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).

ELIT ion selective electrodes

� 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.

Nico2000 Ltd can also supply Mini ISEs with a range of tube diameters and lengths for special applications. These electrodes are fitted with a simple wire connector to allow maximum flexibilty for connecting to other equipment and are not suitable for use with the ELIT electrode head system.

These electrodes are of a robust design with 8mm diameter PVC bodies and gel electrolytes. Made to be compatible with ELIT ISEs, they must also be connected to the measuring system via an ELIT Electrode Head.
Leak rates are very low and the electrolytes do not require replacing, therefore these electrodes are manufactured as maintenance-free sealed units which should have an active (or shelf) life of several years - provided that the porous frit is kept moist by immersing in a solution with the same composition as the gel electrolyte when not in use. This is normally done by keeping a few drops of solution in the protective black plastic cap which must always be replaced when the electrode is not in use.

Two types of Reference electrodes are available.
(Click on code number for operating instructions - pdf files).

• Single Junction potassium/silver chloride.
  (code number ELIT 001n ) This has traditionally been the preferred option because the simple design allows fast and reproducible stabilisation - but it is only suitable for Ba, Ca, F, NO₂, Na, ClO₄) due the possibility of interference from the internal electrolyte on other ionophores. However, it must be noted that the introduction of gel electrolytes means that this risk is very much reduced and some users prefer to use this for all ions - particularly for quick measurements or in free-flowing open systems
  
  
• Double Junction lithium acetate ( 003n) - suitable for all Ion Selective Electrodes.
  The double junction Reference was developed to avoid contamination of samples from the concentrated KCl internal electrolyte by introducing a bridging solution which is less concentrated and does not contain any ions which would interfere with the sample ISE. Traditionally several different bridging solutions have been used for different ions but Nico2000 Ltd recommend only Lithium Acetate because neither ion interferes with any ISE and both have virtually the same mobility (equi-transferrent solution) which minimises the liquid junction potential and stabilises almost as quickly as the single junction.

ELIT pH electrodes are basic combination electrodes with glass membranes and AgCl reference systems. They are fitted with cable and BNC connector for connection to any type of measuring system and do not require an ELIT electrode head connector or separate reference electrode.

pH measuring range: 0 - 14. Dimensions: 120mm long by 12mm diameter.

ELIT P11 has a glass body, refillable liquid electrolyte and annular ceramic junction.
Temp Range 0 - 80�C.

ELIT P2011 is a low-maintenance version with an epoxy body, non-refillable gel electrolyte and porous teflon junction.
Temp Range 0 - 60�C.

For more details please see individual Operating Instructions: PH2011

Three types are available. All are Cost effective Combination Electrodes fitted with 1 m Cable and BNC Connector. Dimensions 12 x 120 mm. Temperature Range 0�C to 80�C. Suitable for Laboratory, Field and light Industrial Applications where the electrode is not submersed or under pressure.


1) Type ORP-31C

Platinum-Rod ORP Electrode for General Applications. Has Gel-filled Plastic Body, Protective Skirt, Single Junction Ag/AgCl Gel Reference with Fiber/Pellon liquid junction.


2) Type ORP-32C

For More Demanding Applications. Platinum-Band (wrapped around a glass dome) ORP Electrode with Gel-filled Plastic Body, Protective Skirt, Single Junction Ag/AgCl Gel Reference with Fiber/Pellon liquid junction.


3) Type ORP-33C

For High Precision Applications. Platinum-Band (wrapped around a glass dome) ORP Electrode with Re-fillable Glass Body, Single Junction Ag/AgCl Solution Reference with annular Ceramic liquid junction.

To prevent the electrodes from drying-out they must be stored in an O-Ring sealed bottle filled with Soaking solution.

For more details please see individual Operating Instructions: ORP-31C , ORP-32C , ORP-33C

• Glass body with platinum black electrodes - cell constant K=0.1
• Glass body with platinum black electrodes - cell constant K=1
• Glass body with platinum black electrodes - cell constant K=10
• Plastic body with graphite electrodes (K=1)
• Plastic body with graphite electrodes (K=10)

  For more details please see Conductivity