Detector Control
Using the Reference Electrode with Alkaline Eluents

The reference electrode inside the ICS-3000 ED or ED40/ED50/ED50A celI is a combination pH-Ag/AgCl electrode. The reference electrode inside the ICS-4000 or ICS-5000 ED cell is either a combination pH-Ag/AgCl electrode or a PdH electrode.

When used with alkaline eluents, the reference potential of pH-Ag/AgCl electrodes undergoes a gradual change. (The PdH reference electrode is largely unaffected by long-term use with alkaline eluents.) As the silver chloride layer on the silver wire is converted to a mixture of silver oxide and hydroxide, the value of the reference potential gradually increases. This gradual change is indicated by the changes in pH readout when pumping a known composition of mobile phase.

For example, the pH readout for 60 mM sodium hydroxide should be approximately 12.8. It thus follows, from the Nernst equation, that a change in pH from 12.8 to 13.3 corresponds to approximately 30 mV of change in the reference potential. By calibrating the pH reference electrode only once, during initial installation, and by monitoring the pH readout, you can determine the status of the reference electrode. On the other hand, repeating the calibration will cause an electronic adjustment of the constant and slope of the Nernst equation in the detector electronics. Hence, the pH readout will be corrected with each recalibration, but the reference potential drift will not be apparent.

When  Defining a Waveform, you must also select the Reference Electrode Mode. If you select the AgCl mode for the combination electrode, only the Ag/AgCl part of the electrode is used. If you select the pH mode, the entire pH-Ag/AgCl electrode is used.

There are several important considerations:

• The same pH-Ag/AgCl combination electrode is used in both the AgCl and pH reference modes.

• The Ag/AgCl half-cell is more commonly used as the cell reference electrode. However, if the pH changes during a separation, then use the pH reference mode instead of the AgCl mode.

• If you select the pH reference mode, always correct the waveform for pH dependence (0.059 V/pH unit). See Correcting a Waveform for pH Dependence for details.

• The pH readout matters, even if the reference mode is AgCl.

If you select the AgCl or pH reference mode, Chromeleon compares the pH value at the beginning of each chromatographic run with the upper and lower pH limits you selected when defining the waveform. If the pH reading falls outside this range, a warning message appears in the Audit Trail. You may want to set the pH limits to define a narrow range and rely on the "out of range" warning to detect any unacceptable shift in the reference potential. To avoid receiving this warning, set the pH limits to a very broad range (for example, 7 to 14).

 Note:

The pH-Ag/AgCl reference electrode is a consumable item and should be replaced whenever the potential has changed by more than 30 mV (typically, after 3 months of use). Continued use of a defective pH-Ag/AgCl reference electrode will cause a passivation (loss of detection response) of the working electrode, and may result in delays and unnecessary service calls.

Recommendations for Specific Operating Conditions

Also refer to:

 Calibrating the ED40/50/50A pH Reference Electrode

Calibrating the ICS-3000/ICS-5000 ED pH Reference Electrode

 Selecting a Default Waveform for Integrated Amperometry.