Embed Size (px)
DESCRIPTION
Potentiometry - ppt, Electro Analytical Method
Citation preview
POTENTIOMETRY
byMr. Shaise Jacob
Faculty, Nirmala College of PharmacyMuvattupuzhaKerala, India
• Measuring the potential or emf of a solution
• Using a set of indicator & reference electrode.
• INDICATOR ELECTRODE• Responds to changes in emf or Ph
of the solution• To indicate emf or ph • Example – Glass electrode,
antimony electrode
REFERENCE ELECTRODE
• Which has a standard potential on its own
• And its potential does not change to whichever solution it is dipped.
• E.g. hydrogen electrode, saturated calomel electrode & silver-silver chloride electrode
• Most commonly used is saturated calomel electrode
Nernst equation
• The Nernst Equation allows us to calculate the voltage produced by any electrochemical cell given Eo values for its electrodes and the concentrations of reactants and products.
• The potential (E) of a metal electrode at 25oC immersed into a solution of its own ions is given by
• Eo – Standard potential of the metal• N – valency of ions• C – concentration of ions
HYDROGEN ELECTRODE
• It can be used as indicator as well as reference electrode
• Platinum coil coated with platinum black and has wire contacts through mercury
• The assembly enclosed in a glass covering through which Hydrogen 99.8% is passed at 1 atm pressure
• The standard H2 electrode potential is defined as the potential that is developed between the H2 gas adsorbed on the pt metal and H+ of the solution when the H2 gas at a pressure of 760 mm of Hg is in equilibrium with H+ of unit concentration
• The magnitude of SHE potential is considered to be zero. It is used-
• 1) For the determination of electrode potential of metal electrode system.
• 2) For the determination of pH of the solution.
Limitations • 1) It is rather difficult to regulate the pressure of
the H2 gas to be at exactly 1atm throughout the experiment.
• 2) If the solution contains any oxidizing agent, the H2 electrode cannot be used.
• 3) Excess of H2 bubbling out carries little HCl with it and hence the H+ concentration decreases. In such a system, it is difficult to maintain the concentration of HCl at 1M.
• 4) Platinum foil gets easily poisoned by the impurities present in the gas and HCl. In fact, the attainment of equilibrium is ensured by trial and error.
SATURATED CALOMEL ELECTRODE
• It contains of an inner jacket and outer sleeve
• Inner jacket has wire contact with Hg and plugged with a mixture of calomel Hg2Cl2 &
KCl• Outer sleeve – crystals of KCl & porous
plug of asbestos• Space b/w inner jacket & outer sleeve is
filled with either saturated KCl or 1N KCl or 0.1N KCl
• Application
• The SCE is used in pH measurement, cyclic voltammetry and general aqueous electrochemistry.
• This electrode and the silver/silver chloride reference electrode work in the same way. In both electrodes, the activity of the metal ion is fixed by the solubility of the metal salt.
• The calomel electrode contains mercury, which poses much greater health hazards than the silver metal used in the Ag/AgCl electrode
MERITS OF SCE
• Ease of construction• Stability of potential
Silver- Silver chloride electrode
• The silver/silver chloride reference electrode is a widely used reference electrode because it is simple, inexpensive, very stable and non-toxic.
• it is mainly used with saturated potassium chloride (KCl) electrolyte, but can be used with lower concentrations such as 1 M KCl and even directly in seawater.
• Silver wire coated electrolytically with silver chloride and dipped into KCl
• It has the advantage that it is easy to use• Demerit is that it is difficult to prepare
Mercurous Sulphate Electrode
• Type – Reference electrode
♠ Similar in construction to the calomel electrode but utilizes dilute sulphuric acid saturated with mercurous sulphate.
Use - I t is used in solution, where silver or lead ions are present
INDICATOR ELECTRODES
INDICATOR ELECTRODES
♠ Indicator electrode indicates the potential or Ph of a solution in comparison to a reference electrode of a known potential.
1. Hydrogen Electrode
2. Glass Electrode Most widely used indicator
electrode
• It is selective to change in conc. of hydrogen ions.
• It consists of a glass tube with a thin Ph sensitive glass bulb at its tip.
• Ag- AgCl wire at the centre of the tube
• Lower tip immerses into the 0.1N HCl Filled in the glass bulb.
• Glass Membrane of the bulb is extremely thin & chemically made up of alumino silicate.
• Potential of the glass electrode given by the following equation –
E = K + 0.0592 (Ph1 – pH2) at 25oC
K = Constant for the electrode, depends on -
1. Thickness of glass bulb
2. Composition of solution
Ph1 = pH of solution in bulb
pH2 = pH of test solution
Now, pH1 is constant for given electrode so,
E = K – 0.0592 pH2
ADVANTAGES
• Response is very rapid• Chemically resistant to oxidizing &
reducing agents, dissolved gases, salts etc.
