Hi – I’m Sherri Renberg from the global liquid analysis marketing group, and I’d like to thank the many liquid analysis experts who have contributed to this blog series. We hope you will enjoy these useful answers to some of the most frequently asked questions we get from users about pH measurement.
While some of the questions are basic, that’s why they’re valuable. pH is a measurement where it never hurts to go back to the fundamentals. We’ll cover a few questions in this blog, and more in future.
Q) What is the shelf life of a pH sensor?
A) pH glass electrodes must remain hydrated which is why all manufacturers ship pH sensors with a cap saturated in a liquid solution. After being on the shelf for some time, the liquid solution inside the sensor cap can go dry, which is the primary reason sensors go bad on the shelf. It’s a good idea to re-saturate the pH sensor cap with a 4-buffer about every (6) months that the sensor remains on the shelf to extend the shelf life of the probe. The best way to determine if the sensor is functioning accurately is to see if it calibrates properly using the two-point calibration method.
Q) What is the proper way to install a pH sensor?
A) Most manufacturers insert an air bubble inside their glass electrodes to allow for temperature and pressure changes. Without this, pH sensors could crack with large temperature or pressure swings. If a sensor is mounted horizontally, the air bubble inside the sensor can move to the tip of the sensor, which can cause poor readings because it can impede the transfer of hydrogen ions. Therefore, pH sensors should be mounted at least 10 degrees above horizontal to ensure correct measurement. Sensors can also be installed vertically.
Q) I have a pH loop and I’m getting a “low slope” error message. What does this mean?
A) If you are getting a “low slope” error message, there are a few possible causes:
• The sensor may be coated or dirty. Try cleaning the sensor and repeating the calibration.
• The glass is dry and needs to be rehydrated before calibration. To rehydrate the sensor, soak it in pH 4 buffer solution overnight. Theoretically, a brand new sensor’s slope should be 59.16mV when the sensor is set to auto-temperature compensate to 25oC, however, a new sensor could potentially have a slope as low as 55mV/pH without causing any problems. Note that the calibration is only as good as the chemicals are fresh. Make sure there are no air bubbles on the glass and that the sensor is left in the solution long enough to stabilize the reading.
• The glass is old and may need replacing.
Q) What affects the accuracy of a pH calibration?
A) The first thing to consider when trying to get an accurate pH measurement is the proper calibration of your equipment. Make sure that you take the appropriate time to calibrate your pH meter or analyzer with a quality standard buffer solution.
Room temperature, buffer temperature, and sample temperature all impact the calibration process. Try to simulate the actual environment the sensor will be operating in for the best calibration results.
As the pH sensor depends on its glass tip to make readings, the cleanliness and the quality of the glass can also impact your accuracy. Time, heat, and harsh chemicals gradually eat away at the glass surface, changing its properties and degrading the quality of the reading.
Q) What buffer calibration errors can occur when calibrating my pH sensor?
A) Buffer solutions have a stated pH value at 25°C (77°F), but when that value is 7 pH or above, the actual pH of the buffer will change with temperature. The values of the buffer solution at temperatures other than 25°C (77°F) are usually listed on the bottle. The pH value at the calibration temperature should be used or else errors in the slope and zero values, calculated by the calibration, will result. An alternative is to use the “buffer recognition” feature on modern pH analyzers, which automatically corrects the buffer value used by the analyzer for the temperature.
Another type of calibration error can result from not allowing enough time for the buffer calibration to complete. If the pH sensor is not given enough time to fully respond to the buffer solution, it can cause errors, especially in the case of a warm pH sensor not being given enough time to cool down to the temperature of the buffer solution. Current pH analyzers have a “buffer stabilization” feature, which prevents the analyzer from accepting a buffer pH reading that has not reached a prescribed level of stabilization.
This is just a start of some of the great questions users have sent us. We’ll share some more in a future blog. What kind of questions do you have about pH measurement?
By Rob Clemons, Sales Manager, PCE Pacific, Inc.
Hi. I’m Rob Clemons, Sales Manager for the Instrument and Automation Division of PCE Pacific, Inc. I’ve got a great customer story to share with you. Chances are, most of you don’t own cruise lines, but as you can imagine, cruise lines have very stringent water quality rules – both for potable water and for water used in recreational facilities like hot tubs. The Center for Disease Control (CDC) places strict regulations requiring the continuous measurement of both halogens, such as chlorine, and pH. So we’re very honored to have been selected by the unique cruise line, Un-Cruise Adventures, as their water quality measurement company. This selection might have implications for your application as well.
