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TURBIDITY-FREE WATER
The definition of low turbidity and turbidity-free water has changed as filter technology
has changed and nephelometric instruments have become more sensitive. At one time
turbidity-free water was defined as water that had passed through a 0.6 micron filter.
Now 0.1 micron filters are available and higher purity water is possible. Water that has
been passed through a 0.1 micron filter could be considered particle free and therefore
turbidity free, 0 NTU water. Turbidity is caused by scattered light. Therefore, low turbidity
water is water without any particles that scatter a measurable amount of light. But water
that passed through a 0.1 micron filter may still have detectable light scatter with modern
instruments. This light scattering can be the result of dissolved molecules or sub-micron
sized particles that can not be filtered out of the water. Because there may still be a small
amount of scattered light from dissolved molecules, high purity water is often called low
turbidity water and assigned a value of 0.01 or 0.02 NTU. However, because this water is
used as a baseline to compare to sample water, the difference between the sample and the
low turbidity or turbidity-free water will be the same whether it is called 0.00 NTU or 0.02
NTU. For design simplicity the 2020t/i BLE uses the term turbidity-free water and the value
of 0.00 NTU.
„ PREPARATION OF TURBIDITY-FREE WATER
A 0 NTU/FNU Standard (Code 1480) is included with the meter. Turbidity-free water
for blanking the meter and dilution of high turbidity samples can also be prepared. The
preparation of turbidity-free water requires careful technique. Introduction of foreign
matter will affect the turbidity reading. A 0.1-micron membrane filter should be used. When
using an apparatus consisting of a filter, filter holder and syringe, the filter, filter holder and
syringe must be conditioned by forcing at least two syringes full of deionized water through
the filtering apparatus to remove foreign matter. The first and second rinses should be
discarded. Turbidity-free water may be stored in the dark at room temperature in a clean
glass bottle with a screw cap and used as required. The storage container should be rinsed
thoroughly with filtered deionized water before filling. The water should be periodically
inspected in bright light for foreign matter.
1.
Remove the plunger from the syringe.
Attach a filter to the bottom of the
syringe.
2.
Pour approximately 50 mL of
deionized water into the barrel of
the syringe. Insert the plunger. Exert
pressure on the plunger to slowly
force the water through the filter.
Collect water in the clean storage
container. Rinse walls of the container
then discard this rinse water.
3.
Remove the filter from the syringe.
Remove the plunger from the barrel.
(This step is required to prevent
rupturing the filter by the vacuum that
would be created when the plunger is
removed.)
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