Electro analysis…Potentiometry..

By Vishaalini

kamali.R Biotech 2 year

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

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

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.

Potentials in electrochemical cell…

• The potential of the cell depend upon the activity of the reactants and products and indirectly to their molar concentrations

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

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

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.

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

Ideal reference electrode..• Reversible and always obeys nerst

eqn

• Exhibits a constant potential• Returns to original potential after

being subjected to small currents

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

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-exchangeCrystallizationComplexation

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-)

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

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

• 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

Biocatalytic membrane electrodes…

• Used to determine the compunds of biological interest

• It combines the selectivity of enzyme catalysed reactions with electrochemical transducers

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

Instruments…• Direct-Reading Instruments• Commercial Instruments– Utility– General-purpose– Expanded-scale– Research

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

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