1. Ion Selective Electrode
-It’s a galvanic cell.
-- Also known as indicator electrodes.
-- Used for direct potentiometric measurement.
By Mohsen Al-Saleh (MSc, Clini. Chem. 2013)

2. Historical approach:
In 1889,Nernst Equation for first time prove that electrical voltage is related to ion concentration 
Nernst equation is then drive for pH or [H+] -> (Home Work; please read)
Difference in voltage of pH electrodes (sensing and reference) are measured
In 1909, Danish chemist work on “pH meter” with principle of potentiometric
[H+] (mmol/L) in a solution led to discover conventional pH scale (1 to 14)
Combine electrode been design since 1940s

3. Definition:
Its transducer or sensor that measures activity of specific ion dissolve in a solution into an electrical potential formed across “the membrane”
This potential can be measured by either voltmeter or pH meter
Voltage (mV) α Log (ionic activity) ; as per “Nernst Equation”
Voltage depends on: (species used, concentration, temperature and pressure)
-There is always two electrodes; reference electrode and analytes (sensing) specific electrode to measure potential
-electrode should always be in wet condition

4. Ionic activity: ISE measures ion activity instead of concentration
Thus, ion activity depends on ionic strength
If ionic strength is fixed to a high value then concentration is directly proportional to ionic activity
Using calibration curve the concentration can be determined from a measured activity

5. Major ISE Parts: ISEs can take many types & shapes
Compose of basic parts:
Reference electrode probe
ISE for specific ion probe
Voltmeter, display and cable wires
Case housing both electrodes

6. ISE diagram: (Combined)
-Salt bridge to balance charges on two solution beakers
-- Internal Reference electrode is inbuilt and also have reference electrode -

7. ISE diagram:
The sensing part of an electrode is usually made of ion specific membrane
Basic example is: pH electrode
Internal (filling) solution
Ion-selective membrane
Internal reference electrode

8. ISE Shapes & Sizes:

9. How it works (Galvanic cell):
ISE works on the basic principle of galvanic cell
More active metals loses its electrons easily
Two electrodes develop a voltage (potential) due to ion exchange occurring between the sample and inorganic membrane
Ecell = EISE – Eref (Click me)
-More active metals loses its electrons easily
-Voltage; Potential between two electrodes is measured
-Current flow from reference electrode to sensing electrode.
Based on static (zero-current) measurements
-Internal solution (solution inside electrode) contains ion of
interest with constant activity
-- Ion of interest is also mixed with membrane

10. Type of ISEs: (analytes)
Ions that can be measured using ISE:
(H+ or pH, NH3,NH4+, Cd+2,Ca2+,CO2, Cl-,Cu+2,CN-,F-,I- ,Pb+2,K+,Ag+, Na+ and ….etc.)
Selectivity: depends greatly on type of selected membranes

11. Type of ISEs: (Electrodes)
Glass electrode: (monovalent cations, thin in size, e.g. pH)
- Most common composition is SiO2, Na2O, and CaO
- dilute HCl solution & inbuilt reference electrode (Ag wire coated with AgCl)
Liquid electrode: (polyvalent cations and some anions)
- Employs water-immiscible substances impregnated in PVC
- The inner solution is a saturated solution of the target ion and inner wire is AgCl
- For sensing organic and inorganic anions like phosphate, carbonate, thiocynate and salicylate
Solid electrode: (that are selective primarily to anions)
- Made of crystals
- Used for Fluoride, pH, Chloride, Thiocynate and cyanide
Examples of Carriers in membranes:
- Monensin for sodium
- Macrocyclic thioethers for Hg and Ag
- Valinomycin for potassium ions
- Calixarene derivatives for lead
- 14-crown-4-ether for lithium

12. Types of ISE membranes:
Glass membranes for pH measurement
Crystalline membranes
Ion exchange resin membranes (Liquid)
Enzyme electrodes
Based on above “membranes” several types of electrode available in the market
1-Glass membrane electrode
2-Solid state electrode
3-Liquid membrane electrode
4-Gas sensing electrode
- Made from a permselective ion-conducting membrane
(ion-exchange material that allows ions of one electrical
sign to pass through)
Membrane is nonporous and water insoluble
Membrane is made of poly(vinyl chloride) (PVC)

13. ISE membranes: Examples of Carriers in membranes: Monensin for sodium
Macrocyclic thioethers for Hg and Ag
Valinomycin for potassium ions
Calixarene derivatives for lead
14-crown-4-ether for lithium

14. Maintenance of ISE: Keep it wet (See this video)
Rinse with distilled water
Polish the silver electrode
Calibrate ISE
Refill always with fresh reference solution
Get rid of bubbles in ISE (See this video)

15. Importance of ISE:
Versatility; (endpoint indicators, direct measurement)
Cost effective
Time (Very fast like glass electrode)
Selectivity; ISE membranes
Non destructive to analytes
Not affected by color or turbidity of sample

16. Importance of ISEs: Exhibit wide response (Sensitivity)
Wide linear range (Stability)

17. ISE applications: Analytical chemistry
Clinical Chemistry (Clinical diagnostic)
Research area
Environmental monitoring & oil samples
Reaction process (Industrial)
POCT (Clinical diagnostic)

18. The End
Questions…