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pH AND ORP (REDOX) ELECTRODE MAINTENANCE This Combination pH/Reference electrode or ORP (REDOX)/Reference Electrode is ruggedly made and easy to use. Because the pH responsive glass bulb or flat surface is relatively thin, care should be taken so that the bulb does not become scratched or broken. It is also important that ORP measuring surfaces are not scratched or gouged. The suggestions in this sheet are intended to help avoid these problems. The built-in sealed reference design of this electrode eliminates the need to add filling solutions and minimizes reference dryout. This design feature also allows the electrode to be used in pressurized systems (refer to specification sheets or consult the factory for maximum pressure/temperature limit information). IMPORTANT CONSIDERATIONS: 1. The pH Electrode is shipped in a plastic bottle or cap containing a solution of 4 buffer and potassium chloride. ORP (REDOX) Electrodes are shipped in caps containing a piece of cotton wetted with tap water. The electrode should remain in the bottle or cap until it is used. If the electrode is used infrequently the bottle or cap and its solution should be saved and the electrode stored in it. 2. Electrodes are in a form of a battery and have limited shelf lives. Electrodes in inventory should rotate so that the older electrodes are used first. 3. Vigorous stirring brings a sample, buffer or rinse solution to the measuring surface more quickly and will improve speed of response. Care must be taken to keep the electrode’s measuring surface from striking a surface and getting scratched or broken. 4. After exposure to a sample, buffer or rinse solution, carryover can be minimized by blotting-never by wiping-the electrode with a clean, non-abrasive paper or a clean cloth towel. 5. As a rinse solution, use a part of the next sample or buffer, which is to be measured. This action also will minimize contamination from carryover. 6. When calibrating, use a buffer close in value to that expected from the sample for 1 point calibrations or as the first buffer for 2 point calibrations (see below). This action will minimize span errors. 7. Readings stabilize faster in some solutions than others; allow time for the reading to stabilize. In general, with new electrodes stable readings in buffer are obtained in 10-15 seconds. 8. All pH electrodes age with time. Aging is characterized by shortened span and slower speed of response. Aging is best detected by the 2 point Calibration method. If the pH meter has manual or microprocessor slope (span) controls, the controls can be adjusted to compensate for electrode span errors (but will not affect the speed of response). 9. Electrodes should be replaced when their readings cannot be corrected by the meter’s controls and/or when their speed of response is too slow for the application for which they are being used. The frequency of electrode replacement is a function of the application; electrodes operating in hot liquids at very high or very low pH values will have shorter lives than those operating at neutral pH and ambient temperature. 10. Coatings on an electrode’s surface prevent new liquids from contacting an electrode’s measuring surface and can mimic the effects of electrode aging. Before concluding that an electrode needs replacing check its surface for coatings. 11. Temperature affects electrode readings in two ways. First, the output of an electrode varies with temperature. For pH electrodes this affect can be corrected by manual or automatic temperature compensation (ORP/REDOX readings are not correctable for the use affect of temperature changes). Second, the real pH or ORP value, independent or the electrode measuring the value or the use of temperature compensation, is temperature dependent. This fact means, for example, that the readings at 25°C and 75°C will be (and, in fact are) different. CALIBRATION As a rule, follow the procedures shown in the pH Meter’s Instruction Manual. These procedures will vary depending on whether the meter is a simple type with manual adjustments, a micro-processor type or a pH transmitter. Frequency of Calibration The frequency of calibration is a function of many factors. These factors include: 1. The accuracy required by the application. 2. The value of the off-specification product versus the cost of calibration. 3. The coating or abrasive nature of the application. 4. The stability of the pH Electrode and pH Meter as a system. The frequency of calibration is really determined by experience. At a new installation calibration might initially be checked every few hours or shift with the calibration changes noted in a log. As a pattern of longer stability is found, the time between calibration checks can be increased to once a day or once a week. Although the frequency of calibration is solely the responsibility of the user, once a week is the longest recommended interval between calibrations.PDF Image | pH AND ORP REDO ELECTRODE MAINTENANCE
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