Maximize pH Sensor Performance

Q: We have a wastewater pretreatment system for the treatment of chrome, nickel and zinc, so we have chrome reduction with acid and metabisulfite followed by precipitation. After our clarifier, the wastewater then discharges to the city.

Over the last several months, we have had difficulty with calibration of our pH sensors, particularly the pH adjustment sensor that controls pH in the range of 9.1– 9.5 for precipitation. Every week we remove both sensors, rinse with water, and then calibrate with buffers 7 and 10. Lately, it has taken many repeated attempts to get the controller to calibrate with these buffers; sometimes when we thought we had the sensor calibrated, we would check it with buffer 7 and the reading would be off by more than 0.5.

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We then thought we had a bad sensor and ordered a new one; however, we experienced the same problem. We have checked the electronics and everything is OK. Somehow the sensor still works well enough for us to achieve metal precipitation to maintain compliance, although our effluent’s metal concentrations are a little higher than in the past. We feel very uncomfortable continuing this way. Any ideas to help? A. J.

A: Our first task is to check the quality of the pH buffers. All pH buffers have a shelf life. Once opened, the buffers begin to degrade. This is especially so for buffer 10. When buffer 10 is exposed to the atmosphere, carbon dioxide in the air dissolves into solution, forms carbonic acid and neutralizes the weak alkali in the solution; over time, the pH of the buffer solution drops. This phenomenon does not occur with buffer 4 because it is already acidic, and is extremely slow to occur with buffer 7.

When using pH buffer solutions, follow these recommendations:

1. Purchase only the amount of buffer solution that can be used by its dated shelf life; reputable suppliers will label their producs with an expiration date.
2. Use buffer solutions that are colored and whose color fades as they lose their strength; this will help you know when to change solution.
3. Periodically check buffer solutions with pH indicating paper to verify accuracy.
4. Keep buffer solution containers closed when not in use.

A second key element to peak pH sensor performance is its cleaning, and your current practice of using water alone is certainly inadequate. pH sensors operate by allowing hydrogen ions (H+) to flow to the sensor; anything that interferes with this flow, such as oil or polymer films, scale, and metal precipitants, will slow down the sensor’s response to the changing pH conditions in the wastewater. If this condition is allowed to deteriorate, it will cause the pH to go out of control and/or excessive chemical (acid and caustic) usage.

For the pH sensor in the pH adjustment tank for metal precipitation, we recommend at least daily cleaning with water to remove the likely collection of loose precipitated metals (zinc hydroxide, iron hydroxide, etc) around the sensor.

At least on a weekly basis we recommend a vigorous cleaning as follows:

1. Rinse sensor with water
2. Never touch sensor with sharp or hard object; if you need to remove a tough material, use a cotton stick
3. Remove oil and polymer films with dilute alkaline cleaner, strong dishwashing solution, or 50/50 water/isopropyl alcohol solution
4. Rinse sensor with water
5. Remove scale and tough precipitated metals by cleaning with dilute (1–3%) muriatic or hydrochloric acid
6. Rinse sensor with water.

Instead of using a cleaner and then acid, we have also found success in using janitorial strength inhibited acid-based toilet bowl cleaner as a one-step chemical that can remove oily films at the same time as scale. The bottom line is to create a thoroughly clean surface for maximum ion transfer.

A third key element to peak pH sensor performance is calibration. Recognizing that pH sensors are in reality batteries with a “life,” the purpose of calibration is not only to keep the sensor accurate, but also to discover when it is nearing the end of its life so that it is replaced before it “dies.”

For pH sensor calibration, we recommend the following general procedure:

1. Thoroughly clean sensor as described above. For the chrome reduction pH sensor use buffers 4 and 7 since your pH control range is 2–3.
2. For the pH adjustment tank for metal precipitation use buffers 7 and 10 since your pH control range is 9.1–9.5.
3. For the chrome reduction pH sensor, use buffer 7 to adjust the “zero” point and buffer 4 to adjust the “slope” or “span;” most of today’s pH controllers have automatic calibration functions so this may not be needed.
4. For the pH adjustment tank, use buffer 7 to adjust the “zero” point and buffer 10 to adjust the “slope” or “span.”
5. Be sure you rinse the sensor very well with clean water between buffer solutions to avoid cross contamination; if in doubt, change buffer solutions.
6. If the pH sensor responds sluggishly during calibration, it is likely nearing the end of its life and needs to be replaced.

I am confident that if you follow our recommendations regarding buffer integrity, pH sensor cleaning, and pH sensor calibration, it will solve your problem and help you achieve maximum performance and efficiency from your wastewater pretreatment system. Let me know your results