3rd.prof mam abida.pptx

Potentiometery
• It is one of the volumetric technique of electroanalytical
chemistry which is used to measure electrochemical
potential of charged particles.
• An electrode system which is connected to potentiometer
used to measure this potential. The potential measured
depends on the activity or concentration of ions , calculated
using Nernst equation, making quantitative analysis
possible, under the conditions when no current or
minimum current passed through the potentiometer circuit.

• Use of high impedance voltmeter ensures that current flow is
negligible. The voltmeter is attached to two electrodes to
measure potential difference between them.

Commercially available potentiometer

( Potentiometery continued)
Potentiometric methods involves two types of electrochemical
analysis:
1 : Determination of pH
2 : Potentiometric titrations which involves measurements of
changes in EMF of cell brought about by adding a titrant i-e
monitoring of the potential serves only to locate the Equivalence
Point for a titration.

(Potentiometery continued)
Apparatus for potentiometric titrations

Electrodes:
Electrodes used for electrochemical measurements in
potentiometery include Reference and Indicator electrodes.
Reference electrode:
A half-cell with an accurately known electrode potential, Eref,
that is independent of the concentration of the analyte or any
other ions in the solution. Always treated as the left-hand
electrode.
Indicator Electrodes:
It is immersed in a solution of the analyte, develops a potential,
Eind, that depends on the activity of the analyte.
It is selective in its response.

Potentiometery continued--
Reference Electrodes:
Commonly used reference electrodes are mentioned below;
1. Silver-silver electrode
2. Saturated calomel electrode (SCE)
3. Mercury (I) sulfate electrode
4. Standard Hydrogen electrode (SHE)

Potentiometery continued-
Indicator Electrodes:
1. Glass electrode
2. Ion-selective electrodes
a) Solid state/ crystal membrane electrode
b) Coated wire electrode
c) Liquid liquid electrode
d) Plastic membrane ionophore electrodes
e) Potentiometric enzyme based electrodes to measure
substrates & many others.

Salt Bridge:
• It is usually a U-shaped glass tube containing a neutral inert salt in an agar
medium , used to obtain electrical neutrality between the two half cells of
a voltaic cell or electrochemical cell.
• Preventing components from mixing with those of the reference
electrode.
• A potential develops across the liquid junctions at each end of the salt
bridge.
• Potassium chloride is a nearly ideal electrolyte for the salt bridge because
the mobilities of the K+ion and the Cl ion are nearly equal.

Salt bridge continued--
WHY DO WE NEED IT ?
 With the passage of time, positive ions accumulate in anode
compartment and negative ions accumulate in cathode
compartment.
 It leads to stoppage of electric current passing through the external
circuit.
 Electrical neutrality of this increased charges or minimum current is
obtained by using salt bridge.

Working of salt bridges:
• They release negative ions into anode or oxidation compartment , and
positive charges into cathode or reduction compartment.
• These oppositely charged ions neutralize the opposite potential created
• The passage of electricity is maintained through the external circuit

Types of salt bridges:
• AGAR SALT BRIDGES
• SOCKED FILTER PAPERS

Characteristics of a good salt bridge:
• It should not provide a pathway to electrolyte of cell to be diffused
from one half cell into other half cell.
• The salt used should be chemically inert , as it should not form any
precipitate with the electrolyte.

Electrode Potential
 A cell is made up of two parts or two half cells, each containing an
electrode, there exist a difference of potential between electrodes and the
solution in which it is dipping and this potential is known as Electrode
Potential , (E); Figure.1(see calculation on coming slide)

Electrode Potential -
The electrode potential depends upon;
 The nature of electrodes
 Concentration of solution
 Temperature

Electrode Potential continued-
Standard Cell Potential Eo:
 When temperature is 25C or 298 K and the solution is one molar
(which is unit activity for all species) or one atmospheric partial
pressure in case of gases (1 Barr), then this potential is known as
Standard Electrode Potential Eo, the following equation is used:
 EoCell = EoCathode − EoAnode (1)
while,
 EoCell is the standard cell potential (under 1M, 1 Barr and 298 K).
 EoCathode is the standard reduction potential for the reduction half
reaction occurring at the cathode
 EoAnode is the standard reduction potential for the oxidation half
reaction occurring at the anode
 The units of the potentials are typically measured in volts (V).

Limitations:
 Electrode potential has certain limits, which are as fallows;
 Electrode potential will predict wheather a given chemical reaction occur
or not, but they indicate nothing about the rate of reaction and also don’t
assure the success of reaction.
 They are useful to predict that a reaction will not occur if the potential
differences are not sufficient.

