A Beginner's Guide to Ion-Selective Electrode Measurements
A comprehensive, award-winning introduction to ISE theory and practice. Selected by StudySphere.com as one of the best educational resources on the web. Written by Chris C. Rundle.
1. Introduction
Ion-selective electrodes have been used for analytical measurements for over 30 years. In that time they have grown from specialist laboratory instruments into robust, widely deployed tools found in water treatment plants, agricultural laboratories, hospitals, food factories, and university teaching laboratories around the world.
The original version of this guide was written by Chris C. Rundle and was selected as one of the best educational resources on the web by StudySphere.com — an award it continues to hold today.
This guide fills the gap between oversimplified product pamphlets and advanced electrochemistry textbooks. It provides sufficient information in relatively simple language to enable the non-specialist to understand what ISE measurements involve, and to achieve the best possible results with whatever equipment they are using. No prior knowledge of electrochemistry is assumed beyond a basic understanding of ions in solution.
The guide covers the theory of how ISEs work, the important distinction between ionic activity and concentration, calibration methods and their practical limitations, step-by-step measurement procedures, and the full range of analytical methods available — from simple direct calibration through to potentiometric titration.
2. Applications of ISEs
Ion-selective electrodes are used across a remarkably wide range of industries and scientific disciplines wherever rapid, inexpensive ion measurement is required.
Pollution Monitoring
CN, F, S, Cl, NO₃Effluents and natural waters — routine surveillance and environmental compliance.
Agriculture
NO₃, Cl, NH₄, K, CaSoils and fertilisers — nutrient management and soil health assessment.
Food Processing
NO₃, NO₂, F, Ca, KNitrate/nitrite in meat products; salt in fish and dairy; fluoride in drinks; potassium in wine.
Detergent Manufacture
Ca, Ba, FMonitoring ions that affect water quality and product formulation.
Biomedical Laboratories
Ca, K, ClBlood, plasma, serum, and sweat analysis; fluoride in dental studies.
Education & Research
Wide rangeClassroom demonstrations, method development, and fundamental electrochemical research.
Key Advantages of ISEs
- 1Relatively inexpensive and simple to use with wide concentration range.
- 2Robust all-solid-state or gel-filled models ideal for field or laboratory use.
- 3Can be used rapidly in favourable conditions — even dangled in rivers for continuous monitoring.
- 4Useful when only an order-of-magnitude concentration estimate is required.
- 5Invaluable for continuous monitoring of concentration changes over time.
- 6Particularly useful in biological and medical applications because they measure ion activity directly.
- 7With careful technique, accuracy of ±2–3% is achievable for some ions.
- 8Measure both positively and negatively charged ions.
- 9Unaffected by sample colour or turbidity — no pre-clarification required.
- 10Operate over a wide temperature range: crystal membranes 0–80°C, plastic membranes 0–50°C.
3. How ISEs Work
The simplest and best-known ion-selective electrode is the pH electrode, and it makes the ideal starting point for understanding ISEs in general. Its glass membrane is selectively permeable to hydrogen ions (H⁺). When the electrode is immersed in a solution, hydrogen ions diffuse through the membrane until an equilibrium is reached. This migration of charge builds an electrical potential across the membrane that is proportional to the logarithm of the hydrogen ion concentration — which is the definition of pH.
A high-impedance millivolt meter measures the potential difference between the ISE and a separate reference electrode whose potential is stable and known. The difference gives the electrode potential, which can then be related to ion concentration via a calibration.
All other ISEs operate by the same fundamental principle. The only difference is the membrane material, which is chosen to respond selectively to a different ion — fluoride, nitrate, calcium, potassium, and so on.
The key phrase is selectively: no ISE membrane responds exclusively to one ion. The differences between pH measurement and other ISE measurements arise from this, and from practical factors described below.
Important differences from pH measurement
4. The Nernst Equation
The quantitative relationship between electrode potential and ion activity is given by the Nernst equation. Understanding it is essential for correct calibration and for diagnosing electrode problems.
E = E⁰ + (2.303RT / nF) × log(A)
| Symbol | Meaning |
|---|---|
| E | Total potential (mV) between the sensing and reference electrodes. |
| E⁰ | Constant characteristic of the ISE and reference electrode pair. |
| R | Gas Constant — 8.314 J / degree / mol. |
| T | Absolute Temperature in Kelvin. |
| n | Charge on the ion (with sign — positive for cations, negative for anions). |
| F | Faraday Constant — 96,500 coulombs. |
| log(A) | Logarithm (base 10) of the activity of the measured ion. |
Practical implications
5. ISE Membranes & Reference Electrodes
Types of ISE Membrane
Highly durable and mechanically robust. Operate from 0–80°C. Generally the most reliable membrane type.
