pH/ REDOX electrode maintenance
The 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 on ths page are intended to help avoid these problems. The built-in sealed reference design of this electrode eliminates the need to add filling solutions and minimises reference dryout. This design feature also allows the electrode to be used in pressurised systems, (refer to specification sheets or consult Sensys 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 a form of a battery and have limited shelf lives. Electrodes in inventory should be rotated so that 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 minimised 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 minimise 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. This action will minimise span errors.

7. Readings stabilise faster in some solutions than others; allow time for the reading to stabilise. In general, with new electrodes stable readings in buffers are obtained in 10-15 seconds.

8. All pH electrodes age with time. Ageing is characterised by shortened span and slower speed of response. Ageing 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 the 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 ageing. Before concluding that an electrode needs replacing, check the surface for coatings.

11. Temperature affects electrode readings in two ways. First, the output of an electrode varies with temperature. For pH electrodes this effect can be corrected by manual or automatic temperature compensation, (ORP/REDOX readings are not correctable for the effect of temperature changes). Second, the real pH or ORP value, independent of the electrode measuring the value or the use of temperature compensation, is temperature dependent. This fact means, for example, that the readings at 25oC and 75oC will be (and, in fact are) different.

12. How far can I run the electrode cable?
Once the electrode signal is amplified, the cable may be extended up to 1000 feet (300 meters). If using a standard, non amplified electrode connected to a preamplifier, the electrode should be located as close to the preamplifier as possible. A maximum of 20 feet is recommended. With the WEL, the amplifier is right in the electrode, for maximum reliability. An electrode cable must be routed away from any AC wiring, fans, motors, or other sources of electrical noise. Keep in mind that the pH/ORP electrode will require calibration on a regular basis. The longer the distance between the electrode and the controller, the more painful the calibration procedure will be. At 1000 feet of separation, you will either need two people with 2-way radios or some good track shoes.

13. How long will my electrode last in my application?
It is difficult to predict pH/ORP electrode life in certain applications. These electrodes are similar to batteries, and eventually they will run down. There are some factors that will influence the life:

a) Temperature: The higher the temperature, the lower the life of the electrode. If the electrode is exposed to temperatures over 60 degrees C on a regular basis, the expected life may only be several months.

b) Shelf Life: The span of a pH electrode, (the mV that the electrode puts out per pH unit) will decrease with time, even if the electrode is stored in the original soaker bottle. Eventually, two pH buffers will create almost the same mV output, and the electrode will fail calibration. Electrodes are warranted to have a 12 month shelf life.

c) Abrasion: Scratches on the pH glass will reduce the span of the electrode. Our flat surface design minimises this effect.

d) Chemical Attack: Acid fluorides will dissolve the pH sensitive glass. Our HF-resistant glass option will minimise this effect. An electrode that might dissolve in a few days with normal pH glass, might last several months with HF-resistant pH glass. Take careful notice of all the wetted materials of construction of an electrode, and make sure that they are compatible with whatever may be found in your solution.

e) Contamination of reference: The pH/ORP electrode utilises a silver/silver chloride reference to measure the process signal against. Certain chemicals, (heavy metals, sulphides, proteins, etc.) will interact with the silver, and either clog the junction or dissolve the reference.

f) Coatings: An application where the electrode frequently coats will require frequent cleaning, and this increased handling will result in increased abrasion, and shorter life. The flat design will reduce the frequency of cleaning. Our probe wash feature will decrease the handling required when cleaning.