• The standard electrode potential of zinc = -0.76V and that of copper is
+0.34V
• The cell potential is the difference between the standard electrode
potential values
= -0.76 - +0.34 = -1.10V
• The zinc half cell has the most negative potential and so the direction of
electron flow would be from the zinc half cell to the copper half cell.
• Reactions occuring in the half cells:
• As the zinc half cell releases electrons then.
Zn------------ Zn2+ + 2e ,(Eo = 0.76 volts (oxidation)
As the copper half cell accept x electrons then.
Cu2+ + 2e----------- Cu ,(Eo = 0.34 volts (reduction)
• Adding these two reactions gives the overall cell reaction as:
Zn + Cu2+---------------- Zn2+ + Cu
STD. ELD. POT.CALCULATION, Exp. 1, Fig.1:

Electrode potential continued-
Standard Electrode Potential (Eo ) = E(red) - E(ox)
= 0.34 – ( - 0.76 ) V
= 1.10 V

Electrode Potential-
Example 2:
Standard Cell Potential Calculation:

Cell Diagram(2nd example)

Standard Cell Potential calculation:
• The example will be using the picture of the Copper and Silver cell diagram.
The oxidation half cell of the redox equation is:
Cu(s) → Cu2+(aq) + 2e- Eo = - 0.340 V
• The reduction half cell of the redox equation is:
( Ag+ + e- → Ag(s) ) x2 Eo = 0.800 V
• Complete redox equation:
• Cu(s) + 2Ag+ + 2e- → Cu2+(aq) + 2Ag(s) + 2e-
• Simplified:
Cu(s) + 2Ag+ → Cu2+(aq) + 2Ag(s)
• Calculations:
• Eo
Cell = Eo(Cathode) - Eo(Anode)
Eo
Cell = 0.800 V - ( - 0.340 V)
Eo
Cel l= 0.800 V + (0.340 V)
Eo
Cell = 1.140 V

Reference electrode:
• Properties of an ideal Reference electrode are as fallows;
• It must have a potential that is accurately known and constant.
• It should be completely insentisive to the composition of analyte solution.
• The electrode must be easy to assemble.
• It should maintain a constant potential while passing a small amount of
current.
• Reference electrodes are necessary to control the potential of a working
electrode or it means to measure the potential of an indicator electrode in
potentiometric measurement.

Calomel reference electrodes:
• A saturated calomel electrode is a reference electrode
consisting of mercury and mercury-chloride molecules. This
electrode can be relatively easier to make and maintain
compared to the standard hydrogen electrode (SHE).
• It consist of a 5cm – 15cm long tube that is 0.5 – 1cm in
diameter.
• It is composed of a solid paste of Hg2Cl2 and liquid elemental
mercury attached to a rod that is immersed in a saturated KCl
solution. It is necessary to have the solution saturated
because this allows for the activity to be fixed by the
potassium chloride and the voltage to be lower and closer to
the SHE. This saturated solution allows for the exchange of
chlorine ions to take place.

Calomel electrode continued
• The saturated calomel electrode (SCE) is the most widely
used because it is easily prepared. The potential is 0.244 V at
25°C.
• The electrode reaction in calomel half-cell (redox reaction):
Hg2Cl2 (s) + 2e- → 2Hg (l)+ 2Cl- (aq)

Calomel electrode continued
• The potential of the calomel electrode depends upon the
concentration of the potassium chloride solution. When
potassium chloride solution is saturated, the electrode is
known as saturated calomel electrode (SCE). All this is usually
placed inside a tube that has a porous salt bridge to allow the
electrons to flow back through and complete the circuit
• Hg|Hg2Cl2 (sat’ d), KCl (XM) ||
• where x represents the molar concentration of potassium
chloride in the solution.Concentrations of potassium chloride
that are commonly used in calomel reference electrodes are
0.1 M, 1 M, and saturated (about 4.6 M).

Calomel electrode continued-
Application:
• The SCE is used for pH measurement, in 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 posses much greater
health hazards than the silver metal used in the Ag/AgCl electrode.

Standard hydrogen electrode
• The SHE is the universal reference for reporting relative half-cell
potentials. It is a type of gas electrode and was widely used in early
studies as a reference electrode, and as an indicator electrode for
the determination of pH values.
• SHE could be used as either an anode or cathode depending
upon the nature of the half-cell.
• It is interesting to note that even though the SHE is the universal
reference standard, it exists only as a theoretical electrode which
scientists use as the definition of an arbitrary reference electrode
with a half-cell potential of 0.00 volts.

Standard hydrogen electrode contd-
• The reason this electrode cannot be manufactured is due to
the fact that no solution can be prepared that yields a
hydrogen ion activity of 1.00M.
• A platinum-black electrode is needed to provide a surface on
which the hydrogen gas can be in contact with the hydrogen
ions(aq)
• Example:

SHE continued-
A positive value for the E value means a favourable reaction is
occurring and therefore the metal is less reactive than
hydrogen.
A negative value for E means an unfavourable reaction and
therefore the metal is more reactive than the standard
hydrogenelectrode.

Silver –Silver chloride Electrode:
It consists of a s silver wire or a silver plated platinum wire
coated along with a thin layer of silver chloride, immersed in a
solution of potassium chloride of known concentration.
Normally this electrode is prepared with saturated potassium
chloride solution and its potential at 25oC is + 0.199 V (vs) SHE
while the potentials with 1.0 M and 0.1 M KCl are +0.237 and +
0.290 V (vs) SHE, respectively.

