Ion-Selective Electrode

Ion-selective electrodes come either as combined electrodes or as half cells. In the first ones, the measuring and the reference electrodes are combined in one sensor. A half-cell comprises the ion-selective element only. A suitable reference electrode must be added to achieve a complete sensor system.

The sensing element of the ISE is the ion-selective membrane, which produces different potentials at different ion concentrations. Hence, the potential difference between the ion-selective and the reference electrode varies accordingly and is measured with an ion meter. This potential difference is proportional to the activity of the selected ion in the solution. The activity of an ion is modulated by its concentration and the ionic strength of the sample solution. In daily practice, instead of activity, the ion concentration is evaluated. The usual concentration units are mol/L, mg/L, or ppm.

Technically, we strongly recommend using ISEs for standards and samples in aqueous solutions only.

Direct measurement in solvents (e.g., ethanol or methanol) can alter the main properties of electrodes, such as sensitivity, selectivity, response time, and lifetime. There are several research works that carried out the behavioral study of ISEs in a variety of organic solvents and their mixtures with water and have reported a decrease in the slope and overall performance of the electrode. Scientifically, non-aqueous solvents have an effect on ionic activities, and thus the change of percent volume by organic solvents with water can alter the electrode potentials. A change in the solvent may cause changes in the thermodynamic and kinetic properties of the ions present. Also, the solubility of the ISE membrane, the stability of other metals, the adsorption of specific ions and/or metal ions on the membrane, and any undefined surface reaction may be strongly solvent dependent and hence requires a proper method development as per the sample.

There are alternate ways to measure such samples by ISE. For example, in the case of inorganic fluoride in non-aqueous solvents, it can be measured using a fluoride electrode after extracting fluoride into aqueous solutions or following diffusion, adsorption, or ashing (whichever is applicable).

All user manuals contain the necessary information about the sensor's short- and long-term storage. Generally, Ion Selective Electrodes should be stored dry for long-term storage.

The potential of the sensor is measured at many different concentrations of the ion of interest. One draws a curve of these mV signals against the concentration (logarithmic). Usually, it is S-shaped: relatively flat at very high and very low concentrations, almost linear in-between. The specified detection limits are defined by the range in which the behavior is more or less linear. To make an ISE usable for a different range, one would have to change the membrane surface (larger for lower concentrations) or use a different ion-selective substance in the membrane.

A sodium-selective electrode is a glass electrode very similar to a pH electrode. pH glass electrodes show a negligible alkali error, and amplifying the "alkali error" leads to sodium selective electrodes, which respond only to changes in the sodium ion concentration at pH values above 7. Therefore, a sodium ISE's lifetime is similar to a pH electrode's lifetime (1-to-3 years) and is influenced by several factors (e.g., elevated temperatures, extreme pH values, etc.).

The most relevant part of an ion-selective electrode is the ion-selective membrane. The composition of the membrane depends on the analyte ion. For routine use, there are three different membrane types.

• Crystalline membrane (solid state membrane)
  The potential difference is measured across a solid mono- or polycrystalline membrane. For example, a monocrystalline lanthanum fluoride LaF₃ membrane is used for the fluoride ISE. Crystalline membranes are robust and provide a long lifetime.

• Polymer membrane (liquid membrane)
  The selective compound (ionophore) is embedded in a polymer membrane, usually PVC. Initially, liquid ion exchangers were used. Later, other organic compounds have been found to be more suitable, e.g., antibiotics or crown ethers. Other ingredients, such as plasticizers, enhance the performance of the ISE. Polymer membranes are delicate. Hence, avert mechanical distortions. They are also sensitive to organic solvents due to swelling of the membrane and elution of the ingredients.

• Glass membrane
  The most common glass membrane ISE is the pH electrode measuring H⁺ ions. Another example is the sodium ISE from METTLER TOLEDO with its membrane glass sensitive to Na⁺. A major advantage of glass membrane electrodes is their chemical resistance.

An ISA (Ionic Strength Adjustment) solution provides a high and constant background ionic strength. The respective ISA solution is chosen depending on the measured ion. ISA solutions are added in the same proportion to the sample and standards. E.g., TISAB II or TISAB III solutions are used for fluoride measurements, adjusting the ionic strength, the pH value, and complex interfering ions.

There is no limit specified for the offset in ISE measurement.
In pH measurement, the ideal offset value for pH electrodes is 0 mV at pH 7 because there is no difference in the H⁺ concentration inside and outside the glass membrane at pH 7. This is achieved because the inner solution (not the reference electrolyte, but the solution inside the glass membrane) is at pH 7 buffer solution. The reading of an ISE is 0 mV if the concentration of the ion of interest is equal inside and outside the membrane. Often, we do not know the composition of the inner solution, i.e., not for perfectION™ electrodes. Therefore, the concentration resulting in a 0 mV reading is unknown and hence not the offset value at any given ion concentration. For the same type of ISE, the offset should always be about the same. But if this value is -300 mV or +650 mV is not relevant. As a consequence, an offset limit is not useful.

While using a half-cell sensor, the requirements are:

1. ISE Half-Cell
2. Electrolyte for DX cell
3. Reference electrode
4. Electrolyte the or reference electrode
5. ISA solution
6. Stirrer
7. Temperature probe

While using a combined (PerfectION™) electrode, the requirements are:

1. Combined electrode
2. Refilling solution
3. ISA solution
4. Stirrer
5. Temperature probe