Potentiometry

 Introduction
 Theory
 Ion selective electrodes and types
 Applications of potentiometry
 Differential Thermal Analysis(DTA)
 Derivative differential thermal analysis(DDTA)
 Thermo Gravimetric Analysis(TGA)
 Differential scanning calorimetry

 Potentiometry is an electrochemical method of Analysis deals with
the measurement of electric potential or emf of an electrolyte
solution under the condition of constant current.
 Potentiometry is the measurement of electrical potential of an
electrolyte solution to determine its concentration.
 The principle is based on the fact that the potential of the given
sample is directly proportional to the concentration of its electro
active ions or its activity (pH)
 When the pair of electrodes is placed in the sample solution it shows
the potential difference by the addition of the titrant or by the
change in the concentration of the ions.

 The theory of potentiometry is based on the nernst equation.It
gives the basic relationship between the potential generated by an
electrochemical cell and the concentration of the ions.
 The potential E ( Half cell potential) of any electrode is given by
nernst equation.
◦ E : Half cell electrode potential
◦ E0 : Standard electrode potential
◦ R : Universal gas constant( 8.3145 j/k/mole)
◦ T : Temperature (298 K or 250 C)
◦ F : Faraday’s constant (96,500 c/mole)
◦ n : No. of electrons transferred in the half reaction
◦ [Ox] : Concentration of oxidized substances (Reducing agent)
◦ [Red] : Conc. Of reduced species (Oxidizing agent)

By combining the constants and converting base e to Log base 10, the
equation becomes as,
Significance of nernst equation
1. It gives the potential of an electrode.
2. It gives the relationship between the electrode potential and
concentration of electroactive ions.
Limitations of nernst equation
1.At lower concentrations activity of ions(pH) is equal to their
concentration, whereas at higher concentration it is not true.
2.The equation is applicable only when no current flows through
the electrodes, otherwise it changes the ionic activity at the
electrode surface and leads to variation in measured potential.

 It is a device which converts Chemical energy into electrical
energy. It is used to generate potential and electric current
from chemical reactions.
 It consists of two electrodes immersed in an electrolyte
solution. The current is generated by the chemical reactions
which involves releasing and accepting of electrons at the two
electrodes (Redox reactions)
 Each electrode of an electrochemical cell is referred to as Half
cell, one half cell loses electrons(oxidation) other half cell
gains electrons(reduction).
 One half cell is called as reference electrode, which has stable
and constant potential and the other is called as indicator
electrode the potential of which is to be determined.

Electrodes: These are mainly used to measure the voltages.
Types of electrodes
1.Reference Electrode
a. Primary Reference electrode : Standard hydrogen electrode(SHE)
b. Secondary Reference electrode
1.Saturated calomel electrode(SCE)
2.Mercury mercurous sulphate electrode
3.Silver silver chloride electrode
4.Mercury mercuric oxide electrode
2.Indicator Electrodes
a. Hydrogen Electrode
b. Quinhydrone electrode
c. Antimony Antimony Oxide electrode
d. Glass electrode
3.Combination pH Electrode
4.Oxidation reduction electrode
5.Ion Selective Electrodes
A. Glass membrane electrode
B. Liquid membrane electrode
C. Solid state electrode/sensor
D. Biocatalytic Membrane electrode

 An ion-selective electrode (ISE), also known as a specific ion
electrode (SIE), is a transducer (or sensor) that converts the activity
of a specific ion dissolved in a solution into an electrical potential,
which can be measured by a voltmeter or pH meter.
 An ideal I.S.E. consists of a thin membrane across which only the
intended ion can be transported.
 The transport of ions from a high conc. to a low one through a
selective binding with some sites within the membrane creates a
potential difference.
 Ion Selective Electrodes (including the most common pH electrode)
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

• The glass contain 60 to 75 mole % SiO2,
2 to 20 % Al2O3or LaF3, 0 to 6 % BaO and
CaO, and a variable amount of a group
1A oxide. Inside the glass bulb contains
a dilute HCl solution and silver wire
coated with a layer of silver chloride.
• By altering the composition of the glass,
it is possible to make the electrode
selective for different ions.
• Glass membranes are selective for
monovalent cations because polyvalent
ions cannot easily penetrate the surface
of the membrane.

 The selectivity of glass electrodes is related both to the ability of
the various Monovalent cations to penetrate into the glass
membrane and to the degree of attraction of the cations to the
negative sites within the glass.
 Glass electrodes which are selective for H+(pH electrode),Li+, Na+,
K+, Cs+,Ag+, Ti+and NH4+are commercially available
 Monovalent cations from a solution into which the glass is dipped
can penetrate into the surface of the glass. The concentration of
the analyzed ion in the sample solution changes from that in the
internal reference solution, a potential difference develops across
the membrane.
 The electrode is immersed in the solution and pH is measured.

• A solid state membrane electrode can be a
single crystal, a pellet made from a sparingly
soluble salt, or a sparingly soluble salt
embedded in an inert matrix, e.g., rubber.
• The single crystal and pellet membranes are
homogenous, electrodes containing them are
referred to as homogenous membrane
electrodes.
• The membrane consisting of the sparingly
soluble salt in the inert binding material is a
heterogeneous membrane electrode.
• The lanthanum fluoride (LaF3) membrane is
the only single crystal membrane that is
widely used in ion selective electrodes.

