Ion-Selective Electrodes

The main advantages of Ion Selective Electrodes are:

  • Inexpensive and simple to use, in the field as well as the laboratory.
  • Wide concentration range – typically from less than one to several thousand parts per million.
  • Unaffected by sample colour or turbidity.
  • Ideal for monitoring environmental pollution or water quality etc. – where the operator simply wants to be sure that a certain ion is below a particular threshold value, or where only the order of magnitude may be required.
  • Ideal for long term monitoring of changes in ion concentration (e.g. for measuring rates of reaction or nutrient uptake etc.) – particularly when used in conjunction with an ELIT Ion Analyser (electrode-computer Interface), to link electrodes directly to a desk-top or lap-top PC.

Note: ISEs are not generally used as precise analytical tools. Nevertheless, routine, rapid measurements can usually be expected to have an accuracy of around � 10% and with careful analysis under ideal conditions some can achieve an accuracy and precision of better than � 2% (1-sigma) – see chapter on Accuracy and Precision in our Beginners Guide to ISE measurements.

Ion-Selective Electrodes measure the activity (or effective concentration) of ions in a solution, rather than the actual concentration; the activity is always less than the concentration because of
inter-ionic interactions and the difference between concentration and activity increases as the
Ionic Strength increases. (Note that the Ionic Strength is a measure of the total effect of all the ions in the solution, both positive and negative, of the measured ion as well as any other contaminants.) In practice, however, the measured activity is not significantly different from the concentration in dilute solutions (i.e. with total Ionic Strength of less than about 0.01 Molar for monovalent ions and 0.001 Molar for divalent ions), and steps can be taken to minimize the difference at higher concentrations (e.g. by adding Ionic Strength Adjustment Buffer, diluting the sample, or, most effectively, by using Standard Addition or Sample Addition techniques).

Other Essential (included) Components of an ELIT Ion Analyser:

Optional Extras:

  • Electrode stand with articulated arm – or a laboratory clamp stand may suffice.
  • Magnetic stirrer.

ELIT ion selective electrodes

These electrodes have a solid state membrane and a solid internal contact – i.e. they do not contain any liquid or gel electrolytes. Their characteristics and advantages are:

All solid-state construction.
Simple gold-plated plug-in connectors.
Robust and Durable.
Easy to handle.
Stable electrochemical potential.
Fast response time.
Function in any position.
No leaking of fluids.
Long shelf life and working life.
Can be stored and transported below 0�C.

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