Measurements · Water Quality
Ion-Selective (Other Ions)
Also known as ion-selective electrode, ISE, total nitrogen
Other ion-selective-electrode measurements (fluoride, potassium, metals, etc.).
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What it is
What is an ion-selective measurement?
An ion-selective measurement uses an ion-selective electrode (ISE), a sensor with a membrane engineered to respond mainly to one dissolved ion, to report that ion's concentration in water, in milligrams per liter. The electrode develops a voltage in proportion to the activity of its target ion, in the same way a pH electrode responds to hydrogen ions, and this general principle underlies the sonde's nitrate, ammonium, and chloride channels as well as electrodes for ions such as potassium, calcium, fluoride, and sodium. This generic ion-selective channel covers the additional or application-specific ions that a monitoring program may need beyond the standard nutrient and salinity measurements. It is the flexible, targeted counterpart to the bulk indicators like conductivity, which sense all ions together rather than one in particular.
Because it can be configured for a range of ions, an ion-selective channel extends a sonde to whatever specific chemistry a site demands.
Health and operational effects
The significance of an ion-selective measurement depends entirely on the ion being measured. Some ions carry direct health or ecological limits (fluoride and certain metals are regulated in drinking water, for example), while others, such as potassium or calcium, describe water hardness, nutrient balance, or industrial suitability.
How it is measured
How Ion-Selective (Other Ions) Is Measured
Each ion-selective channel is one electrode on a multiparameter sonde connected to a Thiamis gateway, carrying a membrane selective for its target ion and read against a reference electrode. The measured voltage is converted to a concentration through calibration against standard solutions, and the reading is temperature-compensated because the electrode response varies with temperature. Ion-selective electrodes share well-known limitations: they drift over time, suffer interference from other ions with similar chemistry, and are affected by biofouling. They need regular cleaning and frequent calibration, and their records are best anchored by occasional laboratory analysis. These maintenance needs are the price of the specificity an ISE provides.
An instrument on Thiamis4
A third-party instrument connected to a Thiamis gateway, reporting into the same Environet account.
Placing ion-selective sensors
An ion-selective sensor is deployed in-situ on the sonde at a representative, well-mixed point chosen to suit the target ion: near a discharge, an intake, or a source under investigation. Continuous logging captures the event-driven and seasonal changes in a specific ion that spot sampling would miss, and reading the ISE alongside the bulk channels such as conductivity helps confirm whether a change is specific to that ion or part of a broader shift. Because ion-selective electrodes drift and foul more readily than physical sensors, frequent cleaning and calibration are essential, and the interval should match how quickly the particular electrode and the local water tend to degrade the reading.
Every device and option that measures Ion-Selective (Other Ions) 4
| Device / option | Type | Base platform | Measures |
|---|---|---|---|
 In-Situ · Level TROLL 300, Level TROLL 400, Level TROLL 500… | Water Quality Sondes | Thiamis | Ion-Selective (Other Ions)Dissolved Ammonia (NH₃)Ammonium (NH₄)Blue-Green AlgaeChlorideChlorophyllConductivity |
 Eureka Water Probes · Manta+ 20, Manta+ 25, Manta+ 30… | Water Quality Sondes | Thiamis | Ion-Selective (Other Ions)Dissolved Ammonia (NH₃)Ammonium (NH₄)ChlorideChlorophyllDissolved CO₂Dissolved Oxygen |
 Spectra Scientific · Spectra Litmus 6, Spectra Litmus 4 | Water Quality Sondes | Thiamis | Ion-Selective (Other Ions)Dissolved Ammonia (NH₃)Ammonium (NH₄)Blue-Green AlgaeChlorideChlorophyllConductivity |
 YSI · 6600 V2, 6600EDS V2, 6920 V2… | Water Quality Sondes | Thiamis | Ion-Selective (Other Ions)Dissolved Ammonia (NH₃)Ammonium (NH₄)Blue-Green AlgaeChlorideChlorophyllConductivity |
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Questions
Ion-Selective (Other Ions) FAQ
What is an ion-selective electrode?
It is a sensor whose membrane is engineered to respond mainly to one dissolved ion, producing a voltage proportional to that ion's activity in the water, the same principle as a pH electrode. This lets a sonde report the concentration of a specific ion, such as potassium, calcium, fluoride, or sodium, rather than the bulk ionic load.
How is an ion-selective measurement different from conductivity?
Conductivity senses all dissolved ions together and reports their total effect, while an ion-selective electrode targets one particular ion and reports its concentration. Conductivity tells you the ionic load has changed; an ISE tells you which ion changed, so the two are complementary.
Why do ion-selective electrodes need frequent calibration?
Because they drift over time, suffer interference from other ions with similar chemistry, and are affected by biofouling, all of which shift the reading. Regular cleaning and calibration against standard solutions, backed by occasional laboratory checks, keep the measurement trustworthy, which is the trade-off for the specificity an ISE provides.
How do I monitor a specific ion?
Deploy a multiparameter sonde fitted with the appropriate ion-selective electrode connected to a Thiamis gateway, logging continuously with regular cleaning, calibration, and occasional laboratory checks. Use the configurator to pair a logger with a sonde covering your target ion alongside nitrate, ammonium, chloride, conductivity, and the other water parameters.
Monitor Ion-Selective (Other Ions) on the Record
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