Silver-silver chloride electrode continued-
• The electrode functions as a redox electrode and the reaction
is between the silver metal (Ag) and its salt — silver
chloride (AgCl, also called silver(I) chloride).
• Commercial reference electrodes consist of a plastic tube
electrode body.
• The electrode is a silver wire that is coated with a thin layer
of silver chloride, either physically by dipping the wire in
molten silver chloride and wire is contained in a Pyrex tube.

Silver silver chloride electrode-
• The Pyrex tube is fitted with 10 mm glass disc or porous
plug of agar gel saturated with KCL on top of the disc to
prevent loss of solution from half-cell.
• The wire is dipped into KCL solution of unknown
concentration which is saturated with silver chloride by
adding 2-3 drops of 0.1M AgNO3.
• The plug can be prepared by heating 4-6 gram of pure agar
in 100 ml of water until solution is completely form and
then add about 35 gram of KCL. A portion of this
suspension is poured into the tube. Upon cooling it solidify
to a gel with low electrical resistance.

Silver-silver chloride continued-

Silver silver chloride continued-
• The electrode functions as a redox electrode as already mentioned.
• The corresponding equation can be written as:
Ag+ + e--------------- Ag(s)
Agcl(s)---------------- Ag+ + Cl-
• The overall reaction can be written as:
Agcl(s) + e ------------- Ag(s) + Cl-
• The potential of this electrode is 0.199v at 250C
• The electrode has many features making is suitable for use in the field:
 Simple construction
 Inexpensive to manufacture
 Stable potential
 Non-toxic components

Silver silver chloride electrode continued
• Biological electrode systems:
Silver chloride electrodes are also used by many applications of biological
electrode systems:
As biomonitoring sensors as part of following:
a) electrocardiography (ECG)
b) electroencephalography (EEG),
c) in transcutaneous electrical nerve stimulation (TENS) to deliver
current.

Mercury (1) sulphate electrode:
 Similar in construction to calomel electrode but utilizes
sulfuric acid (0.05M) saturated with mercury (1) sulphate.
 It is useful for solutions where silver or lead ions are present
and has a potential of 682 mv.

Ion Selective Electrode:
 Ion Selective Electrodes (ISE) are membrane electrodes that
respond selectively to ions in the presence of others.
 Ion Selective Electrodes (including the most common work on
the basic principal of the galvanic cell. By measuring the
electric potential generated across a membrane by "selected"
ions, and comparing it to a reference electrode, a net charge
is determined. The strength of this charge is directly
proportional to the concentration of the selected ion. The
basic formula is given for the galvanic cell:
Ecell = Eise - Eref

Ion selective electrode-
 Ion-selective membranes are currently only available for a
limited number of commonly occuring ionic species.
 CATIONS:
Ammonium (NH4
+), Barium (Ba++), Calcium (Ca++), Cadmium (Cd++), Copper
(Cu++), Lead (Pb++), Mercury (Hg++), Potassium (K+), Sodium (Na+),Silver(Ag+).
 ANIONS:
Bromide (Br-), Chloride (Cl-), Cyanide (CN-), Fluoride (F-), Iodide (I- , Nitrate
(NO3
-), Nitrite (NO2
-), Perchlorate (ClO4
-), Sulphide (S-), Thiocyanate (SCN-).
 Responsive over wide concentration range i.e. 0.1 – 10,000ppm.
 Sample analysis can be complete within 1 –2 minutes.

Measurement considerations:
Agitation -- When carrying out ion selective measurements, it is
important to have good agitation. This allows a fresh supply of ions
to be exposed to the sensing portion of the ISE. It is best to select a
speed that keeps a constant, smooth motion.
pH Adjustment -- In many cases pH control is necessary for
accurate, repeatable measurements.
Response Time -- ISE's require a much longer time for the
readings to stabilize. At least fifteen minutes should be allowed for
equilibrium to be established when measuring standard solutions.
 Establishing a Calibration Curve -- It is
recommended to use three standard solutions reading when
establishing a calibration curve.

ISE continued-
Rinsing -- It is necessary to rinse the ISE between measurements
to insure accurate readings.
Distilled water—
Use a steady stream of deionized or distilled water.
Precaution--
Take care not to rub the electrode with a cloth to dry the probe. It is
usually best to "shake off" any excess water. Take care not to hit the
probe against anything while shaking the electrode.
Conditioning -- The ISE needs to remain moist at all times
even when not in use.

Ion selective electrode-
Types:
 There are three types of ions selective membranes used in ion selective
electrode;
 Glass membrane (It has selectivity for single charge ions H+, Na+, and
Ag+.)
Crystalline membrane (It has selectivity for both cations and anions.)
 Ion exchange resin membrane (It is the most popular one. Its selectivity
changes by changing the composition of resin).
 Ion exchange resin:
It has five types;
 Solid state electrode
 Coated wire electrode
 Liquid-liquid electrode
 Plastic membrane electrode
 Potentiometric electrode (enzymes based electrode to measure the
substrate)

Glass Electrode:
 It is a type of ion-selective electrode made of a doped glass membrane that is
sensitive to a specific ion. It is an important part of the instrumentation for
chemical analysis and physico-chemical studies.
 Almost all commercial glass electrodes respond to single charged ions, like H+,
Na+, Ag+. The most common glass electrode is the pH-electrode.
 Most of the metal cations (e.g. Na+ in the hydrated gel layer diffuse out of the
glass membrane and into the solution (analyte solution).
 Concomitantly, H+ from analyte solution diffuse into the membrane.
 Response of glass electrode (at 25 °C) or potential is given by equation:
 E = Easym + 0.05916log ( AH+ ,outside )
( AH+ , inside )

Glass Electrode continued-
 Glass electrode is mostly used for PH measurements, for pH
measurements only a bulb is needed to be submerged or dipped in the
analyte solution whose pH is to be determined.
 There is internal reference electrode and electrolyte (Ag/ AgCl/ Cl-) for
making electrical contact within the glass membrane and its potential is
constant set by the concentration of HCl.

