Support Home > Technology Document  
Q: ISE Info

Basic Theory
Ion-selective electrodes (ISEs) are membrane based electrochemical sensors. The membrane is the component that makes the electrode selective for a particular ion. Four different types of ISEs can be described, depending on the material of the membrane:
• Glass membrane (such as Na+ or pH)
• Solid state membrane (such as Pb2+)
• Polymeric membrane (such as K+)
• Gas permeable membrane (such as CO2)
A potential difference develops across the membrane when the electrode is placed in a solution. To measure the potential difference when the concentration of the ion of interest changes in the sample, an ISE is used in combination with an internal or external reference electrode. Schematically, the complete set up can be described as:

The relationship between the measured potential, E, and the ion activity in the sample, ai, is mathematically described by the Nernst equation:
E = Eo + slope.log ai
The slope term is equal to 2.3.RT/ziF, where
R is the gas constant, 8.314 JK-1 mol-1
T is the absolute temperature, K
F is the Faraday equivalent,
9.6487.104 Cmol-1
zi is the charge of the measured ion

The slope is equal to 59.16 mV/zi at 25 oC. If the measured ion is a monovalent ion such as potassium (zi = +1), a potential change of 59.16 mV would be observed at 25 oC for a ten-fold change in the concentration. Similarly, the potential change for a divalention such as calcium (zi = +2) would be 59.16/2 = 29.58 mV, also at 25 oC.
Eo is a constant potential difference, which is typical of every ISE/reference electrode combination. The term ai is the activity of the ion, which is equal to the concentration only in diluted samples. The activity of the ion depends on the ionic strength of the sample, which is determined by its ionic content.
Deviation from linearity occurs both at low and high concentrations of the measured ion,and determines the lower and higher limits of detection. Accurate determinations can be done for samples with concentrations as low as 10-6 M. At high concentrations (usually higher than 0.1 M) the deviation from linearity is due to the effect of the counter ion.

Calibration and ISA/Buffers
In order to ensure the same ionic strength in standards and samples, it is strongly recommended to use an Ionic Strength Adjustor (ISA), usually in a 50:1 sample to ISA ratio.
Moreover, some ISEs can be used only in a limited pH range. The addition of a buffer to the ISA will adjust standards and samples to the desired pH value. More complex ISA formulations can include additives that complex possible interferences. The ISA can also contain preservatives such as antioxidant agents.
In order to make accurate determinations, it is strongly recommended to use at least two solutions of known concentration (standards) to calibrate the electrode. The standards are usually prepared by dilution with deionized water a 1000 ppm stock solution of the ion to be measured. The concentration of the sample should be included in the range of concentrations covered by the standards. Ideally, the electrode should be calibrated as often as possible, or at least once a week.

A response curve of the ISE can be defined by plotting the potential readings from the two standards (mV std 1 and mV std2), versus the logarithm of the standard concentrations (Std 1 conc and Std 2 conc).
Graphically, the concentration of the ion in the sample (sample conc) can be determined by measuring the sample potential (mV sample) and interpolating that value in the response curve defined by the two standards.
Mathematically, from the Nernst equation:
mV sample = Eo + slope Log (sample conc)
sample conc = 10^[(mV sample)-Eo]/slope
where the Eo and the slope terms can be calculated from the standard concentrations and mV readings.

The response of an ISE can be affected by the presence of other ions in the sample.
The effect of these interfering ions can be quantified for every particular ISE and if the concentration of the interference is known, it can be corrected for. In other cases it is possible to analyze the sample by other methods, such as the Standard Addition method. This method in particular is very accurate when the sample matrix is complex or contains a high level of interfering ions.
A good working ISE response stabilizes within one or two minutes. However the ISE can become sluggish if the membrane becomes dirty from grease or particles in the sample. The tip of a glass electrode can be cleaned with alcohol or a mild detergent, such as the one used to clean lab glassware.
Polishing strips with very fine particles (e.g.3 micron aluminum oxide) can be used to restore the surface of solid state ISEs. A PVC ISE should just be rinsed with water. After thoroughly rinsing with deionized water, the electrodes should be reconditioned by soaking for 2-4 hours in the lowest standard used for calibration. If the slope or the response time of the electrode is out of specification, longer soaking times (e.g.overnight) may be required before a new calibration is attempted with solid state styles.
Regarding storage, glass ISEs can be stored in a low concentration standard. Solid state,
PVC and gas ISEs can be kept in a low concentrated standard between samples, otherwise, these electrodes should be stored dry if not in use for several hours for solid state and several days for PVC and gas ISEs. Please refer to the Instruction Manual for particular recommendations for each ISE.

ISE Quick Reference Information

How do they work or what is an Ion-Selective Electrode?
An Ion Selective Electrode measures the potential of a specific ion in solution. This potential is measured against a stable reference electrode of constant potential. The potential difference between the two electrodes will depend upon the activity of the specific ion in solution. This activity is related to the concentration of that specific ion, therefore allowing the end-user to make an analytical measurement ofthat specific ion.

How Does the mV Reading Correspond to the
Concentration? Standard solutions of known concentrations must be accurately prepared. These solutions are then measured with the pH/mV meter. The mV reading of each solution is noted and a graph of concentration vs. mV reading must be plotted. Now the unknown solution can be measured. The mV value of the unknown solution is then located on the graph and the corresponding solution concentration is determined.

Several types of sensing electrodes are commercially
Available. They are classified by the nature of the membrane material used to construct the electrode. It is this difference in membrane construction that makes an electrode selective for a particular ion.

1. Polymer Membrane Electrodes (Organic Ion Exchangers and Chelating Agents) -- Polymer membrane electrodes consist of various ion-exchange materials incorporated into an inert matrix such as PVC, polyethylene, polyurethane or silicone rubber. After the membrane is formed, it is sealed onto the end of a PVC tube. The potential developed at the membrane surface is related to the concentration of the species of interest. Electrodes of this type include potassium, calcium, fluoroborate, nitrate, perchlorate, and water hardness.

2. Solid State Electrodes -- Solid state electrodes utilize relatively insoluble inorganic salts within a membrane. Solid state electrodes exist in homogeneous or heterogeneous forms. In both types, potentials are developed at the membrane surface due to the ion-exchange process. Examples include silver/sulfide, lead, cupric, cyanide, thiocyanate, chloride and fluoride.

3. Gas Sensing Electrodes -- Gas sensing electrodes are available for the measurement of dissolved gas such as ammonia, carbon dioxide, dissolved oxygen, nitrogen oxide, sulfur dioxide and Free Chlorine. These electrodes have a gas permeable membrane and an internal buffer solution. Due to their construction, gas sensing electrodes do not require an external reference electrode.

4. Glass Membrane Electrodes -- Glass membrane electrodes are formed by doping the silicon dioxide glass matrix with various chemicals. The most common of the glass membrane electrodes is the pH electrode. Glass membrane electrodes are also available for the measurement of sodium ions.

Polymer Membrane Electrodes (Organic Ion Exchangers and Chelating Agents)

Solid State Electrodes

- Back -