Ionic Strength Adjustment Buffers (ISAB’s)

Ion Selective Electrodes measure Activity. That is they can only measure what comes in contact with them. High concentrations of ions in solution effect the mobility of all  other ions in that solution. As the Ionic strength rises the activity of each ion decreases. This means that two samples containing the same amounts of e.g. Fluoride ions with different ionic strengths will give different results. This can be corrected by using ISAB’s. 

Image showing a bottle of standard solution.

Ionic Strength Adjustment Buffers (ISAB)

Ion selective electrodes (ISEs) measure Ion activity in solution. However if you calibrate an ISE in pure stand curve will be non-linear. This is because ions collide with eachother in solution and each collision reduces the activity. Put simply the ions in low ionic strength solutions move faster and therefore the concentration appears higher than those in highly ionic solutions.

Ideally this needs to be corrected for accurate work. The simplest way around this problem is to make sure the ionic strength is similar in all standards and samples. This is achieved by adding a known volume of ISAB to a known volume of each standard and sample.

A typical addition is 2ml of ISAB to 50ml of standard and sample. This is not a set rule and will vary with the application. e.g. the higher the ionic strength of the sample the more ISAB is added.

Note always add the same volume to all standards and samples.

EDT directION manufacture a full range of ISABs for ISEs. These will have a high Ionic strength typically 1-3 moles/litre and be made from a salt that is inert to the ION being measured.

Below is a list of common ISABs that are used for each ISE:

Lithium Acetate Solution:           ISE Species:   Ammonium, Nitrate, Nitrite, Potassium, Sodium

Potassium Chloride Solution:     ISE Species:   Cadmium, Calcium, Cyanide, Lead, Mercury

Sodium Nitrate Solution:             ISE Species:   Bromide, Chloride, Copper, Iodide, Thiocyanate, Silver

TISAB (Total Ionic Strength Adjustment Buffer- includes pH and complex correction):  Fluoride ISE only

SAOB (Sulphide Anti Oxidant Buffer):      Sulphide ISE only

Sodium Hydroxide Solution (to raise pH above pH12):   Ammonia ISE only

A full range of ISABs are available in 500ml bottles from EDT directION.

If in doubt contact EDT directION for specific Method advice at info@edt.co.uk