Article | June 16, 2015

Essential Answers To Important pH Questions – Part 1

Source: Emerson
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pH measurement plays an important role in virtually every industrial process and an equally essential part in environmental regulatory compliance. Many of the below questions pop up every day whether in chemical processing plants, power plants, water and wastewater plants or biopharmaceutical processing. (Read Part 2 here.)

Why is it important to measure pH?
The major function of pH in industry is process control. Controlling pH ensures product quality, reduces corrosion and scaling in equipment, and protects the environment by monitoring and regulating product waste. It’s important that the person performing pH measurements understands how the measurement is made, how to calibrate the instrument, and how to recognize and avoid common problems.

What is the shelf life of a pH sensor?
pH glass electrodes slowly deteriorate in storage overtime and no specific expiration date is given. The best way to determine if the sensor is functioning accurately is to see if it calibrates properly using the two-point calibration method.

What is the two-point calibration method?
This method uses two buffer solutions, usually at least 3 pH units apart, which allows the pH analyzer to calculate a new slope and zero value to be used for deriving pH from the millivolt and temperature signals. The slope and zero value derived from a buffer calibration provide an indication of the condition of the glass electrode from the magnitude of its slope, while the zero value gives an indication of reference poisoning. Ideally, the pH electrode will have a slope of 59.16 mV/pH, but in practice, a well-functioning electrode has a slope of 54 to 59 mV/pH.

How often do I need to calibrate my pH sensor?
The frequency at which sensors should be calibrated can be determined only by experience. Many factors influence calibration frequency. Sensors installed in a dirty or corrosive process stream usually require more frequent calibrations than sensors used in clean water. Sensors measuring extreme pH values also require more frequent calibrations than sensors measuring mid-range pH.

Why isn’t a pH 10 buffer solution recommended for calibrating?
A pH 10 buffer solution absorbs carbon dioxide (CO2) from the air, which depresses the pH. When CO2 is absorbed in water, it forms carbonic acid, which in turn lowers the pH of the buffer. Thus, the true pH is less than the expected value, and the calibration slope is low. If the pH 10 buffer gives a low slope, repeat the calibration using a lower pH buffer. For example, use pH 4 and 7 buffer instead of pH 7 and 10 buffer.

What is the best way to clean a pH sensor?
Again, the frequency at which a sensor should be inspected and cleaned can be determined only by experience. If the process liquid coats or fouls the sensor, frequent cleaning may be necessary. If the process does not contain a high level of suspended solids, the need for regular cleaning will be less. Sensor diagnostic measurements, if available, can also help indicate when a sensor needs cleaning. Often, an increase in glass or reference impedance indicates the electrode is becoming fouled. To remove oil deposit, clean the electrode with a mild non-abrasive detergent. To remove scale deposits, soak electrodes for 1 to 5 minutes in a 5% hydrochloric acid solution. Remember to always recalibrate the sensor after cleaning. If the sensor was cleaned with detergent or acid, soak the sensor in pH 4 buffer for at least an hour before calibrating.

What is the slope of a sensor?
The slope (sensitivity) indicates how well the sensor responds to changes in pH. A theoretically ideal electrode slope has an mV change of 59.16 mV/pH at 25°C. As the electrode ages, its slope decreases and a sensor should be replaced when the slope reaches 48 to 50 mV/pH.

To determine what kind of pH sensor would work best for your application, check out our pH Sensor Selection Tool HERE.