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Complete Article from Edition 46 APG eNewsletter
Article written by Jeffrey Stewart, QC Chemist at APG
pH analysis is one of the most widely performed tests. The concept behind pH analysis is that it is the measurement of hydrogen ion activity. At 25º C , pH 7 is considered neutral. This means that the hydrogen ion activity is equal to the hydroxyl ion activity. The neutral point is temperature dependent and is pH 7.5 at 0º C and pH 6.5 at 60º C. Natural waters usually have pH values between 4 and 9 and most are slightly basic due to the presence of carbonates and bicarbonates.
How does one go about setting up a pH meter? The first step is to obtain pH buffers for standardization. The most commonly used pH buffers are 4, 7, and 10. With most natural waters having a pH value between 4 and 9, these standards will be acceptable to use. One can either obtain the pH buffers from a supplier that are already made, or one can obtain the appropriate salts from NIST(National Institute of Standards and Technology). Either step up is acceptable depending on specific requirements from accrediting authorities.
The next step is to make sure that the pH buffers are at room temperature. The determined pH value is assumed to be at a temperature of 25º C. The temperature of the pH buffers will affect the slope of the pH curve. As the temperature of the pH buffers decrease, slope flattens out more causing the pH value to be read higher for the sample. The pH buffers at a higher temperature will cause the slope to shift to the lower end of the pH scale; causing the pH values to come in lower.
After the pH meter has been standardized with the pH buffers, the sample is then tested. The easy thing about testing is that the sample is poured into a beaker, and the electrodes are placed into the sample. The pH meter then reports a value for the sample. Depending on the model of the pH meter, the value can be expressed up to three decimal places. This seems to be fairly straightforward, but problems can arise during this test.
One problem is if the sample is stirring too fast. As the sample is being stirred, CO2 from the air is mixed into the sample. This will cause the pH value to decrease over time. A way to minimize this is to allow the sample to mix at very low speed. You want the sample to come into contact with the electrode but also minimize the addition of CO2 to the sample.
The analysis for pH is pretty simple. Most issues affecting the accuracy of pH measurement can be eliminated during the setting up of the meter. You need to make sure that everything is at room temperature before starting to minimize the effect of temperature on the analysis. There are limitations to the effectiveness of automatic temperature compensation probes. The process by which temperature compensation occurs is based on mathematical calculations from the Nernst equation. Keep in mind that the factors used in the equation may be hard coded in the pH meter and may be relative to temperatures of calibration at 25ºC. Therefore, the calibration may not deliver the necessary accuracy when there are significant differences between the temperature of the calibration buffers and the samples being tested. The pH meter is trying to match the set of conditions from the calibration to the conditions of the determinations. Therefore, the closer you can make the conditions of the calibration to the conditions of the sample determinations the better the pH meter will perform. We recommend performing the calibration with pH buffers at temperatures as close as possible to the temperature of the samples to be tested. The differences may not seem significant on your regular testing, but a difference of 0.05 pH units may be significant when your PT sample acceptance range is only ±0.2 pH units. A published reference indicates the error in pH measurements between pH of 6-8 at 5ºC is 0.06 pH units. This indicates if your calibration buffers are being stored refrigerated and are not allowed to equilibrate to room temperature before you use them to calibrate the meter, then you analyze a sample at room temperature, your pH measurement could be as much as 0.06 pH units higher than actual. We have verified this to be true with our own testing, even when using an automatic temperature compensation probe.
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