Glass electrode-
GLASS ELECTRODE

Glass electrode-
Construction:
 The glass membrane, is manufactured by blowing molten glass into a thin-
walled bulb with a wall about 0.1 mm thick. The bulb is then sealed to a
thicker glass or plastic tube, and filled, for example, with a solution of HCl
(0.1mol/dm3). In this solution, immersed a silver/silver chloride electrode
with a lead to the outside through permanent hermetic seal. The filling
solution has constant Cl- concentration, which keeps the Ag/AgCl inner
electrode at fixed potential.
 The pH sensing ability of the glass electrode stems from the ion exchange
property of its glass membrane.
 There are two main glass forming systems.
A : Silicates based on molecular network of silicon dioxide with additions of
other metal oxides as Na, K, Li, Al, B, Ca .
B : Chalcogenide matrix based on molecular network of AsS, As Se.

Glass electrode-
 Glass has a silicate skeleton that forms a thin hydrated layer on contact with
aqueous media. The glass structure is softened in this hydrated layer, i.e.
ions can penetrate this thin layer and alter the electrochemical properties of
the glass.
 This hydration of the glass surface is essential for the use of glass as the
material for pH glass electrodes, as without this hydrated layer no pH
measurement would be possible. The structure of glasses used for pH glass
electrodes (mainly lithium silicate glasses) has been optimized so that, as far
as possible, only protons (H+) can penetrate the glass membrane.
 The hydrogen ions diffuse through the hydrated layers, in the direction of
lesser concentration and in the process replace Na+ ions and other metallic
ions contained in this structure of glass membrane.
 Net result of this diffusion and ion exchange process a plot boundary
potential is set up on each side of the glass membrane (asymmetric
potential).

Glass electrode-
H+ replaces metal cations bound to negatively charged oxygen
atoms. The PH of the internal solution is fixed. As the PH of the
external solution (sample) changes the electric potential
difference across the glass membrane changes.
Fig: Ion exchange equilibria on the inner and outer
surfaces of the glass membrane:

Glass electrode-
Applications:
Glass electrodes are commonly used for pH measurements. There are also
specialized ion sensitive glass electrodes used for determination of
concentration of lithium, sodium, ammonium, and other ions. Glass
electrodes have been utilized in a wide range of applications — for pure
research, control of industrial processes, to analyze foods, cosmetics and
comparison of indicators of the environment and environmental
regulations: a microelectrode measurements of membrane electrical
potential for biological cell, analysis of soil acidity, etc.
Interfering Ions:
Because of the ion-exchange nature of the glass membrane, it is possible for
some other ions of similar size to interact with ion-exchange centers of the
glass, notably the alkali metals.

Glass electrode-
Storage:
 Between measurements or after the electrode should be kept
in the solution of its own ion (Ex. pH glass electrode should be
kept in 0.1 mol/L HCl or 0.1 mol/L H2SO4).
 It is necessary to prevent the glass membrane from drying
out.
 For the glass itself the optimal storage medium would be
distilled water (important).

Glass electrode-
Storage:
If a glass electrode is stored for a long time in a solution containing plenty of
sodium or potassium ions then these will penetrate the glass membrane
and result in an increased response time of the glass membrane as the
protons must first replace the “foreign ions” from the hydrated layer.

Indicator electrode-
Solid state electrode:

Solid state electrode-
 The most typical and successful example is fluoride electrode. It is one of
the most successful and useful electrode for determination of fluoride
ion which is difficult to determine by others.
Construction:
 Membrane consist of single crystal of Lanthanum Fluoride doped with
Chromium II to increase the conductivity of the ion. This electrode has at
least 1000 folds selectivity for fluoride ion over chloride, bromide, iodide,
nitrate, sulfate, mono hydrogen, bicarbonate and phosphate ions.
 A useful solution of minimizing hazards with fluoride electrode consist of
a mixture of an acetate buffer having pH 5 – 5.5, this solution of NaCl
and cycloxylene dinitro acetic acid (CDAA) available in market as Tisah.

Liquid-liquid electrode-
Construction:
 Potential determining membrane is a layer of water immiscible liquid ion
exchanger held in place by an inert/ porous membrane. Porous
membrane allow contact between the test solution and ion exchanger
but minimize mixing. It is either a synthetic, flexible membrane or a
porous glass frit.
 Internal filling solution contains;
1. The ions for which ion exchanger in specific
2. A halide ion for the internal reference
 The filling solution contains usually a chloride salt of a primary ion. The
chloride established the potential of the internal Ag/ AgCl electrode. CaCl2
for Ca+2 electrode and KCl for K+ electrode.
Example:
 The common example is Ca+2 selective electrode, ion exchanger is Ca+2
organophosphorous compounds.

