ION SELECTIVE POTENTIOMETRY

ByArvind Singh HeerM.Sc.-l Sem-II (ANALYTICAL CHEMISTRY.)Mithibai CollegeION SELECTIVE POTENTIOMETRY

ContentsLiquid-Liquid electrodesEnzyme electrodesGas sensing electrodes

Introduction What is Ion selective potentiometry?Various type of membrane electrodes have been developed, in which the membrane potential is selective towards a given ion or ions. Just as the potential of the glass membrane of a conventional glass electrode is selective towards the hydrogen ions. None of these electrodes is specific for a given ion , but each will possess a certain selectivity towards a given ion or ions so they are properly referred to as ion selective electrodes.

Liquid- liquid membrane electrodes

The basic construction of these electrodes is as shown in figure . Here the potential determining membrane is a layer of water immiscible liquid ion exchanger held in place by an inert porus membrane. The porus membrane allows contact between the test solution and ion exchanger , but minimises mixing.

It is either a synthetic , flexible membrane or a porus glass prit . The internal filling solution contains the ion for which the ion exchanger is specific plus a halide ion for the internal reference electrode .

An example of this is calcium-selective electrode .The ion exchanger is a calcium organophosphorus compound .The sensitivity of the electrode is governed by solubility of the ion exchanger in the test solution it can be used over the pH range of 5.5 to 11 .Above pH 11,calcium hydroxide precipitates

A divalent cation ion exchange electrode that responds to several cation is available its response is nearly equal for calcium and magnesium and it is useful the measurement of . water- hardness .A copper and lead electrode are also available Anion -selective electrode of this type are available for Nitrates,perchlorate, and chlorides.

ApplicationBy taking advantage of the technology of atomic emission spectrum based on liquid electrode discharge, the detecting limit of heavy metals such as copper, mercury, and argent as well as active metals ion including sodium, potass, and magnesium can achieve microg x L(-1).

Enzyme Electrode Ion selective electrodes used in conjunction with immobilized enzymes can serve as the basis of electrodes that are selective for specific enzyme substrates. Enzymes are proteins that catalyses specific reactions to a high a degree of specificity . The reactants are the substrates. Consider the hydrolysis of substrate urea in the presence of enzyme urease NH2CONH2 + 2H2O + H 2NH4 + HCO3A urea electrode can be prepared by immobilizing urease in a and gel coating it on the surface of a cation sensitive type glass electrode ( that respond to monovalent cation ) when the electrode is dipped in the solution containing urea, the urea diffuses into the gel layer and the enzyme catalyses its hydrolysis to form ammonium ions.

These ions diffuse to the surface of the electrode where they are sensed by cation sensitive glass to give a potential reading . After about 30 to 60 second , steady state reading is reached which over a certain working range is a linear function of the logarithm if the urea concentration . By appropriate choice of immobilized enzyme and electrode a number of substrate selective enzyme electrode have been described

ApplicationCoupling of appropriate amperometric electrode processes with enzyme system, e.g urease or aminopeptidases, result in a of simplification operation Coupled enzyme raection of the sequence or competition type are successfully used for extension to new analytes e.g inhibitors, cofactors, or alternative substrate.

Gas - sensing probes

Gas sensing probes are remarkably selective and sensitive devices for determining dissolved gases or ions by pH adjustment Membrane probe design:- The figure is a schematic showing details of a gas sensing thin porus membrane which is easily replaceable.

This membrane separates analyte solution from internal solution containing NaHCO3 and NaCl . A pH sensitive glass electrode having a flat membrane is fixed in position so that a very thin film of the internal solution is sandwiched between it and the gas permeable membrane .A silver chloride electrode is also located in the internal solution . It is the pH of the film and the liquid adjacent to the glass electrode that provides a measure of CO2 content of the analyte solution on the other side of membrane

REFERENCES Insrumental methods of analysis willard

-THANK YOU