ION SELECTIVE ELECTRODES

InBrief

OUTLINES

• Introduction• Reference Electrodes• Types of Ion selective electrodes• Applications• Potentiometric titrations

IntroductionWhen a metallic rod is dipped in its salt solution some potential is developed on the surface of the metallic rod. This system is called single electrode and the potential developed is called single electrode potential.

The difference in the Single Electrode Potentials of the galvanic cell is called Electromotive force, which is the driving force for the flow of electricity through the external circuit. This happens only when EMF is negative.

• A standard electrode is used and EMF is measured by using a test electrode, whose single electrode potential is calculated.

• Standard Hydrogen electrode or Calomel electrode is used as Reference electrode.

• The ultimate reference electrode is Standard Hydrogen Electrode.

Standard Hydrogen Electrode

Calomel Electrode

The combination of the Reference electrode and the indicator electrode measures the electro motive force of the cell.

Ion Selective Electrodes

• An electrode that responds to a particular ion’s activity is called ion-selective (or) ion-sensitive electrode (ISE).

• Principle: When ever two solutions of different concentrations are separated by a membrane, a potential difference is set up across the membrane due to the unequal distribution of ions in the solutions. This potential difference is known as membrane potentials. This membrane potential difference will be measured by ion-selectometers with the help of ion selective electrodes.

Types of Ion Selective Electrodes

There are six types of ISE’s based on the membranes used in the electrode: 1. Glass membrane electrode2. Liquid membrane electrode3. Solid membrane electrode4. Gas – sensing electrode5. Enzyme – based electrode 6. Bio catalytic membrane electrode

Glass Membrane Electrode

• The electrode has a thin glass membrane which is selective to various univalent cations, by suitable changes in he glass composition.

• The best and the oldest glass membrane electrode is Glass Electrode which is responsive towards the activity of hydrogen ions.

Glass Electrode (Combined)

• The nature of the glass used for construction of the glass electrode is very important. If a lime soda glass of approximate composition of SiO2 72%, Na2O 22% and CaO 6% used for manufacturing a glass electrode, such electrodes are satisfactorily work over the pH range 1-9.

• By replacing most of all the sodium content of glass by lithium content, found suitable to measure in the pH range upto 14.

Solid membrane Electrode

• In solid membrane electrodes, the glass membrane is replaced by an ionically conducting membrane.

• The LaF3 electrode for fluoride determination is an example for the solid membrane electrode.

Fluoride Electrode:In the fluoride electrode, the active membrane is a single crystal of LaF3 doped with europium(II) (to lower its electrical resistance and facilitate ionic charge transport).The LaF3 crystal, seated into the end of a rigid plastic tube, is in contact with the internal and external solutions. Typically the internal solution is 0.1M each NaF and NaCl; the fluoride ion activity controls the potential of the inner surface of the LaF3 membrane, and the chloride ion activity fixes the potential of the internal Ag/AgCl reference electrode the electro chemical cell for this electrode.

Ag/AgCl(s), Cl-(0.1M), F-(0.1M)/LaF3 crystal/test solution //Reference electrode

E = Constant + (RT/F)

Since [] is contant, E = Constant + 0.0591 pF

The fluoride electrode also responds to hydroxide ion concentration. Hence, the hydroxide ion concentration is kept constant with buffer solutions. It contains of 0.25M acetic acid, 0.75M sodium acetate, 1M sodium chloride and 1M sodium citrate. Sodium citrate masks Al3+ and Fe3+ which interfere by complexing Fluoride. The buffer controls the overall ionic strength as well as the pH. This buffer is also known as Total Ionic Strength Adjusting Buffer (TISAB) solution.

Liquid Membrane Electrode

• Liquid membrane or ion exchange electrodes are prepared using an organic liquid ion-exchanger which is immiscible with water, or with ion sensing material is dissolved in an organic solvent which is immiscible with water.

• The solvent is placed in a tube sealed at the lower end by a thin hydrophobic membrane such as cellulose acetate paper, aqueous solutions will not penetrate this film.

• Example for Liquid membrane Electrode is Calcium Responsive Electrode.

It contains calcium salts of bis (2 ethyl 1 hexyl) phosphoric acid (d2EHP) dissolved in straight chain alcohols (or) di decyl hydrogen phosphate dissolved in di-n-octyl phenyl phosphate.

The neutral undissociated molecules of Ca(d2EHP) diffuse easily in the solvent saturated pores of the membrane, but are insoluble in water.

Gas Sensing Electrode

Dissolved ammonia from the sample diffuses through a gas permeable fluoro carbon membrane until a reversible equilibrium is established between the ammonia level of the sample and internal filling solution. Hydroxide ions are formed in the internal filling solution by the reaction of ammonium water.

NH3 + H2O NH4+ +OH-

The hydroxide level of the internal filling solution measured by the internal sensing element and is directly proportion to the level of ammonia in the sample.

Enzyme Based Electrode

• These electrodes make use of an enzyme to convert the substance to be determined into an ionic product the which can itself be detected by a known ion selective electrode.

• A typical example is the urea electrode in which the enzyme urease is employed to hydrolyse urea and the progress of the

CO(NH2)2 + H2O+H+ 2NH4+ +CO2

• Reaction can be followed by means of glass electrode which is sensitive to ammonium ions.

• The Urease is incorporated in to a polyacrylamide gel which is allowed to set on the bulb of the glass electrode and may be held in position by nylon gauze.

• Then the electrode is inserted in to a solution containing urea, ammonium ions are produced, diffuse through the gel and cause a response by the ammonium ion probe.

Bio-Catalytic Membrane Electrode

In bio catalytic membrane electrodes, a biocatalyst is immobilized at the surface of an electro chemical sensor (membrane electrode) the membrane electrode may be ion-selective (glass, Solid (or) polymer) or gas sensing (NH3, CO2 (or) H2S) electrode. The biocatalysts may be enzyme, tissue, bacteria etc.

Bio-catalytic electrode life times are dependent on stability of the bio-catalyst, which in turn depends on(1) Methods of the biocatalyst immobilization (2) Solution pH, (3) Storage conditions and (4) Presence of activators (or) inhibitors.

Potentiometric Titrations

In potentiometric titrations, the change in the electrode potential upon the addition of the titrant is noted against the volume of titrant added. At the end point the rate of change of potential maximum. (The end point is determined by plotting a curve of potential versus the volume of titrant and recording the inflation point). Volume of the Titrant

EMF

in m

Volts