Liquid-Liquid electrode-
Selectivity:
 Selectivity of this electrode is 3000 for Ca+2 over Na+ or F-,
similarly 200 for Mg+2 and 70 over Sr+2.
 It can be used over the pH range of 5.5 – 11.0
 A divalent ion exchange electrode that respond to several cations
is also available.
 It responses equally for Ca+2 & Mg+2 and useful for measuring
water hardness.
 A copper and lead electrode are also available. Anion selective
electrode of this type are available for nitrate, perchlorate and
chloride, same in principles except that a liquid anion exchanger is
used instead of cation exchanger.

Plastic membrane electrode:
 An alternative method instead of using wet liquid membrane electrodes, is
to use a polymeric membrane, which is composed of a polymer such as
polyvinylchloride PVC (33%), a plasticizer (65%), and the ion carrier or
exchanger. The response of these electrodes is highly selective and they
have replaced many liquid membrane electrodes. Polymer electrodes have
been used to determine ions such as K+, Ca2+, Cl-and NO3-.
 Ionophores, or chelating agents, that selectively complex with ions and the
antibiotic valinomycin. The important feature of the neutral carrier molecule
is its cavity which has dimensions approximately that of a molecule or ion.
The valinomycin electrode was one of the first polymer membrane
electrodes and is routinely used to determine potassium.
 The electron-rich center of valinomycin efficiently extracts K+ ions due to the
similarity between the diameter of K+ and the inner diameter of the
valinomycin molecule. The outer lipophilic part of the valinomycin molecule
allows it remain in the polymeric membrane. In the United States alone,
nearly 200 million measurements are made annually of blood potassium
levels using this electrode (see on next slide).
.

Potentiometric Electrodes (enzyme
based electrodes to measure the
substrates)

Pot.electrode-
Construction:
 Urea electrode can be prepared by immobilizing urease in a gel
and coating it on the surface of a cation sensitive type glass
electrode (respond to monovalent cations).
Working:
 When electrode is dipped in a solution containing urea, the urea
diffuse into the gel layer and the enzyme catalyzes its hydrolysis to
form ammonium ions.
 The ammonium ions diffuses to the surface of the electrode
where they are sensed by the cation sensitive glass to give
potential reading.
 In this way urea can be determined by this electrode.

NERNST Equation:
 The equation was given by Walther H. Nernst, he was awarded Nobel
Prize in 1920.
The NERNST equation enables us to determine the;
 Electromotive force EMF of many processes
 Cell potential of a system called Galvanic cell
 Energy of a chemical reaction
 Most of our daily used, practically important electrochemical cells do
not operate under standard conditions.
 STANDARD CONDITIONS refers to a cell which is operated at 25⁰C and
concentrations of electrolytes are Unimolar, i.e 1M.

Nernst equation-
The final form of Nernst Equation is
𝐸 = 𝐸0
−
0.0592
𝑛
𝑙𝑜𝑔𝑄
Where,
E= Electrode potential or emf of cell under non-standard conditions
E⁰= Standard electrode potential or standard cell potential
n= No. of electrons transferred during the cell reaction(may be redox ,
oxidation or reduction reaction)
Q= Reaction Quotient or Position of equilibrium, whereas at any time
𝑄 =
𝑃𝑟𝑜𝑑𝑢𝑐𝑡𝑠
𝑅𝑒𝑎𝑐𝑡𝑎𝑛𝑡𝑠
Cell Potential calculation:

Nernst equation-
Gibbs Energy For Spontaneous Reactions:
“Gibbs energy is defined as the total energy either of a reactant or a
product in a chemical reaction given as ;
𝑮 = 𝑯 + 𝑻∆𝑺
DERIVATION:
 For a Galvanic Cell operating under Non-Standard conditions the
Gibbs Energy Equation would become ,
 ∆𝑮 = ∆𝑮𝟎
+ 𝑹𝑻𝒍𝒏𝑸
 ∆G = Gibbs energy change under non standard conditions
 ∆G⁰ = Gibbs energy change under standard conditions
 Q = Reaction Quotient or Position of reaction
 R = Gas Constant
 T = Absolute Temperature
Derivation of Nernst Equation:

Nernst Equation-
From Faraday’s Laws of Electrolysis ,
∆𝐺 = −𝑛𝐹𝐸 and ∆𝐺⁰ = −𝑛𝐹𝐸⁰
Therefore , Gibbs energy equation becomes
−𝑛𝐹𝐸 = −𝑛𝐹𝐸0
+ 𝑅𝑇𝑙𝑛𝑄
Dividing both sides of equation by – 𝑛𝐹 we get
𝐸 = 𝐸0
−
𝑅𝑇
𝑛𝐹
𝑙𝑛𝑄
But the original equation has log 𝑄 value instead of 𝑛𝑄 , so here
ln 𝑥 = 2.303𝑙𝑜𝑔 𝑥
Therefore ,
𝑬 = 𝑬𝟎
−
𝟐. 𝟑𝟎𝟑𝑹𝑻
𝒏𝑭
𝒍𝒐𝒈𝑸
Above equation is known as the Nernst Equation , It can further be
elaborated as follows.

