Potentiometry and electroanalysis basics
- 2. ELECTRO ANALYTICAL CHEMISTRY…. • Consists of group of quantitative methods of analysis • It uses the electrical properties of the analyte to determine 1.the concentration of each of the chemical species (unlike other techniques that can only find – the total conc) 2.stochiometry of the sample 3.rate of charge transfer or mass transfer 4.extent of adsorption or chemisorption 5. rates and equi constants
- 3. advantages • Allows to find the conc of each chemical species than as a whole • Most important: it can find the activity of ions unlike others…(activity is more important than the conc) • The cost is low compared to other spectroscopic instruments
- 4. Electrochemical cells • It is a device capable of either generating the electrical energy from the chemical reactions or vice versa • Consists of electrodes, electrolyte, power supply, voltmeter, wattmeter, salt bridge.
- 7. Potentials in electrochemical cell… • The potential of the cell depend upon the activity of the reactants and products and indirectly to their molar concentrations
- 8. what is activity??? • Measure of the effective conc of the species in a mixture • Dimensionless quantity • Depends on temp, pressure, standard state of species and conc
- 9. What is standard state? • It is a reference point used to calculate the properties of the materials under different conditions. • The standard potential is equal to the cell potential when the activity is unity, ie, when a=1, then Ecell= E°cell
- 10. Potentiometry.. • Methods of analysis based on the measurement of potential of electrode system • consists of two electrodes called reference and indicator electrode, potentiometer and a solution of analyte • enable selective detection of ions in presence of other substances.
- 11. Reference electrodes • Highly desirable: to know the half cell potential of one electrode • Thus the electrode whose half cell potential is- constant and completely insensitive to the composition of analyte under study- reference electrode
- 12. Ideal reference electrode.. • Reversible and always obeys nerst eqn • Exhibits a constant potential • Returns to original potential after being subjected to small currents
- 15. Ion selective electrodes.. • The membrane electrodes- allow rapid and selective determination of numerous cations and anions by direct potentiometric measurements • Also known as specific ion electrode • Converts the activity of a specific ion dissolved in the solution into an electric potential that can be measured by voltmeter
- 16. Properties.. Properties: • Minimal solubility. • Electrical conductivity. • Selective reactivity with the analyte. • A membrane or some species contained within the membrane matrix must be capable of selectively binding the analyte ion. • Three types of binding: Ion-exchange Crystallization Complexation
- 17. Types of ion selective membrane electrodes… • Crystalline: 1.single (ex:LaF3 for f-) 2. polycrystalline (ex:Ag2S . for Ag+) • Non crystalline: 1. glass (ex: silicate glasses for Na+ and H+) • 2.liquid: (ex: liq ion exchanger for Ca2+ and K+) • 3.immolized liquid in a rigid polymer ( ex: PVC matrix for Ca2+ and NO3-)
- 19. Molecular selective electrodes…. • These are selective towards certain of molecules • Two types: • 1. for determination of dissolved gasses • 2. determination of organic compounds like glucose and urea
- 20. Gas sensing probes… • An indicating electrode is first placed into a specific solution • On the opposite side there is a permeable membre • When the solution containing Co2 is brought into contact with the porous membrane, it passes through and equilibrium is established
- 21. • in the internal medium another equilibrium is established • This causes change in the pH of the internal medium to change and the glass electrode immersed in the film of internal solution detects this pH change
- 23. Biocatalytic membrane electrodes… • Used to determine the compunds of biological interest • It combines the selectivity of enzyme catalysed reactions with electrochemical transducers
- 24. Mechanism.. • When the sample is brought into contact with an immoblized enzyme, the analyte undergoes a catalytic reaction to yield species like ammonia, Co2, H+ ions,or H2O2 • The conc of this product is directly proportional to the analyte conc that is determined by the transducer
- 25. Instruments… • Direct-Reading Instruments • Commercial Instruments – Utility – General-purpose – Expanded-scale – Research
- 26. Potentiometer • Instrument used to measure the EMF, TPD, internal resistance of a cell. • It consists of a board where a tungsten or manganese wire is fitted on it • Driving cell (E) of some EMF which is always greater than the EMF (E’) to be measured is used to send current through the circuit
- 29. Principle and working.. • Generally when the cells of same emf if connected to the galvanometer shows zero deflection • But if emfs vary then deflection is observed • the potential dropped between two points in a wire of uniform cross section is directly proportional to the distance between the points