The analyzer that proved to be ideal for the Un-Cruise Adventures application is the 56 Advanced Dual Input Analyzer configured for FCL (free chlorine) measurement. The 56 is a benchmark in ease of use and met the stringent requirements of the CDC.
The CDC regulations regarding measurement of halogens states:
A HALOGEN analyzer-chart recorder must be installed at a distant point in the POTABLE WATER distribution system where a significant water flow exists and represents the entire distribution system. In cases where multiple distribution loops exist and no pipes connect the loops, there must be an analyzer and chart recorder for each loop. Potable Water; 48 184.108.40.206.2 Data Logger Electronic data loggers with certified data security features may be used in lieu of chart recorders.
Needless to say, the choice of an electronic data logger is far superior in terms of precision, ease-of-use, and reduction in maintenance time. The CDC specifications go on to say:
220.127.116.11 Operation 18.104.22.168.1 Maintenance. A manual comparison test must be conducted daily to verify calibration. Calibration must be made whenever the manual test value is > 0.2 ppm higher or lower than the analyzer reading. 22.214.171.124.2 Calibration (06) The daily manual comparison test or calibration must be recorded either on the recorder chart or in a log. 126.96.36.199.3 Accuracy (05) The free residual HALOGEN measured by the HALOGEN analyzer must be ± 0.2 MG/L (ppm) of the free residual HALOGEN measured by the manual test.
The built-in data logger of the 56 again saves time, and enhances accuracy.
Un-Cruise Adventures is using the 56 equipment to provide CDC-mandated chlorine and pH control of three public hot tubs as well as for ensuring CDC compliance with onboard potable water systems on two passenger vessels. Since pH measurement is part of the free chlorine measurement on the 56, the system saved both time and money. To accommodate Un-Cruise Adventure’s space requirement, the back-panel of the assembly was trimmed down on one side and the 56 analyzer re-installed in order to fit inside a weather-proof enclosure they supplied. Our team is available to customize a system for you. Are there any water quality monitoring issues you have?
Dan Emigh, Port Engineer of Un-Cruise Adventures, stated, “I chose to use the Rosemount analytical equipment from Emerson for this important function based on my past experiences when installing this panel on our first vessel back in 2000. I had fantastic support from the rep and the equipment worked precisely as expected. When it became necessary to add the same type of equipment to the second vessel in 2014 I started by contacting Emerson and again received immediate and full support from the technical staff. As a direct result of the fantastic customer and technical service I have always received from everyone, I have no reason to consider anyone else for our water treatment needs. One phone call to Emerson and questions are answered, issues are resolved, and parts are on their way to our distant ships. Thanks for all the past and present support.”
And we didn’t even have to twist Dan’s arm to say that. Remember, the next time you’re sailing with Un-Cruise Adventures, the quality of the onboard water is in good hands.
by Barry Wallen
Hello, and welcome to Analytic Expert! I’m Barry Wallen, Senior Sales Engineer at Emerson Process Management. Today I’d like to talk about measuring pH in corn slurry in an ethanol plant. Historically, this has been a tough measurement due to a variety of factors including heat, viscosity, abrasion, and contents of the stream. The problems included shortened probe life, lack of accuracy across a useful pH range, and sluggish response to process changes. The use of a sodium reference pH sensor made meaningful 2 point calibrations impractical. The need is for accurate pH measurement across a wider range, and quicker response times.
Here’s a meaningful solution. The Rosemount Analytical 3300HTVP and the 1056 analyzer have been performing well in dozens of ethanol plants beginning with a trial in Hudson, South Dakota. With these technologies, plants are getting consistent accurate pH values as well as longer sensor life.
The 3300HTVP is a robust sensor with a rebuildable reference electrode. This extends the life of the sensor as the reference electrode is usually the first part of the sensor to “die.” Typically, plants are doing a reference junction rebuild monthly and seeing probe lives of about a year.
Generally, the electrode is mounted in a tee in a recirculating loop beside the tank; however there is a retractable version that can be mounted through a ball valve directly into a tank. Ideally, the fins on the electrode protecting the glass bulb of the sensor should be oriented so they are upstream and downstream, not on the sides of the stream. This gives the glass measurement electrode protection from abrasion caused by the slurry as well as any metal pieces that may have made it this far into the process.
On initial power up, the 1056 will walk through a quick start menu. This menu allows operators to rapidly confirm a few parameters including language, measurement (in this case, pH), temperature units, and operating Hertz. The manual includes detailed instructions on advanced set up – there is very little that would need to be changed. The display allows for two (2) large display items (usually pH and temperature in a single channel unit) and four (4) small ones. These are all user selectable. For additional information on the application, please click HERE.
You can then perform an initial calibration and start receiving reliable accurate pH values.