Nernst Equation-
Nernst equation can be elaborated as follows:
 The value
𝟐.𝟑𝟎𝟑𝑹𝑻
𝑭
is a constant value. And putting the values of all
constants in it , we get
𝟐. 𝟑𝟎𝟑𝑹𝑻
𝑭
= 𝟎. 𝟎𝟓𝟗𝟐
 So , final form of Nernst Equation can be written as
𝑬 = 𝑬𝟎 −
𝟎. 𝟎𝟓𝟗𝟐
𝒏
𝒍𝒐𝒈𝑸
 Above equations are used to calculate the cell potential or the electrode
potential under non standard conditions.

Applications of Nernst Equation:
 (a) Calculation of Cell-Potential under Non-Standard conditions:
 Example No.1:
 Consider a Electrochemical Cell in which following reaction takes place
𝑪𝒓𝟐𝑶𝟕
−𝟐
+ 𝑰−𝟏
↔ 𝑪𝒓𝟑+
+ 𝑰𝟐
 Calculate the cell potential if ,
𝑪𝒓𝟑+
= 𝟏. 𝟎 × 𝟏𝟎−𝟓
−𝟐
= 𝟐. 𝟎 𝑰−𝟏
= 𝟏. 𝟎
𝑯+𝟏 = 𝟏. 𝟎
 SOLUTION:
−𝟐
+ 𝟏𝟒𝑯+𝟏
+ 𝟔𝑰−𝟏
↔ 𝟐𝑪𝒓+𝟑
+ 𝟑𝑰𝟐 + 𝟕𝑯𝟐𝑶
 Now find value of Q for balanced redox reaction , we can write it as
𝑸 =
𝑪𝒓+𝟑
−𝟐 𝑯+𝟏 𝟏𝟒
𝑰−𝟏 𝟔

Application N.E-
Calculation of cell potential:
Putting the values,we get
=
𝟏. 𝟎 × 𝟏𝟎−𝟓
𝟐 𝟏. 𝟎 𝟏𝟒 𝟏 𝟔
= 𝟓. 𝟎 × 𝟏𝟎−𝟏𝟏
Now find out Standard Electrode potential of the cell from individual
half cell reaction’s potentials, as we know from our previous knowledge
that
𝐸𝐶𝑒𝑙𝑙
⁰
= 𝐸𝑐𝑎𝑡 ℎ𝑜𝑑𝑒
⁰
+ 𝐸𝑎𝑛𝑜𝑑𝑒
⁰
= 1.33 − 0.5345
= 0.796

Calculation of cell potential
 Now put the values in Nernst equation , we have
E = E0 −
0.0592
n
logQ
= 0.796 −
0.0592
6
log 5.0 × 10−11
= 0.796 − 0.009867𝑙𝑜𝑔5.0 × 10−11
= 0.796 − −0.0918
= 0.796 + 0.0918
𝐸𝐶𝑒𝑙𝑙=0.888𝑉
 Example 2: Calculation of half-cell potential:
 What is electrode potential of a Zinc Electrode dipped in 0.01M 𝑍𝑛𝑆𝑂4
solution at 25⁰C. Whereas 𝐸𝑐𝑒𝑙𝑙
⁰
= 0.763𝑉.

Calculation of cell Potential
SOLUTION:
 Step-1:
 For evaluation of 𝒏 write the chemical equation and balance it.i.e
𝒁𝒏 ↔ 𝒁𝒏+𝟐 + 𝟐𝒆−
 As there has been a net transfer of two electrons , therefore 𝒏 = 𝟐
 Step-2:
 Now calculate the value of 𝑸, as
𝑸 =
𝒁𝒏+𝟐
𝒁𝒏

Calculation of cell Potential:
 As concentration of solid metal is zero in electrolyte , i.e 𝒁𝒏 = 𝟎,
Therefore
𝑸 = 𝒁𝒏+𝟐 = 𝟎. 𝟎𝟏
 Step-3:
 Standard Electrode potential is given in the question. 𝑬°
= 𝟎. 𝟕𝟔𝟑
 Step-4:
 Now put the values in Nernst Equation, i.e
𝑬 = 𝑬⁰
−
𝟎.𝟎𝟓𝟗𝟐
𝒏
𝒍𝒐𝒈𝑸
𝑬 = 𝟎. 𝟕𝟔𝟑 −
𝟎.𝟎𝟓𝟗𝟐
𝟐
𝒍𝒐𝒈𝟎. 𝟎𝟏
𝑬 = 𝟎. 𝟕𝟔𝟑 − 𝟎. 𝟎𝟐𝟗𝟔𝒍𝒐𝒈𝟎. 𝟎𝟏
𝑬 = 𝟎. 𝟖𝟐𝟐𝟏𝑽

PH:
 Measuring pH: How to Calibrate a pH Meter
 pH is a measure of acidity or basicity. An acid has a pH less than 7, a neutral compound (like water) has
a pH near 7, and a base has a pH from 7-14. pH can be measured using either litmus (or indicator) paper,
which changes color based on the acidity of a solution, or by using a pH meter. A pH meter is a more
accurate means of measuring pH because it can be calibrated to measure one tenth of a pH unit,
whereas the indicator paper only measures to one pH unit.
A pH meter uses an electrode to measure the pH of a solution. The electrode is stored in distilled water in
order to keep it at a neutral pH.

