IAC SERIES 1100 SODIUM TECHNICAL INFORMATION

Why Measure Sodium? | Principle of Operation | Simple and Reliable Operation | Description of the Measuring System | Sodium Ion Selective Glass Electrodes

Sodium Ion SELECTIVE GLASS ELECTRODES

Glass Sodium ion selective electrodes are the method of analysis of all Sodium Analyzers. The electrodes response mechanism and theory of operation are not fully understood by most operators.
The most important factor of a Sodium electrode is the requirement for periodic conditioning of the electrode bulb surface. This process provides the reliability of detecting extremely low levels of concentration.

The mechanism of Sodium glass electrodes is complex. Glass electrodes undergo continuous corrosion in water. Contact with water results in a change in structure and composition of the surface glass layer. Alkali metal ions leach into the aqueous solution and are replaced by hydrogen ions; hydrogen bonds form, resulting in a more open yet less conductive structure. The penetration of the layer further into the glass by ions is a continuous process, its rate dependant on temperature and even the composition of the sample solution.

As the ions penetrate deeper into the layer, it is simultaneously dissolving. Since the solubility of the layer is relatively high, it reaches a steady-state thickness.

However, the equilibrium reaction potential of the glass electrode does not depend on the thickness of the layer. This potential is developed at the interface of the layer and sample solution. Therefore, when a step change in concentration occurs, it results in a longer response time as the layer thickens.

In most Sodium analyzers the electrode requires periodic etching of the electrode to increase the rate of response and accuracy. This etching is accomplished by emersing the electrode in a solution at pH 5. The electrode is emersed in the solution for no longer than 60 seconds. The electrode‘s initial layer is removed resulting in a regenerated layer of ion receptors.

However, more advanced Sodium electrodes and analyzers (IAC Model 1101) requires less use of the etchant.  Electrode response is optimized by exposing the Sodium electrode to a specific high concentration of Sodium ions at the beginning of every calibration. This process results in a longer life of the electrode and a greater accuracy due to the fact that the electrode layer is not destroyed as with etching. The speed of reponse and reproducibility of the measurements are also increased.

Glass Sodium electrodes have a remarkable dynamic range. Analyzers based on glass Sodium electrodes routinely measure in the part per trillion range. Since part per trillion standards can not be made or maintained reliably, verifying such results proves to be difficult.

A simple and proven technique to verify low levels is to run a series of known concentrations at part per billion concentrations as grab samples. The properly operating analyzer will reproduce these concentations accurately. Verifications using lab analysis techniques and other methods are always questionable due to the exposure during collection, storage and transport of low concentration samples.