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?
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 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) 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?
A) 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?
A) 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.