Potentiometric Titrations:
 It involves the measurement of an indicating electrode potential against
a reference electrode and plotting the changes of this potential
difference against volume of titrant used.
 Potentiometric titrations involves the measurements of potential of a
suitable indicator electrode as a function of titrant volume.
 Advantages:
 Titrations that use chemical indicators the potentiometric end points
provides more reliable data.
 These are particularly useful for the titration of colored and turbid
solution.
 Useful for detecting the presence of unsuspected species in a solution.
 Main disadvantage is that, more time consuming than those titrations
using indicators, on the other hand they are readily automated.

Potentiometric titration contd--
Process:
• The process usually involves measuring and recording the cell potential (in
units of millivolts or pH) after each addition of the reagent.
• The titrant is added in large increments at the outset. These volumes are
made smaller as the end point is approach sufficient time must be allowed
for the attainment of equilibrium after each addition of reagent.

Potentiometric titrations cont--
End point determination:
• Several methods can be used for the determination of end point. The most
straight forward method involves a direct plot of potential with respect to
reagent volume.

• As fig the midpoint is the steep rising position of the curve is the
estimated visually and taken as the end point.
• Various graphical methods have been proposed to aid in the
establishment of the midpoint but is doubtful that these procedures
significantly improves its determination (equivalence point).
• Various titrations can be performed by this method;
Acid base titrations
Redox titrations
Precipitation reaction titrations
Complex formation titrations
Differential titrations

Applications:
Assay of Caffine
Assay of phenobarbitone
Assay of thonzylamine hydrochloride
Assay of tetrahydrozoline hydrochloride

Polarography:
Polarography, also called polarographic
analysis, or voltammetry, in analytic chemistry,
an electrochemical method of analyzing solutions
of reducible or oxidizable substances.
• In general, polarography is a technique in which
the electric potential (or voltage) is varied in a
regular manner between two sets of electrodes
(indicator and reference) while the current is
monitored. The shape of a polarogram depends
on the method of analysis selected, the type of
indicator electrode used, and the potential ramp
that is applied.

Polarography:
The solution to be analyzed is placed in a glass cell containing two electrodes. One electrode
consists of a glass capillary tube from which mercury slowly flows in drops, and the other is
commonly a pool of mercury. The cell is connected in series with a galvanometer (for measuring
the flow of current) in an electrical circuit that contains a battery or other source of direct current
and a device for varying the voltage applied to the electrodes from zero up to about two volts

 With the dropping mercury electrode connected (usually) to the negative side of the
polarizing voltage, the voltage is increased by small increments, and the corresponding
current is observed on the galvanometer.
 The current is very small until the applied voltage is increased to a value large enough to
cause the substance being determined to be reduced at the dropping mercury electrode.
The current increases rapidly at first as the applied voltage is increased above this critical
value but gradually attains a limiting value and remains more or less constant as the
voltage is increased further.

 The critical voltage required to cause the rapid increase in current is characteristic of, and
also serves to identify, the substance that is being reduced(qualitative analysis). Under
proper conditions the constant limiting current is governed by the rates of diffusion of the
reducible substance up to the surface of the mercury drops, and its magnitude constitutes
a measure of the concentration of the reducible substance (quantitative analysis).
 Limiting currents also result from the oxidation of certain oxidizable substances when the
dropping electrode is the anode.
Polarography:

Polarography:
 When the solution contains several substances that are reduced or oxidized at
different voltages, the current-voltage curve shows a separate current increase
(polarographic wave) and limiting current for each.
 The method is thus useful in detecting and determining several substances
simultaneously and is applicable to relatively small concentrations—e.g., 10−6 up
to about 0.01 mole per litre, or approximately 1 to 1,000 parts per 1,000,000.

Polarography:
Polarography offers a new and exciting analytical tool that allowed
for the first time for the measurements of concentrations at
submilimolar level without heroic efforts.

Polarography:
Characteristic features of polarography :
 Applied Voltage :
It varies from 0 -2.5 voltes.
 Current value:
Applied current:
0.12 -100 micro ampere.
 Half wave potential: (Fig. 1)
It is an oxidation a reduction potential at the current mid point of a polarographic curve
It is the characteristic of an eletrolysable species and is independent of concentration.

Polarography:
• Oxygen waves:
 Dissolved oxygen in solution to be analyzed polarographically often
interferes and determination of other species becomes difficult.
 Removal of oxygen is usually done by deaeration of sample before
analysis.
 For this purpose Ultrasonification, vacume or purging with Nitrogen for
several minutes.
 Oxygen interferences due to two equal oxygen waves.
 First oxygen wave is produced by its reduction to hydrogen peroxide and
second by further reduction of hydrogen peroxide to water.

