By Mary Vessele
Measuring high-purity water can be a challenge
If you work in a laboratory you understand the importance of using high-purity water and the challenges of keeping that water pure. Ensuring your high-purity water is not contaminated is most likely very important to you and requires specialized equipment to ensure you are able to produce repeatable and reliable results. So why is this so hard when we all know that pure water has no ion interference and should be neutral pH7.0?
Water that has very few ionic species is hard to work with as it is said to be low in alkalinity, ionic strength, and has low conductivity/high resistivity. Due to the varying junction potentials that develop across the reference junction, when attempting to measure high-purity water it is common to attain different pH values even with new, sealed electrodes that calibrate perfectly in pH buffers. This can be very frustrating especially if you just purchased a new piece of equipment to accomplish this and you can’t get the same results repeatedly. Our number one goal is to be able to measure our pure water sample and produce the same result repeatedly.
How to measure high-purify water
The best choice for measuring pH in a high-purity water sample uses a refillable, liquid filled electrode, ideally made of low resistance glass with a fast leak, big liquid junction. A flowing reference junction has a higher flow rate to minimize junction potentials. A fast leak rate is desirable with pure water so that a pH potential can be established more quickly.
As if high-purity water being a bad conductor isn’t challenging enough, it can also be a source of static potentials. These static potentials may present problems while measuring pH as well. To compensate for this interference, pH electrodes are available with a special shielded or grounded compartment. Such electrodes are expensive and may not be an answer for everyone. Here are a few additional tips for measuring pH in high-purity samples.
- It is beneficial to measure pH in the smallest sample volume possible. Direct pH measurement in large volume samples, such as drums or tanks and other samples with flowing or moving water, produces results that tend to fluctuate and will require excessive stabilization time.
- Addition of a tiny amount of KCl will increase ionic strength to the sample and improve response time. However, only high purity KCl should be used as trace contaminants, as low-grade KCl can artificially alter the pH.
- Maintain good laboratory practices (GLP), including using clean glassware, avoiding cross-contamination after calibration by rinsing thoroughly with DI water, and using only certified calibration standards.
- Temperature compensation should be used during measurement and calibration. Calibrate daily at multiple pH calibration points.
- Minimize exposure of your sample with air. CO2 gas absorption can decrease pH.
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Mary Vessele is the product marketing manager for Test & Measurement at Cole-Parmer. She can be reached at firstname.lastname@example.org.