Prepare Dilution Water; Prepare The Sample; Prepare A Representative Sample; Remove Air Bubbles From The Sample - Hach 2100AN IS User Manual

Prepare dilution water

Dilution water is used when indexing a sample cell or matching sample
cells and to prepare formazin standards.
1. Collect at least 1000 mL of high-quality, low-turbidity water (i.e.,
distilled, demineralized or deionized water or filtered tap water).
2. Measure the turbidity of the water using the turbidimeter. Refer to
Turbidity measurement on page 25.
3. If the turbidity of the water is greater than 0.5 NTU, filter the water
using the sample filtration and degassing kit. Refer to the user
instructions provided with the sample filtration and degassing kit.

Prepare the sample

Proper sampling techniques are important to get accurate
measurements.

Prepare a representative sample

A representative sample accurately reflects the true condition of
the water source from which the sample was taken.
To prepare a representative sample:
• Gently but fully mix every sample before collecting aliquots (sample
portions). Mix by gentle inversion only. Do not shake.
• When collecting a sample from a water tap in a distribution system or
treatment plant, turn the water on for at least five minutes, then collect
the sample.
• When collecting a sample from a body of water (e.g., a stream or
storage tank), collect at least one liter (1 quart) and fully mix before
taking an aliquot for measurement. If the quality of the sample source
is not constant, collect samples at many locations at different depths
as necessary. Then, mix the samples together to prepare one sample
for measurement.

Remove air bubbles from the sample

If readings are not stable, air bubbles may be the cause. Remove air or
other gases from the sample before measurement even if no bubbles
can be seen.
The methods typically used for degassing are:
• Let the sample stand for several minutes

• Apply a vacuum

• Use an ultrasonic bath

• Apply heat
Let the samples stand for several minutes, then gently invert two or three
times before measurement.
In some cases, more than one method may be necessary to remove
bubbles (e.g., the use of heat with an ultrasonic bath may be necessary
in some severe conditions). Use care with these methods as sample
turbidity can be changed if these methods are not used correctly.
Apply a vacuum
Apply a vacuum with any available, clean, oil-free vacuum source, such
as the sample degassing kit, or an electric or hand-operated pump
equivalent to those in Accessories
atmospheric pressure above the sample letting trapped gas bubbles exit.
Vacuum works well with samples that are not viscous, such as water,
and do not contain volatile components. Application of vacuum to
viscous, volatile samples (i.e., paint resins) may cause volatile
components to come out of solution, and increase the bubbles.
Use an ultrasonic bath
An ultrasonic bath removes gas bubbles from most samples, especially
viscous liquids. The time necessary to remove bubbles may be a few
seconds to a minute or more.
To identify the time necessary for ultrasonic treatment:
1. Apply ultrasound to the sample for a short period of time, then
measure turbidity. Record the value and the treatment time.
2. Do step 1 again until there is no change in the turbidity of the
sample.
Note: In some instances, the use of ultrasound may divide gas bubbles and make
them more difficult to remove.
To use an ultrasonic bath:
on page 47. The vacuum lowers the
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