Process Water Analysis with Ion-Selective Electrodes (ISE)

How does an ion-selective electrode work?

An ion-selective electrode (ISE) is a potentiometric sensor that responds to the activity of a specific ion in solution. The selective membrane at the tip of the electrode interacts only with the target ion, forming a potential on the membrane surface dependent on that ion's concentration. This potential is measured against a reference electrode and converted to ion concentration using the Nernst equation.

In process analyzers, measurement takes place in a flow cell with the electrode, reference electrode and a temperature compensation probe (ATC). In some applications, the sample's pH is adjusted with a reagent; for example, pH is raised in low-level sodium measurement, and ammonium is converted to ammonia in ammonium measurement.

What the ion-selective electrode measures and where it is used

The ISE method measures ions such as sodium, fluoride, chloride, ammonia/ammonium, calcium and sulfide. Application areas:

• Monitoring boiler water, steam, condensate and demineralized water in power plants (sodium, chloride)
• Dosing control in drinking water fluoridation and monitoring of fluoride in wastewater discharge
• Nitrification control in wastewater treatment (ammonium)
• Hardness tracking at softener and reverse osmosis outlets
• Sulfide measurement in refinery water and process streams

Where the ion-selective electrode works well and where it struggles

The ISE method can measure continuously over a wide concentration range, is instrumentally simple and has low consumable costs. Because it senses only the target ion, it is unaffected by colored and turbid samples.

Limitations: electrode response drifts over time, so periodic calibration and membrane/electrode replacement are needed. Interfering ions, temperature and pH can affect the measurement. Response time can lengthen at low concentrations. Tracking the reference electrode filling solution is part of maintenance.

Choosing an instrument for the ion-selective electrode

When selecting, the target ion, concentration range, sample pH and required reagent and dosing arrangement are assessed together. Regular electrode calibration and standard solution kits are part of the maintenance plan for sustaining measurement reliability.

Tetra and ISE analyzers

Tetra supplies, installs, commissions, calibrates and services ion-selective electrode process analyzers in Türkiye. The product range includes Thermo Orion 2111XP sodium, 2109XP fluoride, 2117 chloride, 2110XP ammonia and 2120XP calcium hardness analyzers and the Galvanic AccuSeries LPA potentiometric wet chemistry analyzer. Tetra also integrates these instruments into SWAS water-steam analysis systems.