• When Lithia -silica glasses are used, it can be used over the entire Ph range.
Use – for pH measurement
DISADVANTAGES
1. It is extremely fragile
2. Minute abrasions on the surface of the tip, damages the electrode
3. It cannot be used with simple potentiometers, because of the high resistance.
Antimony – Antimony Oxide Electrode
• it consists of a antimony rod dipped into a solution, whose potential or Ph to be determined.
• Antimony oxide is formed on exposure to air
• ADVANTAGES1. It can be used from Ph 3 to Ph 8. it
can be used even up to Ph 122. It is not easily poisoned or
damaged3. Can be used even with viscous
fluids
DISADVANTAGES
1. This electrode cannot be used in presence of dissolved oxygen,
oxidizing agents, complexing agents etc..
ION SELECTIVE ELECTRODE • a) General• i. electrodes based on determination of
cations or anions by the selective adsorption • of these ions to a membrane surface.• ii. Often called Ion Selective Electrodes
(ISE)• iii. Desired properties of ISE’s• ‚ minimal solubility – membrane will not
dissolve in solution during • measurement• – silica, polymers, low solubility inorganic
compounds •• ‚Need some electrical conductivity• ‚Selectively binds ion of interest
ION SELECTIVE ELECTRODE
ION SELECTIVE ELECTRODE
• This is a extension of concept of glass electrode
• Electrodes which is specifically permeable to specific types of ions only & other ions are resisted.
• Now-a-days verities of ion selective electrodes are available
1. SOLID MEMBRANE ISE
solid substance of crystal structure is used to select any ion
Specifically designed liquid is filled in specific plastic material or resin material, - semi permeable membrane, highly selective for particular ions.
LIQUID MEMBRANE ISE
• LIQUID IS FILLED IN PLASTIC OR RESIN MATERIAL , WHICH HAS SEMI PERMIABLE MEMBRANE - SELECTIVE FOR PARTICULAR ION.
• Membrane” usually consists of organic liquid (not soluble in sample) held by porous disk between aqueous reference solution and aqueous sample solution.– ‚ Membrane has ability to selectively
bind ions of interest
Example: Calcium dialkyl phosphate Liquid membrane electrodes
ENZYME ELECTRODE
• Enzyme is chemically bonded to some inner surface
• They are very selective for reaction with substrate
• Thus, this electrode is useful for measurement of insulin or adrenaline etc.
QUINHYDRONE ELECTRODE
• This electrode is used as a substitute of hydrogen electrode to overcome its disadvantages.
• Bright platinum wire dipped into the test solution which has been saturated with quinhydrone.
PLATINUM ELECTRODE
• MOST SIMPLE ELECTRODE• It has a platinum plate or wire or ring• Platinum is the most inert & non
reactive metal – which is a good character for stable electrode
• USE –
It is used in all redox titrations
pH METERS
• Same as potentiometers• pH meters consists of two
electrodes
1. Saturated Calomel electrode
2. Glass electrode
☼ USE – they are used to measure pH.
* here instead of platinum electrode, glass electrode is used
• Bze for measurement of Ph, we want to measure selectively only H+
Ion conc. & glass electrode is H ion electrode.
FEATURES / ADVANTAGES OF pH METERS
Due to the following features
1. temperature control knob
2. Calibration knob
3. Temperature display
4. Internal calibration, without external calibration using buffers
POTENTIOMETRIC TITRATIONS• End point of titrations can be
determined by measuring changes in potential of a solution caused by addition of titrant.
ADVANTAGES1. Colored solutions, dil. Solutions or turbid
suspensions can be titrated.2. Titration can be automated3. Mixture of components can be titrated4. Inexpensive & more accuracy5. Reference electrode potential need not
be known - constant
APPARATUS
Method of detecting end point
• Indicator method not suitable• Potentiometer – determining end
point graphically by using –
1. Normal titration curve
Emf vs vol. of titrant
2. First derivative curve
• A plot of ▲E/▲V VS Vol. of titrant
Second derivative curve
• A plot of ▲2E/▲2V VS Vol. of titrant
• At the end point , the rate change of potential is maximum.
APPLICATIONS
Following types of titrations can be done by potentiometry.
1. Acid base titrations
2. Redox titrations
3. Diazotisation titrations
4. Precipitation titrations
5. Complexometric titrations
Dead stop end point technique ( Biamperometry)
E.g. – determination of water ( moisture content) by KARL FISCHER REAGENT
It contains two platinum electrodes
b/w which Small emf is applied
No current flows till the solution is free from polarizing substances
Current flows only when both electrodes are depolarized
