pH Electrodes Designed & Manufactured by Hanna Electrode Cleaning Cleaning Procedure The most common cause for pH measurement inaccuracies is an unclean or improperly cleaned electrode. This is very important to note, because during calibration, the instrument assumes that the electrode is clean and that the standardization curve created during the calibration process will remain a valid reference until the next calibration. pH meters on the market today will allow an offset voltage of approximately ±60 mV. The deviation from 0 mV is not unusual but ideally should be no greater than ±30 mV. The calibration process compensates for the change in offset voltage. Calibrating a meter with a dirty electrode will result in inaccurate readings. If the mV offset continues to deviate with a properly cleaned electrode, it is a good indication that the electrode may need to be replaced. In time, particles during routine measurement can contaminate the sensor tip. Mishandled and aged solutions can also be affected. A proper cleaning and fresh solution ensures the whole surface of the sensor tip is reading correctly, ensuring an accurate calibration. Your meter can still be calibrated even if the electrode sensor tip is not properly cleaned before calibration. If the contamination dissapates, the calibration is no longer valid and the readings are inaccurate. General Cleaning Soak in Hanna HI7061 General Cleaning Solution for approximately 30 minutes to dissolve mineral deposits and other general coatings. Protein Coating Soak in Hanna HI7073 Protein Cleaning Solution for 15 minutes to enzymatically dissolve deposits from protein sources. Inorganic Soak Soak in Hanna HI7074 Inorganic Cleaning Solution for 15 minutes. This cleaner is especially effective at removal of precipitates caused by reaction with the silver in the filling solution that may form on a ceramic junction. Oil and Grease Rinse Oil and grease removal require the correct chemicals to solubilize the coating, but are mild enough to leave the electrode unaffected. Use Hanna HI7077 Oil and Fat Cleaning Solution. After performing any of the cleaning procedures, rinse the electrode thoroughly with purified water and then soak the electrode in HI70300 or HI80300 storage solution for at least 1 hour before taking measurements. Troubleshooting Drifting/Erratic Readings Potential problems include: Build up on glass electrode – Clean electrode Clogged junction – Depending on the material clogging the electrode, use application specific cleaning solutions. It may be possible to dissolve in high purity water or place in an acid such as 0.1M HCl or 0.1M HNO3 at elevated temperature (50°C) for about an hour to clear the clog. If the junction is constantly clogging due to measuring in semi solids or viscous samples, use a pH electrode that has an open junction design or cloth junction. Low conductivity solution – Use an electrode that has a high flow rate or add high purity KCl to sample to increase EC. Electrode is not properly hydrated – Soak in storage solution for at least 1 hour, if not longer. Frozen pH Reading Broken electrode – Possible short between internal pH electrode and reference. pH meter displays the same value when placed in different buffers. The electrode should then be replaced. Inaccurate Reading: Improper calibration – Make sure that pH electrode was rinsed with purified water between buffers to prevent cross-contamination and the electrode is at thermal equilibrium with the buffer. Check offset and slope of electrode. Offset mV value in pH 7.0 should be ±30 mV; if outside of this range, try cleaning the electrode. Slope (difference in mV from pH 7.0 to pH 4.0) must be greater than 150 mV (85%). If the slope is less than 85% then use fresh buffers, change fill solution, and clean electrode. If the slope cannot be increased to an acceptable value, replace electrode. Important note: A low slope can be due to a bad buffer. If calibrating to pH 7 and 10, it is possible that pH 10 buffer is no longer valid. pH 10 buffer is susceptible to diffusion of CO₂ from the air. When this happens, the pH 10 buffer will have a lower pH value and result in a low slope percentage value. Tracking the mV values of the buffer by writing the value on the bottle when opened is a way to have a reference point of a good buffer. 85% slope is the absolute threshold of an acceptable slope percentage. There are industries that require a slope of 90% or higher. 2 pH 2.131 www.hannainst.com | electrodes
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