May 6, 2015

Essential Answers to Important pH Questions – Part 2

By Jill Jermain, Senior Product Manager

FAQtwoHi. 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 jill.jermain@emerson.com.

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