Measuring the pH of pure DI water is easy when you know what to expect. In theory, pure water should have a pH of 7. When you actually measure the pH, it will most likely fall between 5.5 and 7 due to its absorption of CO2 from the atmosphere. This natural occurrence forms carbonic acid in the water, lowering the pH. Since DI water is pure, there is nothing to buffer it and stabilize the pH. Below are a few tips to increase the accuracy of your pH measurements.
Tips for accurate pH readings
- First and foremost, use a high quality ph meter and ensure that it is properly calibrated with pH buffer solution. Check the manufacturer’s recommendations for calibration. The Ultrameter II 6P and the Techpro II TPH1 are portable pH meters that are extremely accurate and easy to use.
- When using a portable pH meter, avoid cross-contamination by thoroughly rinsing with the DI water that you will be sampling. If a glass beaker or cup is to be used, rinse that as well.
- Use small samples and minimize exposure to air, as this will lower the pH value. Taking samples from an open-air drum or tank will typically give erroneous readings. Collect samples from a sample port if possible.
- If you have access to high-purity reagent grade KCl (Potassium Chloride) salts, then you can buffer the DI water to stabilize the pH. Adding a tiny amount to the pure DI water sample will increase the ionic strength and reduce the absorption of CO2 from the atmosphere. Be careful not to contaminate the KCl salts. Use proper tools/utensils to add the KCl salts
- If no salt is available and all you need is a quick check of your system, you can flow the water from a sample port into your portable pH meter to measure the pH values. This will take slightly longer to stabilize. Be sure to use an accurate, waterproof pH meter and hold it closely to the sample port.
- Changes in temperature can affect the pH. Use a pH meter that is temperature compensated to remedy this issue.
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