can be followed. First, apply ultrasound until all visible bubbles
are absent. Then measure the sample turbidity. Apply ultrasound for a
short time period and again measure turbidity. Continue for several
repetitions, noting the treatment time and turbidity readings. If turbidity
begins to increase instead of decrease, the ultrasound waves have
probably started to alter the suspended particles. Note the time it
takes for this to occur and record it as the maximum time limit for
ultrasonic treatment.
Ultrasonic baths effectively remove gas bubbles from most samples,
especially viscous liquids. However, the ultrasonic waves which cause
degassing may also alter the characteristics of the particles causing the
turbidity. Turbidity depends on the size, shape, composition and
refractive index of the suspended particles. Excessive ultrasound
application may alter particle size and shape, thus changing sample
turbidity. In some cases, ultrasound may aggravate air bubble removal
by fracturing the bubbles, making degassing more difficult.
1. Fill a clean sample cell to the line with sample. Leave uncapped.
2. Immerse the cell (1/2 to 2/3 immersed) in an ultrasonic bath and
allow it to stand until visible bubbles are expelled.
3. Remove the cell, cap, then thoroughly dry the cell. Apply silicone
oil as directed.
2.3.5.4 Application of heat
Whenever possible, avoid using heat to degas samples because heat may
change the characteristics of the suspended particles and cause volatile
components to come out of solution. Gentle heating may be helpful for
degassing some very viscous samples when combined with application
of vacuum or ultrasound. If heat is necessary, heat the sample only until
degassing occurs. The simplest technique is to prepare a warm water
bath and partially immerse the filled sample cell. Use the shortest time
necessary for expelling visible bubbles. Cool sample to original sample
temperature before taking measurements.
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