December 20, 2017

The ABCs of pH (part 2)

By Marc Mason, business development manager, liquid analysis, and Gregory Taylor, sales manager, Emerson Automation Solutions

I wanted to share some insights from our liquid analysis experts into the top questions asked by users like you about pH measurement. Today, I’ve selected a few of the questions asked the very most. Chances are, you or someone in your plant have had one or more of these questions puzzling you –

Q) What affects the accuracy of a pH calibration?
) 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 is the slope of a sensor?
A) 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 (77°F). As the electrode ages, its slope decreases and a sensor should be replaced when the slope reaches 48 to 50 mV/pH.

Q) 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. It’s important to keep in mind, however, that a 10 pH buffer is more susceptible to cross contamination between buffers during calibration and more affected by temperature than a 4 or 7 or other lower pH buffers, so at different temperatures, a 10 pH buffer may not actually be a 10. Applications with higher pH values often times are better served using a 10 pH buffer.

Q) 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. A pH sensor that has a cracked glass will often read pH around 7, so if the pH meter is consistently reading 7 pH, it would be a good idea to calibrate the sensor to see if it is truly working. 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.

Q) What causes pH sensor poisoning?
) 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.

Hopefully, the answers to these FAQs have touched on areas that you need to know. More information can be found here.

I’ll be back with more user questions here at Analytic Expert soon.

December 6, 2017

Wireless Technology Changes the Bottom Line for Industrial Wastewater Applications

By Aryanto Wibisono, deputy manager, analytical measurement division, PT Control Systems

It’s easy to think of wastewater as the forgotten kid of industrial and municipal applications. The lower budgets and constrained personnel power often means wastewater is the last to take advantage of new technologies and solutions. And sometimes this can be a costly mistake as an installation at an Indonesian Liquid Natural Gas plant shows.

This LNG plant, the largest in Kalimantan, Indonesia, has a Corporate Social Responsibility (CSR) to ensure its wastewater quality meets environmental regulatory requirements before they discharge effluent into the sea – a demand shared by most industrial enterprises. This plant has been granted a gold rating in the Indonesian government PROPER* program and, in order to maintain its gold status, the company must continually strive to utilize innovative methods that ensure environmental compliance and sustainability. However, its wastewater treatment pond is located remotely, about 1 kilometer (km) from the LNG plant. Due to physical constraints and economic considerations, it had not been possible to implement online effluent pH measurement. The monitoring and reporting of process wastewater was done manually via a third-party Health, Safety & Environment (HSE) engineer. This method requires the HSE engineer to commute to the wastewater pond at least two to three times daily, collecting effluent grab samples and reporting data back to the local environmental agency.

From both the efficiency and compliance points of view, this method was unsatisfactory. The manual pH recording method is extremely time-consuming and accuracy of sample data can be unreliable. The risk of poor quality effluent being discharged between manual recording intervals is also a serious concern. Sample recordings can be missed when the HSE engineer is unable to go onsite due to safety issues such as severe weather conditions. Any uncaptured data poses a risk of violating regulatory statutes, resulting in penalties or operation suspension. Many municipal and industrial wastewater installations find themselves with this kind of challenge.

To solve the problem, this LNG plant implemented wireless – a technology some wastewater installations assume is out of their reach. The plant used the Rosemount™ 56 Dual Channel Transmitter with the Emerson Wireless 775 THUM™ Adapter and gained access to real-time online effluent pH monitoring. Previously, manual sample recordings took one to one-and-a-half hours to complete, and this was carried out two to three times per day, year-round. Replacing the manual method with a wireless solution saved approximately 1,000 hours of labor and travel time to the site. Yes, 1,000 hours! The return on investment was huge and immediate.

The remote diagnostic features of the wireless Rosemount 56 Liquid Analyzer enable maintenance engineers to quickly and easily identify and determine the cause of an issue, such as poor wastewater quality or a device malfunction. The data logger function provides data redundancy, mitigating the risk of losing data in the event of a power failure, and offers data recording for environmental audit reporting. Maintenance engineers can also download the process data and event logger from the analyzer to a memory stick for further analysis. Due to the success of this wireless solution, the plant plans to expand the monitoring scope to include turbidity and dissolved oxygen (DO) monitoring.

