Scientists using sensitive techniques in analysis and bio-science need a supply of top-grade water containing less than 0.1µg/l of ionic impurities. How can they be sure that ionic purity of the water is good enough? Dr Paul Whitehead reports.
Modern highly regenerated premium-grade ion-exchange resins can reduce the ionic content of water to below 0.1µg/l until the resin capacity is becoming used up. The difficulty is to detect when the resin pack needs to be changed while ensuring the purity of the product water.
Laboratory water purifiers rely on the electrical resistivity of the product water as a measure of its ionic purity.
But, as the resistivity approaches that of absolutely pure water, 18.2Macm at 25oC, it is a poor indicator of residual ion levels due to errors in measuring resistivity, and the complex relationship between resistivity and low ionic concentrations.
A new laboratory water purifier offers an alternative solution to the problem. Instead of relying on final resistivity measurements, it uses two purification cartridges and resistivity cells in a unique design to guarantee the lowest levels of residual impurity ions in the ultra-pure water.
Measuring resistivity
Resistivity measurements depend not only on the ionic content of the water, but on the electrode materials, surface cleanliness, exact cell geometry, and the temperature measurement accuracy. Typical errors are over ±0.2Macm.
Resistivity and trace ions
The resistivity of absolutely pure water of
18.2Ma-cm is due to hydrogen and hydroxyl ions from the slight dissociation of water.
As other ions are added, they also affect the dissociation of the water. As a result, significant concentrations of cations can be present even at
18 Mohm-cm. Some examples are given in Table 1 above.
As shown in Fig. 1, as an ion exchange cartridge produces purified water, its resin capacity is gradually used up.
After about 45 per cent of the total capacity is used there is no longer enough new resin to stop impurity ions getting into the product water. This can be detected as a drop in product water resistivity but this is far from reliable.
In the new purifier, the Purelab Ultra, water flows through a first cartridge and then through an intermediate resistivity cell before entering a second apolishing' cartridge. Finally, it flows through a filter and a second resistivity cell before being available for use.
As shown in Fig. 2, as the capacity of the resins in the first cartridge is used up, impurity ions are released into the water, causing the intermediate resistivity to fall.
Second cartridge
However, the purity of the product water is protected by the second cartridge. This will barely have begun to be used when the first cartridge is discarded.
The second cartridge then replaces the first and a fresh polishing cartridge is fitted. As a result, water produced by the Purelab Ultra is close to absolute purity at all times.
As shown in Fig. 3, silica and organic contaminants are among the first to be released as the resin starts to exhaust; even at resistivity values close to 18.2 Mohm-cm. These impurities are removed by the second resin pack in the Purelab Ultra, whereas in traditional water purifiers it is impossible to avoid such contamination.
This system provides the user with key benefits. It guarantees the ionic purity of the ultra-pure water, and also ensures consistent organic purity and maximum utilisation of the ion-exchange resins along with scope for in-line TOC monitoring and enhanced UV photo-oxidation. u
ENQUIRY No 88
Dr. Paul Whitehead, is R&D Laboratory Manager with ELGA LabWater, High Wycombe, Buckinghamshire, UK. www.elgalabwater.com
