LaMotte 2020i Turbidimeter - Manual (Page 22)

Dilution Procedures
If a sample is encountered that is more than 4000 NTU, a careful dilution with 0 NTU
or very low turbidity water will bring the sample into an acceptable range. However,
there is no guarantee that halving the concentration will exactly halve the NTU value.
Particulates often react in an unpredictable manner when diluted.
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 scattred 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
2020 uses the term turbidity-free water and the value of 0.00 NTU.
Preparation of Turbidity-Free Water
A 0 NTU Standard (Code 1480) is included with the meter. An accessory package
(Code 4185) is available for preparing turbidity-free water for blanking the meter and
dilution of high turbidity samples.
The preparation of turbidity-free water requires careful technique. Introduction of
foreign matter will affect the turbidity reading. A filtering device with a special
membrane filter is used to prepare turbidity-free water. 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 b discarded. Turbidity-free water as prepared below 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 for foreign matter in bright light.
PROCEDURE
1. Remove the plunger from the syringe (0943). Attach the
filter to the bottom of the syringe.
22 ANALYSIS & CALIBRATION