pH Electrodes Designed and Manufactured by Hanna Maintenance and Storage General Maintenance Tips Periodically check the oset and slope characteristic of the pH electrode. If your meter does not have GLP (Good Laboratory Practice) capability to display this information, see below on how to use the mV function of a pH meter to determine oset and slope characteristics. A probe should have an oset (pH 7.01) voltage of ± 30 mV. Values outside this range could indicate that an electrode needs to be cleaned or the reference ll solution is contaminated. A probe should have a slope greater than 85% (50 mV/pH @ 25°C). Many Hanna meters will alert the user if the oset exceeds ±8.0 mV or if the slope is less than 94%. If it is not possible to check offset and slope of the electrode with your meter, it is recommended to change the pH electrode yearly to ensure that accurate readings are obtained. How to calculate oset and slope • Must have a pH meter that can be placed in mV mode • Must use fresh buffers The procedure below is based on calibration buers at 25°C. At this temperature the theoretical 100% slope is 59.16 mV/pH change from pH 7.0. A pH electrode in calibration buer at 50°C will generate 64 mV/ pH, while at 0°C the response will be 54 mV/pH. Step 1 Measure mV of pH 7.01 buer and record value Step 2 Measure mV value of pH 4.01 buer and record value Step 3 Calculate the absolute mV dierence (pH 4.01 value – pH 7.01 value) Examples: Electrode 1 pH 7.01 = -15 mV pH 4.01 = +160 mV Absolute mV dierence is +160 mV – (-15 mV) = +175 mV Electrode 2 pH 7.01 = +15 mV pH 4.01 = +160 mV Absolute mV dierence is +160 mV – (-15 mV) = +145 mV At 25°C pH 7.01 (oset) = ±30 mV. The absolute mV dierence should be 150 mV (85% slope) to 186 mV (105% slope). Conclusion: Electrode 1 is working properly while electrode 2 has an unacceptable slope. Try cleaning and if possible replace ll solution. If slope is still low then replace the pH electrode. Important note: A pH 7.01 mV value outside ±30 mV is an indicator of a build up/coating on the pH bulb. The electrode should be cleaned. Electrode Storage Solutions To minimize junction clogging and ensure fast response time, always keep the glass bulb and the junction of your pH electrode hydrated. For benchtop meters used in the lab pour a small amount of the HI70300 storage solution in a small beaker and lower the electrode into it making sure that the junction is covered. For portable meters, store the electrode with a few drops of HI70300 storage solution in the protective cap. Storage solutions are designed to keep the pH electrode hydrated while minimizing growth on the electrode from bacteria and algae. Placing a probe in water will result in a growth on the electrode that might not be visible to the naked eye. This growth will aect the performance and accuracy. To minimize growth it is recommended to use pH 4 buer if storage solution is not available. Solutions with lower pH values can inhibit growth. If pH 4 buer is not available, it is advisable to use pH 7 buer. Never store a pH electrode in purified water as it will dehydrate the bulb. The concentration of the fill solution is 3.5M KCl. The reference cell with this concentration generates a specific voltage. Placing a probe in purified water will have an osmotic effect causing water to move into the reference cell. There will also be a higher rate of diffusion of electrolyte from the reference cell into the water due to a concentration gradient. Both will result in a different reference electrolyte concentration, which will result in a change in the reference potential. If using a non-refillable probe in which the reference electrolyte cannot be changed, storage in purified water may result in premature failure and ultimately replacement of the electrode. Inspect the electrode for any scratches or cracks on the bulb or stem. If any are present, replace the electrode. 2 pH 2.136 | www.hannainst.com electrodes
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