purity ultrapure water SNWPMYTHS

purity ultrapure water SNWPMYTHS
Q
A
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When I check the purity of ultrapure water in
a beaker, it does not match the purity on the
water system display. Is there a problem with
my water system?
This is an ultrapure water myth. When the
purity on the display doesn’t match the purity in the dispensed ultrapure
water, this is not necessarily an indication that a problem exists. The
truth is the purity of ultrapure water can quickly change after being
dispensed from a lab water system. CO2 in the air is readily absorbed in
the pure water and creates carbonic acid, which lowers the purity and
can even lower the pH.
When there is no air and no CO2 in contact with ultrapure water, the
expected conductivity of the water is 0.055 µS/cm (18.2 Mohm-cm).
For outdoor or ambient air, 300 to 500 ppm CO2 is typical. When the
ultrapure water is in contact with ambient air containing up to 500 ppm
CO2, the expected conductivity of the water rises to around 1 µS/cm
(or 1 Mohm-cm). What causes these varying CO2 levels? The season
of the year, plant respiration, human respiration, decaying vegetation,
ventilation in a room, and more. As these CO2 levels vary in the air, so
does the CO2 level and the conductivity reading of your ultrapure water
in a beaker.
myths
Understanding common myths and truths of ultrapure water
Measuring conductivity, resistivity, and pH in ultrapure water leads to frequent questions and “lab myths” about the
measurements and what they mean to the lab. The following are some common myths and truths related to ultrapure water:
MYTH
TRUTH
Ultrapure water
conductivity after
dispensing should
match the water
system display
Ultrapure water immediately picks up carbon dioxide and conductivity upon dispensing.
Due to this volatility, the conductivity level of the ultrapure water once dispensed will not match what is on
the display. However, the CO2 can be easily accounted for in the water as described in USP <645> Water
Conductivity.
Fresh ultrapure water can read anywhere between pH 5.0 and 8.0.
The pH of ultrapure The pH level can quickly decrease after the water has been dispensed from the water purification system.
water should be 7.0 This is can be attributed to innocuous exposure to CO in the air. In addition, pH is not considered a
2
useful indicator of ultrapure water quality, because of the expected CO2 absorption.
An ultrapure water
system display will
measure all the
impurities in water
The purity displayed on the water system is based on specific impurity measurements in water.
Resistivity or conductivity measures the Total Ionized Solids (TIS) or the positive and negative ions and
metals in the water, organics are not included in this measurement. Total Organic Carbon (TOC) can be
measured and displayed if the water system has a TOC option. Impurities such as bacteria, particulates,
endotoxins, and nucleases need to be measured with additional tests.
Accessories could adversely affect purity depending on where it is added to the system.
Accessories added
to ultrapure water
systems will always
improve purity
Accessories such as filters and cartridges can further purify the water for specific impurities, when well
chosen and well placed. However, non-recirculated water or even using additional external tubing to the
system could leach ions and/or organics into the water or encourage the growth of bacteria. Check with
the manufacturer before adding any additional components to the ultrapure water system to determine
how it could affect the water quality.
How can I verify the purity on my display is correct?
The best way to measure the conductivity or resistivity of ultrapure water is with an inline measurement using a conductivity
meter with a flow cell to exclude air and preserve the ionic purity. The purity display does not represent all impurities; therefore,
additional testing based on the applications may need to be done. To prevent inaccurate results, poor analytical baselines and
down time, consistent maintenance of the water system is highly recommended.
Summary
Safeguard your critical research and experiments. Separate fact from
fiction when it comes to ultrapure water.
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