ISE Selectivity and Interferences

As indicated in their names Ion Selective Electrodes (ISE) are selective and as such can suffer interference from the presence of some ions in the sample solution. Though it is theoretically possible to correct for these interferences it is better to be aware of the magnitude of the problem them and then take sensible measures to reduce their effect. This can be by the addition of suitable Ionic Strength Adjustment Buffers (ISAB’s), the use of an incremental technique or dosing standards with the interferent to correct the error in the standards. The below table allows you to identify any issues before they arise.

Image showing a process electrode.
Image showing a sodium flow cell.

Ion Selective Electrodes. Interference and Selectivity

No ISE is perfectly selective. Even the pH electrode suffers interference from Sodium in high pH solutions. Below is a table of selectivity coefficients
which will help you decide whether or not your analysis is being comprimised by the presence of other ions that you are not wishing to measure.

The Selectivity Coefficient quantifies the is the apparent increase in the measured concentration caused by 1 unit of the interferent.
Multiply the SC by 100 to give the % error when analyte and interferent are the same concentration.

Ammonium:   Potassium (0.1), Sodium (0.002), Magnesium (0.0002), Calcium (0.00006), Lithium (0.00003).
Bromide:         Chloride ( 0.002), Hydroxyl (0.00003). Silver, Cyanide, Sulphide and Iodide must be absent.
Cadmium:       Iron (10)or Lead (10) these should be less than 1 /100th of the Cadmium conc. Silver, Sulphide, copper and Mercury must be absent.
Calcium:          Iron Fe+2 (0.02) Strontium (0.008), Barium (0.005) Copper (0.002), Sodium, (0.0005), Mg (0.0006), K (0.00005), NH4 (0.00003),
                          Lithium (0.00001), Al +3 (5) Only tolerated in low concentrations.
Chloride:         Iodide, Bromide, Cyanide, Sulphide, Silver must be absent, or only present in insignificant amounts compared to the Chloride ion.
Copper:           Bromide (>1), Chloride (>1). Silver, Sulphide and Mercury must be absent.
Cyanide:          Silver, Sulphide and Iodide should not be present
Fluoride:          Hydroxyl (0.1)
Iodide:             Bromide (0.0004), Chloride (0.000001). Silver Sulphide and Iodide should be absent.
Lead:               Cadmium (>1) . Silver Sulphide, Copper, Iron II, Iron III and Mercury Should be absent.
Mercury:         Silver and Sulphide must be absent
Nitrate:           Chloride (0.006) Bicarbonate (0.005), Nitrite (0.001), Acetate (0.0005), Fluoride (0.0001), Sulphate (0.00001).
Nitrite:            Acetate (0.001), Fluoride (0.0008), Chloride (0.00005), Nitrate (0.00001), Sulphate (0.00001). Cyanide must be absent
Perchlorate:   Thiocyanate (0.03), Iodide (0.02), Nitrate (0.02), Chloride (0.0003), Phosphate (0.0002), Acetate (0.0001).
Potassium:     Rubidium (2), Caesium (0.4), Ammonium (0.01), Sodium (0.0004), Calcium (0.0003), Magnesium (0.0003), Lithium (0.0001).
Silver:              Sulphide and Mercury should be absent or in very low concentrations relative to Silver
Sodium PVC:  Potassium( 0.6), Ammonium (0.2), Calcium (0.02), Magnesium (0.03).
Sulphide:        Silver and mercury should be absent
Thiocyanate:  Silver, Sulphide, Chloride and Iodide must be absent. Small interference from Bromide and Thiosulphate.