Ion Selective Electrode -It’s a galvanic cell. (No oxidation or reduction reaction take place) – its movement of ions. -- Also known as indicator electrodes. -- Used for direct potentiometric measurement. --- We are concern here with “ION” not a protein. ISE: Ion for +/- ions and selective : for specific ion by membrane used in it and utilize electrode for measuring the potential by voltmeter. ---- Specificity and sensitivity = ISE = separation and detection By Mohsen Al-Saleh (MSc, Clini. Chem. 2013)

Historical approach: In 1889,Nernst Equation for first time proved that an 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+] in (mmol/L) in a solution led to discover conventional pH scale (1 to 14) Combine electrode been design since 1940s - Voltage = It’s a potential difference between two points. An electromotive forces.

Conventional pH electrodes -RE = is at constant potential voltage -Sensing electrode = varies in its potential

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 Transducer = any device that convert physical quantity into electrical signal or vice versa (like heat, pressure, brightness, ….etc)

Ion activity α Electrical potentialαion concentration Reagent Sample That’s why electrode should always be in wet conditions. Ion activity α Electrical potentialαion concentration

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

Major ISE Parts: Compose of basic parts: Reference electrode probe Specific ion probe Voltmeter, display and cable wires Case housing both electrodes, internal buffer Ion selective membrane in the side the probe

ISE diagram: (Combined) -Salt bridge to balance charges on two solution beakers -- Internal Reference electrode is inbuilt and also have reference electrode -Reference electrode does interact with solution by pouring more ions in the solution for compensation however; measuring electrode only interacts with specified ion through membranes -Reference electrode function is to keep constant current in solution -pH electrode is close to outside world

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

ISE Shapes & Sizes:

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

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

Type of ISEs: (Electrodes) Glass electrode: (monovalent cations, thin in size, e.g. pH) - Most common composition is SiO4 , 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 infused 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 -PVC: polyvinyl chloride – the worlds 3rd most poplar plastic materials been used.

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)

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

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)

Importance of ISE: Versatility; (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

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

ISE applications: Analytical chemistry Clinical Chemistry (Clinical diagnostic) Agricultural and research Environmental pollution monitoring & oil samples Food/water process (Industrial) POCT (Clinical diagnostic)

The End: Questions…?