Table of Contents

• Ion-selective Electrodes
• Electrode Structure
• Utilisation
• Selectivity and Cross Dependencies
• Maintenance

The iRAS Solid State Sensor Technology

Ion-selective Electrodes

allow the direct determination of the ion activity or ion concentration in a liquid, irrespective of its colour or its turbidity. An ion-selective electrode and a reference electrode are immersed into the solution of interest and the cell voltage is measured.

Silver/silver-chloride and calomel electrodes are preferred as reference electrodes. A dc voltmeter with an input resistance >10 gigaohm can be used as the instrument in all measurements. High-accuracy amplifiers of pH meters with mV readings or IONOMETERs are most suitable.

Ion-selective solid state membrane electrodes are composed of insoluble metal salts with ionic conductivity and make use of the principle of free ion passage through the phase boundary defined by the electrode membrane and the solution of interest, where an electrochemical difference develops.

In the ideal case, the potential at the electrode membrane depends upon a single species of ions. The NERST equation describes the relation between activity and electrochemical potential:



where E₀ denotes a constant depending upon the electrode system and the temperature; R, the gas constant; T, the absolute temperature, z_i, the charge of ion species i; and a_i, the activity of the ion in the solution (ion of interest); the positive sign holds for cations, the negative, for anions.

It follows that the cell voltage changes by 59.12 mV when the activity of a potential-defining monovalent ion changes by the a factor of ten.

When certain assumptions are made, the activity of the ion of interest can be replaced by its concentration.

There exists no „ideal selective" electrode in practice. Since other ion species contribute to the potential in all solution of complicated composition, the activity of interest or the concentration in a solution are determined after specific prior treatments and with the aid of calibration and evaluation procedures.

Electrode Structure

All ion-selective solid-state membrane electrodes are formed by a plastic shaft portion and a sensor module permanently embedded in a special resin compound. The sensor membrane of high mechanical strength is a die-formed part made of insoluble salts of silver, copper or lanthanum with ionic conductivity. Contact to the electrochemical potential is established by silver contacts on the die-formed part proper and by a shielded cable.

The „liquid membrane" electrodes (NO3, Ca) are made from special organic ion exchangers which are dissolved in an organic solvent and inserted in a PVC matrix. A chloride-selective solid-state membrane is used as a permanent contact for the discharge of the equilibrium voltage developing on the sensor membrane. In this way the electrodes are pressure-resistant and do not need any maintenance work.

The miniature embodiments of the ion-selective electrodes are solid-state membrane electrodes of reduced size in low flow resistance. These electrodes are therefore suitable for incorporation in channels of flow and pipelines and are used for continuous control and monitoring.

The shaft is made of plastics.

The protective cover provides protection against dust and mechanical loads and may also help to condition the electrode when a piece of soaked foam plastic is inserted.

A shielded cable is used for connection to the meter. The cable is fastened at the electrode by an embedded compound or connected via a BNC connector; a BNC connector or a socket meeting the customer's requirements is used at the meter.

Utilisation

Ion-selective solid-state membrane electrodes are used preferably for determining anion or cation concentrations in aqueous solutions or in mixtures of organic and aqueous solutions with pH 2-14 for electrodes of type Cu, pH 2-14 for types Ag/S, Cl, Br, I/CN, and pH 5-8 for fluoride.

Electrodes with a „liquid membrane" such as the direct-contact matrix membrane electrodes for determining NO₃ are stable in the pH range 3-10; those for determining Ca have stable operation in the pH range 3-9.

The maximum temperature in the use of standard electrodes must not exceed 50 °C.

„Liquid membrane" electrodes should not get in contact with organic solvents since the PVC matrix membrane will swell. The measurements are also affected by anionic surfactants.

Various factors influence the upper and lower limits of electrode operationfor concentration measurements.

For example, in case of liquid membrane electrodes, the lower limit is determined by the solubility of the membrane’s ion exchanger in the solution to be analysed and by the pH. The upper limit is at 1 mole/L in the case of liquid-membrane electrodes. It is recommended to dilute the solution to be analysed at the upper limit in order to increase the accuracy of measurements.

Information on the chemical behaviour of the solution to be analysed must be available for proper work with ion-selective electrodes. In each measurement, the pH, the total ionic strength, and the presence of interfering and complexing agents in the solution must be taken into account.

Erroneous results are obtained when ions forming complexes or precipitates with the ion to be analysed are present.

It should be noted that some peculiar ions have a detrimental influence upon the electrodes. For example, a high concentration of interfering ions poisons the solid-state membrane. If there is the risk of poisoning a solid-state membrane, the interfering ions must be removed chemically before the actual measurement.

Excessive cyanide concentration must not be present in cyanide concentration measurements since in such a case the life of the I/CN electrode will be greatly reduced.

Selectivity and Cross Dependencies

The following selectivity data hold for the various types of electrodes. The concentration of the ion of interest was normalised to „1"; the data indicated refer to the concentration of ion of interest.

Maintenance

All ion-selective electrodes are formed by a plastic shaft and a sensor module firmly embedded in epoxy resin. In regard to the medium into which they may be immersed, it is noted that certain organic solvents must be excluded.

The compounds used in the sensor material, such as silver sulphide, silver halides, copper selenide, lanthanum fluoride, are stable in organic solvents and in acids and bases of low concentration.

The possible presence of inst important values to determine.

Metallurgy

Ion-selective electrodes for the analyses of silver, copper, fluoride, chloride, and cyanide are used in the metallurgical industry for monitoring, etching, pickling, rinsing, and plating solutions.

Ion selective electrodes are also used in environmental protection, geology, the pharmaceutical industry, and science training activities.