Dealing with Interfering Ions
Understanding selectivity coefficients and practical strategies for minimising ionic interference in ISE measurements.
Unlike the pH glass membrane — which is effectively perfectly selective for H⁺ — most ISE membranes allow some passage of ions other than the target ion. When these interfering ions are present in a sample, the electrode gives a reading that reflects the total response to all species, not just the target ion.
Result: the apparent measured concentration is higher or lower than the true target ion concentration, depending on the charge and direction of response to the interferent.
The degree to which interfering ion j affects the measurement of primary ion i is described by the selectivity coefficient K_ij (also written as SC in manufacturer datasheets). The full Nikolsky–Eisenman equation is:
For practical purposes, when the interferent concentration is small relative to the primary ion, the error can be estimated directly:
Each ion guide page provides an interference table listing the interferent name, selectivity coefficient, and notes on special conditions. Here is an example interpretation for the Ammonium electrode:
| Interferent | K_ij (SC) | Practical effect |
|---|---|---|
| K⁺ | 0.1 | 10 ppm K⁺ in a 100 ppm NH₄⁺ sample causes ~10% apparent increase |
| Na⁺ | 0.002 | Even at 10× the NH₄⁺ concentration, Na⁺ causes only ~2% error |
Published K values are approximate. In practice they depend on:
- Concentration of both the primary and interfering ions
- Temperature of the measurement solution
- Ionic strength of the solution
- Age and condition of the electrode membrane
- Method of measurement (direct calibration vs. standard addition)
For high-precision work, it is better to measure the selectivity coefficient directly for your specific conditions rather than relying on the published value.
- Complexation: add a reagent that binds the interferent — e.g. TISAB for fluoride also removes Al³⁺ and Fe³⁺ which would otherwise complex fluoride itself
- Precipitation: add a reagent that causes the interferent to precipitate out of solution
- Ion exchange resins: selectively remove specific interfering ions before measurement
Diluting the sample reduces both primary and interferent concentrations. However, if the interferent is in large excess, the ratio [interferent]/[primary] may remain problematic — dilution alone is rarely a complete solution.
These methods inherently reduce the impact of constant background interference, because calibration and measurement occur in the same matrix. The interferent is present at the same level in both the baseline and the spiked reading.
If the interferent concentration is known — measured separately by another method — calculate its contribution using the selectivity coefficient and subtract it from the apparent primary ion reading.
For some systems, changing pH can convert the interfering ion to a non-interfering form. Example: at high pH, ammonium (NH₄⁺) converts to ammonia (NH₃), which does not affect the electrode response — useful when ammonium is itself the interferent for another electrode.
Some ISABs contain masking agents designed specifically for common interference problems. Example: CuSO₄ ISAB for ammonium uses Cu²⁺ to complex with certain interferents in the matrix.
The most common interference problem in environmental and agricultural analysis. Potassium is the primary interferent for the ammonium electrode (SC ≈ 0.1).
Standard correction using the published selectivity coefficient is usually adequate — measure K⁺ in the sample and subtract the calculated contribution from the apparent NH₄⁺ reading.
Measure the actual selectivity coefficient for your samples or spike all standards to the same K level as the samples.
- 1Measure both K⁺ and NH₄⁺ in a representative sample
- 2Spike additional K⁺ to approximately double its concentration
- 3Recalculate SC from the observed change in apparent NH₄⁺ reading
Interference has a particularly severe effect near the detection limit of the electrode. The interferent can effectively mask the primary ion response, making reliable measurements below a certain concentration impossible without first removing the interferent.
Use this table as a starting point when deciding how to handle interference in your specific application.
| Situation | Recommended Strategy |
|---|---|
| SC < 0.01 and [interferent] < 10× [primary] | Ignore — error <10% |
| SC 0.01–0.1, low [interferent] | Standard correction using published K |
| SC > 0.1 or high [interferent] | Measure K directly for your conditions |
| Interferent can be removed chemically | Chemical removal (complexation, precipitation, resin) |
| Complex or unknown matrix | Standard addition or sample addition methods |
| Highest precision required | Triple electrode with correction software |
- Ion-Specific Guides — each guide includes a full interference table with selectivity coefficients specific to that electrode
- Beginner's Guide — Chapter 5 covers the modified Nernst equation and interference theory