For complete instructions on operation, maintenance, and steps in rebuilding the reference electrode, please click HERE. And for additional information on other food and beverage applications, please click HERE.
By Jill Jermain, Senior Product Manager
Hi. Welcome back to Analytic Expert. As promised, this is the second in a two-part series answering questions about pH analysis in various types of industrial process control applications. I’m Jill Jermain and I’m happy to be your analytic expert on this important topic. If you missed Part 1, please click HERE and read through it, since some of those answers will help you understand these better.
What causes poisoning of the sensor?
The common reference electrode used in pH measurements consists of a silver wire coated with silver chloride in a fill solution of potassium chloride. The purpose of the potassium chloride is to maintain a reproducible concentration of silver ions in the fill solution, which in turn, results in a reproducible potential (voltage) on the silver-silver chloride wire. The mechanism of reference poisoning is a conversion of the reference from a silver-silver chloride based electrode to an electrode based on a different silver compound. The ions (bromide, iodide, sulfide) form less soluble salt with silver than does chloride. When these ions enter the fill solution, they form insoluble precipitates with the silver ions in the fill solution. But there is no initial effect on the potential of the reference, because the silver ions lost to precipitation are replenished by silver ions dissolving off the silver chloride coating of the silver wire. It is not until the silver chloride coating is completely lost that a large change in the potential of the reference occurs. At this point, the reference electrode must be replaced.
How can I keep my sensor from frequent coating?
Coating typically increases the response time of the pH sensor and can cause instability in pH control. A process flow velocity greater than 5ft/sec will help minimize coating. Also, be sure to check on the placement of the sensor in the process as this may contribute to the rate of coating as well. For example, if the sensor is barely peeking out into the process, there isn’t a great deal of flow versus if the sensor sticks out a few inches more into the process. The higher flow rate would help the sensor from not coating as quickly since it would act as an artificial scrubber.
Why should I monitor glass impedance and reference impedance?
Glass impedance refers to the impedance of the pH-sensitive glass membrane. The impedance of the glass membrane is a strong function of temperature. As temperature increases, the impedance decreases. The impedance of a typical glass electrode at 25°C is about 100MΩ. Most Rosemount Analytical pH sensors are 50 to 200MΩ, with exception of the PERpH-X pH sensors, which measure 400 to 1,000MΩ. A sharp decrease in the temperature-corrected impedance implies that the glass is cracked. High glass impedance implies that the sensor is nearing the end of its life and should be replaced as soon as possible.
The major contributor to reference impedance is the resistance across the liquid junction plug. The resistance of the liquid junction should be less than 40kΩ. High impedance readings around 140+kΩ typically indicate that the junction is plugged or the filling solution/gel is depleted.
What is a ground loop?
A ground loop exists when a circuit is connected to earth ground at two or more points. Because the potential of the earth varies from point to point, two or more connections to ground cause currents to flow. If the current flows through a signal carrying wire, the result is a noisy, offset signal. The classic symptom of a ground loop is a sensor that reads correctly in buffers, but gives a reading grossly in error when placed in the process liquid. These ground loop currents find a good conductor in the reference electrode and make this electrode part of the current loop. Because the voltage is in series with other cell voltages, the ground loop current causes the pH reading to be substantially different from the expected value. The source of the ground loop could be any pump, motor, or other electrically powered device. To eliminate a potential ground loop, don’t attach any electrically powered device to the same ground on your meter as the pH electrode.
What is a sodium error?
More correctly called alkali ion error, sodium ion error occurs at high pH, where hydrogen ion concentration is very low in comparison to sodium ion concentration. Sodium errors come about because the potential of the glass membrane depends not only on the concentration of hydrogen ions, but also on the concentration of other metal ions, for example, sodium. The sodium ion concentration can be so high relative to hydrogen ion concentration that the electrode begins to respond to the sodium ion. This results in a reading that is lower than the actual pH. Depending on the glass formulation, this can occur as low as 10 pH.
I hope these answers have been helpful to you in understanding and operating your pH sensors. Do you have more questions? Check out our pH Sensor Selection Tool HERE, and feel free to ask any questions here and I’ll answer as soon as possible. Or you can contact me at email@example.com.
Hi and welcome to Analytic Expert! I’m Jill Jermain, senior product manager at Emerson Process Management. Today’s blog is the first in a two-part series answering essential questions about pH analysis. 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, or biopharmaceutical processing. Keep this blog handy to refer to regularly. Here we go:
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.
And here’s Part 2 of this blog series, as we continue to address important pH questions – like what is a ground loop, typical causes of sensor poisoning, and what’s a sodium error.