Ion-Selective Electrode

How to store Ion Selective Electrodes correctly?

All user manuals come with the necessary information about the short and long term storage of the respective sensor. Generally Ion Selective Electrodes should be stored dry for long term storage.

What response time is reasonable for an ion selective electrode?

The response time depends on the type of ion as well as its concentration.

Whereas the relatively fast sensors for Na⁺ and F^- feature response times of 1-3 minutes at higher concentrations, the response time increases to 5-10 minutes when measuring in the range of the lower detection limits.

The size of an ion has an important impact on the speed of a measurement. That is why most of the other ions take longer to be measured. Analysis of heavy metals such as Cu²⁺ or Cd²⁺ can even take 20-30 minutes when performed near the lower detection limit.

A gentle and constant stirring in ion measurement is crucial for achieving a stable signal as fast as possible. Stabilization is reached faster, when moving from a lower concentration to a higher one than going the inverse way. Therefore, it is recommended to always start with the lowest concentration when calibrating and measuring.

What is the detection limit of an ion selective electrode?

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). Normally, it is S-shaped: rather flat at very high and at 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.

What is the average lifetime of sodium ISE?

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