Polarography contd--
• Techniques or types of polarography:
• Direct current polarography
• Sample DC polarography
• Single sweep cathode ray polarography
• Pulse polarography
• Differential pulse polarography

Potentiometery contd--
• Applications:
• The majority of heavy metals ions, some nonmetals like oxygen ,chlorine
, ozone, bromine can be determined by polarography in trace amounts.
We can also find out alkaline earth metals like cr2o7-2, k2cr2o7etc.
• Quantitative and Qualitative Analysis :
• The current voltage-curve which has characteristic s-shape may be used
for the qualitative and quantitative analysis of electro reducible and
electro – oxidizable substances or ions.
• Biological Application:
• In biological fluids, oxygen can be determined both in vivo and vitro.
• Environmental Application:
• Metals such as Pb and Cd present in samples of chemical or biological
origin are readily determined by polarographic analysis.

Polarography contd--
• Analysis of organic compound:
• For the analysis of organic compounds the following groups ; azo, diazo,
nitroso, dioxy can be determined by polarography. Simple functional
group can also be determined by polarographic analysis.
• Inorganic applications:
• Polarographic method is widely applicable for analysis inorganic species,
e.g ions of the alkali and alkaline earth metals .

Radiochemical method of analysis:

Neutron Activation Analysis:
Advantages
• Wide possibilities of applications for different types of samples (matrices).
• Low detection limits down to 10-6 mg·kg-1.
• The virtual absence of an analytical blank.
• The relative freedom from interference and matrix effects.
• The possibility of non-destructive analysis (so-called instrumental neutron
activation analysis - INAA).
• The high specificity based on characteristics of induced radionuclides.
• A completely independent nuclear principle, in contrast to electron nature of
most of other analytical methods.
• The method is theoretically simple and well understood, which makes it possible
to evaluate and model sources of uncertainty of results.
• The neutron energy and neutron flux density can be chosen to certain extents,
which allows obtaining of the best results using simple optimization means, such
as selective activation.
• The isotopic basis, which makes it possible to use analytically independent routes
of determination of many elements and to cross-check the results, obtained
using so-called internal self-verification principle.

Neutron Activation Analysis:
• Disadvantages:
• The need of a nuclear reactor.
• The work with radioactive materials and the resulting consequences
(the need of disposal of radioactive waste, radiation protection, and
dose regulation of personnel).
• Industrial applications are not frequent.
• The feasibility of determination of traces of some toxicologically
important elements, such as lead is limited, because they do not
form radionuclides with suitable properties.
• Samples for NAA should preferably be dried (or freeze-dried),
because in the presence of water radiolysis occurs on irradiation
in a nuclear reactor. This may result in increased pressure in the
sample container and the risk of explosion on handling.
• The time of analysis can be quite long (about six week) for
elements, which produce long-lived radionuclides.
• NAA is not available as a simple apparatus with software, which
can easily be operated in any analytical laboratory

NAA:
• Sources of neutrons:
• Nuclear reactor: It may produce neutrons flux
up to 1014 Neutrons cm2/sec.
• Particle accelerator: The mixture of beryllium
contained in a block of paraffin produces a
flux of neutrons up to 107 Neutrons cm2/sec.

Radioactivity contd--
• Uses:
The analysis is used for the determination of cadmium in food products:
• The analysis is used for the determination of arsenic in sea food.
• The analysis is used for the determination of mercury in fish.
• The analysis is used for the determination of Nitrogen , Oxygen ,
Fluorine , in organic compounds.
• The analysis is used for the determination of metal ions in water
(mercury, arsenic, zinc, cadmium, antimony).

Radio Immune Assay
• IMMUNE REACTION:
• When a foreign biological substance enters
into body blood stream through non oral
route, body recognizes the specific chemistry
on surface of foreign substance as antigen and
produces specific antibodies against the
antigen so as nullify the effects and keep the
body safe. The antibodies are produced by
body immune system so, it is an immune
reaction. Here the antibodies or antigens bind
move due to chemical influence.

RDA:
A competitive binding or competitive displacement reaction:
• This is a phenomenon wherein when there are two antigens
which can bind to same antibody, the antigen with more
concentration binds extensively with the limited antibody
displacing other. So here in the experiment, radiolabelled
antigen is allowed to bind to high affinity antibody. Then
when patient serum is added unlabelled antigens in it start
binding to the antibody displacing the labeled antigen.
• Measurement of radio emission:
• Once the incubation is over, then washings are done to remove
any unbound antigens. Then radio emission of the antigen
antibody complex is taken, the gamma rays from radio labeled
antigen are measured.

RDA
• The method can also be useful for quantitative analysis of various drugs,
steroids, and hormones.
• Drugs Steroids Hormones
• Diazepam Estrogen Adrenocorticotrophic hormones ACTH
• Nicotine Progesterone Follicle stimulating hormone FSH
• Penicillin Corticosteroids Glucagon
• Morphine Growth hormones
• Gentamicin Insulin

Radioactivity contd--
Autoradiography:
• The method is used for the detection of separated compounds on the
surface of thin layer plate or paper can be done by this technique.
• Example:
• 16 different amino acids can be located and separated on thin layer plate
by this method.

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