The most common type for cations and many anions. Operate from 0–50°C. Simple to manufacture and replace.
High selectivity for H⁺ and Na⁺. Require high-impedance measurement electronics.
Reference Electrodes
Every ISE measurement requires a reference electrode to complete the electrochemical cell. The reference electrode must maintain a stable, known potential that is unaffected by the composition of the sample.
Single-junction references are suitable for most routine measurements. They consist of a single porous frit through which the inner reference filling solution makes contact with the sample.
Double-junction references are essential when the inner filling solution (typically KCl or Ag/AgCl) would react with sample components. This includes samples containing proteins, sulfides, or silver ions. The outer chamber provides a buffer between the inner solution and the sample.
The outer filling solution of a double-junction reference should ideally be “equi-transferrent” — meaning both the cation and anion have similar mobilities, so neither diffuses faster than the other at the junction. This minimises the liquid junction potential, a source of systematic error. Lithium acetate and potassium nitrate are both commonly used for this purpose.
ELIT Reference Electrodes
| Model | Type | Notes |
|---|---|---|
| ELIT 001 | Single junction (Ag/AgCl) | Suitable for most general-purpose applications. |
| ELIT 002 | Double junction, potassium nitrate outer filling | Use when inner filling solution would react with sample. |
| ELIT 003n | Double junction, lithium acetate outer filling | Recommended for most ISE work — lithium and acetate have similar mobilities, minimising junction potential. |
6. Activity vs Concentration
ISEs do not measure the total concentration of an ion — they measure its electrochemical activity, which represents the ion’s “effective” concentration once the effects of interactions with other ions in solution are taken into account.
In dilute solutions these are essentially identical. In more concentrated or complex solutions they can differ significantly, because charged ions exert mutual electrical forces that reduce their effective free concentration.
The mathematical relationship is:
a = γ × C
where a = activity, γ = activity coefficient (always ≤ 1), C = true molar concentration
The ionic strength (I) of a solution quantifies the total contribution of all ions to these effects:
I = ½ × Σ(cᵢ × zᵢ²)
cᵢ = molar concentration of each ion, zᵢ = charge number
As ionic strength increases, activity coefficients decrease, and the measured activity falls further below the true concentration. Divalent ions (z = 2) contribute four times more to ionic strength per mole than monovalent ions.
Ionic Strength Adjustment Buffer (ISAB)
ISAB is a concentrated inert electrolyte solution added in equal proportions to all standards and samples. Its purpose is to raise the ionic strength of every solution to the same high level — dominated by the ISAB — so that variations in the sample’s own ionic strength become negligible.
When ionic strength is effectively fixed at a constant high value, the activity coefficient γ is constant for all solutions. The calibration therefore describes the relationship between concentration and millivolts, not activity. This is what most laboratories need.
A secondary benefit is that ISAB also helps stabilise the liquid junction potential of the reference electrode, reducing a significant source of systematic error.
| Situation | Recommendation |
|---|---|
| Sample IS < 0.01 M (monovalent) | ISAB optional |
| Sample IS 0.01–0.1 M | ISAB recommended |
| Sample IS > 0.1 M | ISAB may be ineffective — consider dilution or addition methods |
| Biological/clinical activity required | Do NOT add ISAB |
7. Calibration Theory
Calibration involves measuring the electrode potential (mV) in a series of standards of known concentration and plotting these values against log(concentration). In the linear range the result is a straight line — the calibration graph — whose slope and intercept define the electrode’s response.
The theoretical slope is approximately 54 mV/decade for monovalent ions (positive for cations, negative for anions) and approximately 27 mV/decade for divalent ions. In practice, good electrodes typically achieve 95–100% of the theoretical slope.
Linear range and detection limit
The linear range is the concentration range over which the calibration data follows the straight-line regression to within ±2 mV. For many electrodes this extends from around 0.1 M down to 10⁻⁶ or 10⁻⁷ M — a dynamic range of five to six orders of magnitude.
The limit of detection (IUPAC definition) is the concentration at which the electrode potential deviates from the linear prediction by more than 18 mV. Below this point the electrode becomes progressively unresponsive to concentration change. Measurements below the detection limit are unreliable.