This example shows that few industrial applications can ignore the potential benefits of wireless technology. Wireless makes possible levels of automation unthinkable only a short time ago. Where are you using wireless in your installations?

November 13, 2017

What Causes Damage to pH Sensor Glass Electrodes?

by Marc Mason, business development manager, liquid analysis, Emerson Automation Solutions

Hi and welcome. pH sensors are a small thing that can have a huge impact on the smooth operation of your plant. When they’re operating correctly, you barely notice them. If they aren’t working well, they can shut you down. Various factors can cause damage to pH sensor glass electrodes – the process, temperature, sodium error, caustic, and hydrofluoric acid (HF).

  • The Process: A glass pH electrode consists of an inert glass tube with a pH sensitive glass tip – either hemispherical (bulb) or flat in shape. The tip contains a fill solution with a known pH, and it is the influence of this solution on the inside of the glass tip versus the influence of the process solution on the outside that gives rise to its millivolt potential. Ideally, the pH electrode will have a slope (response) of -59.16 mV/pH, at 25C, but in practice, a new electrode may only have a slope of -57 to -58 mV/pH. As the electrode ages, its slope decreases.
  • Temperature: In addition to changing millivolt output of the pH electrode, elevated temperatures accelerate the aging of the electrode. Extremely high or low temperatures can alternatively boil the fill solution or freeze it, causing the electrode tip to break or crack. Elevated temperatures can also affect the interior and exterior of the pH electrode differently, giving rise to asymmetry potential, which shifts the zero point of the pH electrode and changes its temperature behavior, thus leading to temperature compensation errors.
  • 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. 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 pH glass formulation, this can occur as low as 10 pH. Where accurate high pH readings are required, the upper pH limit of the pH electrode should be checked and a specially formulated, high pH electrode used if necessary. Compared to sodium, lithium ions will produce a larger error, while the effect of potassium ions is negligible.
  • Caustic Components Attacking pH Electrodes: As noted earlier, high concentrations of hydroxyl ions shorten the life of pH electrodes. Solutions that approach 14 pH (equivalent to 4% caustic soda) can destroy a pH electrode in a matter of hours. There is nothing that can be done to prevent this, short of simply avoiding pH measurements in these solutions and using conductivity instead.
  • Hydrofluoric Acid (HF): HF also dissolves pH glass, but there are pH glass formulations designed to resist destruction by HF which, when used within their limits, can give satisfactory electrode life. It is important to note that, while only HF attacks glass and not the fluoride ion (F-), hydrofluoric acid is a weak acid. Therefore, a solution can contain a relatively high concentration of fluoride ion at a high pH and do no damage to the electrode. But if the pH of the solution decreases, the fluoride ion will combine with hydrogen ion to form HF, which will damage the electrode. So, it is important to look at both pH and HF concentration to determine the impact on pH glass.​​​​​​​​​​​​​​​​​​

What kinds of problems have you encountered with your pH sensor glass electrodes?

August 24, 2017

The ABCs of pH (Part 1)

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?
) 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?
 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?
 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?
 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?
) 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?

November 4, 2015

Choose the 56 FCL Measurement and You’re Cruising!

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 endeavourtubs. 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.

  • High resolution full-color screen: easily viewed process measurements and on-screen data trend graphs
  • User help screens: detailed instructions and troubleshooting in multiple languages
  • Data logger and event logger: download process data and alarm conditions with time and date stamps via USB 2.0 data port
  • Control: PID and time proportional capabilities; also includes synchronized interval timers and four special application functions
  • Digital Communications: HART® and Profibus® DP communications with full features and functions

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 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: Operation 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. Calibration (06) The daily manual comparison test or calibration must be recorded either on the recorder chart or in a log. 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.

What kinds of precision water quality measurement applications do you have?