Take Some Basic Steps with pH Measurements

A number of factors contribute to achieving high performance of pH loops

By Bhupen R. Patel, and Fred Kohlmann, Endress+Hauser, Inc.

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Glass electrode. This also is known as the measuring electrode. A special glass acts as a membrane interacting with process H+ ions and the alkali metal ions in the glass membrane. The interaction creates an electrical potential on the glass surface that changes as the hydronium ion concentration changes in the process fluid.

Two kinds of glass membranes are mainly available, ordinary and “B” types. Ordinary glass has cross-sensitivity towards other cations such as sodium ions. In salt solutions the pH may be neutral; however excess sodium ions from NaCl ionization can fool the ordinary glass membrane. The Na+ ions mimic the effects of hydrogen ions. As such, the sensor reads a lower than actual pH. This is common with such membranes in highly caustic fluids and is commonly referred to as sodium ion error.

“B” glass membranes, which contain a lithium impurity, are less sensitive to alkali metal ions. “B” glass formulation can minimize sodium ion error.

Reference electrode. The pH measurement is a half-cell reaction and requires an electrical circuit to be completed. The electrolyte from the reference cell completes the circuit with an ionic bridge. This electrode provides a reference voltage for comparison with the potential delivered from the glass electrode. The electrolyte is pressurized up to 6 bars. Often it’s in polymerized gel form to prevent rapid depletion.
Reference junction. The process fluid and the reference electrolyte interface at the reference junction. The junction permits the electrolyte to create the ionic cloud around the glass electrode to complete the electrical circuit.

Several types of junction interfaces are available; the longevity of a sensor depends upon selecting the proper reference junction. This can be as simple as an open hole allowing electrolyte interface with the process, or can use porous materials such as ceramic, polytetrafluoroethylene (PTFE) or wood, which allow the electrolyte to dilute into the process.

Sensor issues
The nature and care of the sensor and its need for maintenance and calibration pose a number of challenges.

Measuring glass issues. The measuring glass membrane can become dehydrated or can get coated with process material. The glass membrane sensitive to pH changes requires periodic service in many process conditions. To clean, soak it in pH 4 buffer and dab it with a clean cloth. If it doesn’t respond or calibrate, then use hot pH 4 buffer. There’re many other ways to clean a pH sensor; cleaning is application specific. It’s also important to note that deionized (DI) or demineralized (DM) water isn’t necessary to rinse the probe; clean tap water works fine. In addition, don’t store the pH sensor in DI or DM water. Doing so actually will shorten the sensor’s life. When the probe is in DI water, the reference fluid that’s in contact with DI water will equalize its ionic strength. This will deplete the electrolyte — it becomes diluted, resulting in shorter probe life.

Reference electrode challenges. When the reference voltage deviates from 0 mV, an error in pH measurement occurs. Depending on the process fluid characteristics, the reference electrolyte can get diluted (contaminated) through the interface with the process. The rate of reference contamination can be fast or slow depending on the permeability of the reference diaphragm. For example, open aperture reference junctions may get diluted comparatively faster — but also have relatively faster response to the pH changes in the process environment. In extreme cases, where the reference diaphragm is a large disk-type surface, scraping can expose a new surface.

Reference electrolyte contamination. The eluting of the reference electrolyte out of the reference diaphragm into the process to form an ionic cloud can lead to dissimilar ions in the reference electrolyte. When the junction potential is changing due to reference contamination, it will appear as a drift in pH. The solution to this ranges from using a different type of junction material, to multiple junction reference assemblies or a pressurized free-flowing reference/junction.

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