A complete calibration should include at least two standards within the linear range and two standards near or below the detection limit. This ensures the shape of the curve is well characterised and the detection limit can be identified accurately.
8. Calibration Practice
A typical calibration series uses standards at 1000, 100, 10, 1, and 0.1 ppm, prepared by serial dilution from a single stock solution. If the approximate sample concentration is already known, bracket it closely with standards — using standards that span the expected range more tightly reduces interpolation errors.
Large errors result from extrapolating beyond the calibration range. Standards and samples should always be within ±2°C of each other, because the Nernst slope is temperature-dependent (see the Nernst equation above).
Minimising drift
Measure standards in ascending concentration order (e.g. 1→5), then repeat in descending order (5→1) and average the two sets. This technique cancels much of the systematic drift that occurs when an electrode equilibrates in a new solution.
One-point recalibration
Once the slope has been established for a particular electrode and measurement session, a single standard close to the expected sample concentration can be used to renormalise the calibration axis without re-running the full curve. This is faster and reduces reagent consumption for large batches.
Calibration frequency
Ideally calibrate before each batch of samples. In practice, one calibration per session of several hours is acceptable if the electrode is stable. Always repeat calibration if electrodes have been stored dry, after cleaning, or if a systematic offset is suspected.
9. Making Measurements
Add ISAB equally to all standards and samples — for example, 2 ml of ISAB to 100 ml of solution. The proportion is approximate; what matters is that it is consistent. If ISAB volumes are not identical for all standards and samples, systematic errors will result.
Stirring and timing
Three approaches are in common use:
- 1. Continuous stirringat 50–100 rpm using a magnetic stirrer (no heat). Gives fastest response but requires care to avoid air bubbles at the membrane.
- 2. Still solutionImmerse the electrode and wait for equilibration. Slower but avoids stirring artefacts.
- 3. Swirl and standThe most commonly used method in practice. Swirl briskly to ensure good electrode contact, then set down and allow to stand still while reading. Takes the reading after a fixed time (typically 1–2 minutes, electrode-specific) or when the reading is stable.
Whichever method is chosen, use it consistently for all standards and samples in a session. Inconsistent technique is one of the most common sources of poor precision.
Cleaning between samples
Rinse the electrode with a jet of deionised water and dab dry with a lint-free tissue — do not rub or wipe, as this can damage the membrane surface and generate electrostatic charge.
For the highest precision work, soak the electrode in deionised water for 20–30 seconds between each measurement. This ensures that each measurement is approached from the same direction (low concentration) rather than from the residual concentration of the previous sample, which reduces hysteresis errors.
10. Methods of Analysis
Four analytical methods are available with ISEs. The best choice depends on sample matrix complexity, the number of samples, required accuracy, and electrode condition.
Immerse the electrode in the unknown sample, read the potential, and determine the concentration from the calibration graph. Best suited to large batches of similar samples over a wide concentration range. Simple and fast. Susceptible to ionic strength differences and junction potential variations between standards and samples — mitigated by ISAB.
Measure a large known volume of sample, then add a small volume of concentrated standard and measure the change in potential. Calculate the original concentration mathematically from the known volumes and potential change. Key advantages: the electrode remains immersed throughout (stable junction potential), calibration and measurement occur in the same solution (eliminating matrix mismatch), and the method works reliably even with an old electrode whose slope is below theoretical.
The reverse of standard addition: add a small volume of unknown sample to a large volume of standard. Useful when the sample volume is limited or when the unknown is too concentrated to measure directly. The electrode and standard solution remain undisturbed, minimising junction potential drift.
Add the sample to a reactive standard that consumes the analyte — the endpoint is detected potentiometrically. This extends ISE analysis to ions for which no direct ISE exists: sulphate, for example, can be measured via a barium ISE after precipitation as BaSO₄. Potentiometric titration determines endpoints with high precision from volumetric measurements, often giving better accuracy than direct methods.
11. Meters & Software
The device used to measure and process the electrode signal has evolved considerably over the decades that ISEs have been in use.
Software functions (ELIT Ion Analyser)
Continue Learning
Deep dive into ionic activity, activity coefficients, and ISAB use.
Direct calibration, standard addition, and sample addition explained.
Selectivity coefficients and strategies for minimising ionic interference.
Detailed protocols and specifications for all 15 measurable ions.