 In the process of “doping”, the resistance of the LaF3 crystal is
decreased by replacing a relatively small number of La3+ions in
the crystals with Eu2+ions(Ionic charge transport)
 Because fluoride can selectively migrate to the crystal, the
lanthanum fluoride membrane is selective for fluoride.
 Vacancies in the crystalline structure have exactly the proper
size, charge, and shape to hold a fluoride ion.
 Fluoride ions migrate from vacancy to vacancy in the defective
LaF3crystal. As a fluoride ion abandons one position in the
crystalline structure, it leaves a hole into which another fluoride
can migrate.
 The result is a crystal which exhibits ionic conductivity.
 The conductance to the membrane, as well as the potential
across the membrane, can be related to the analyte concentration
for many solid state membrane electrodes.

 The membrane of the electrode uses an ion exchanger
permanently embedded in a plastic material that is sealed to
the electrode body.
 The membrane separates the internal filling solution and
reference from the external sample solution.
 The electrode resembles that of solid state electrode.
 The sites are free to move in the active phase(the membrane)
this makes the electrodes selective for multivalent ions over
univalent ions.
 These liquid membrane electrodes are responsive exactly to
those ions ( Ca2+ , ClO4
-, NO3
- and BF4
_ ) That are extremely
difficult to monitor by other techniques.

 A gas permeable membrane is used to isolate the analytes from
possible interferences in the sample.
 A thin buffer layer is used to trap the analyte gas and covert it to
some ionic species that can be detected potentiometrically.
 Gas sensing electrodes are available for the measurement of carbon
dioxide, nitrite and sulphur dioxide.
 These are simple and reliable but they tend to have a relatively slow
response and recovery time(often 30 seconds to 5 minutes)
 They are used to assay the gases dissolved in aqueous solutions.
 It is constructed by enclosing the glass pH membrane in a second,
gas permeable hydrophobic membrane.
 A thin layer of an electrolyte solution is held between the two
membranes.
 They also have a small reference electrode enclosed within the gas
permeable membrane.

 The potential between the internal ISE and the reference
electrode within the outer membrane is monitored.
 The gas permeable membrane holds a constant volume of
solution around the internal ISE into which the gaseous analyte
can diffuse.
 The hydrophobic gas-permeable membrane can be composed
of substance which allows passage of dissolved gas but
prevents the solution within the membrane from escaping
 The materials used are silicon rubber, Teflon polypropylene,
fluorinated ethylene propylene, polyvinylidene fluoride etc…
 Gas from the sample solution passes through the submerged
gas permeable membrane and equilibrates in the electrolyte
solution between the two membranes.

 The gas reacts reversibly with the electrolyte solution to form an ion
to which the ion selective electrode responds.
 Because the activity of the ion that is formed between the two
membranes is proportional to amount of gas dissolved in sample,
the electrode response is directly related to the activity of the gas in
the sample.
 The gases (primarily NH3, SO2 and CO2) which are detected by gas
sensing electrodes based on the pH electrode equilibrate with the
electrolyte solution to alter its pH:
 NH3+ H2O = NH4++ OH-
 SO2+ H2O = HSO3-+ H+
 CO2+ H2O = HCO3-+ H+
 H2S, HCN, HF and chloride can be assayed by using internal
homogenous membrane electrode containing the appropriate silver
salt.
 Disadvantage possesses relatively long response time i.e; require 1-
7minutes after insertion in to a sample solution to reach equilibrium

 They are another form of gas
sensing electrodes invented by
Ruzicka and Hansen.
 A very thin layer of an
appropriate electrolyte solution is
adsorbed on the surface of the
membrane of the glass electrode.
 The electrolyte solution is
adsorbed on glass membrane when
membrane comes in contact with
the sponge containing the
electrolyte solution and a wetting
agent.
 The reference electrode makes contact with the adsorbed
electrolyte layer through a small, porous, ceramic salt bridge.

 The air gap electrode is used to assay ionic species which can be
chemically converted to gases,e.g. HCO3
-
 The HCO3-solution is placed in the sample holder and an acid is
added to convert HCO3
-(aq) to CO2(g).
 The sample holder is placed in position under the electrode and
stirred with a magnetic stirrer and stirrer bar.
 Carbon dioxide which is emitted during the chemical reaction
equilibrates with the electrolyte solution on the glass membrane
and alters the pH of the solution.

 The glass electrode measures the pH of the resulting solution.
 The electrolyte solutions used with air gap electrode are the same as
those used with other gas-sensing electrodes.
 The air-gap electrode has a faster response time due to the thinner
layer of electrolyte solution and a longer lifetime than most of the
other types of sensing electrodes.
 A typical response time for an air-gap electrode is less than a
minute.
 Air-gap electrode is primarily used for analysis of NH4
+, HSO3
-.
 As an example they can be used for the determination of urea in
blood.

 They have the same design as liquid-ion-exchanger membrane
electrodes.
 The liquid-ion-exchanger is replaced in neutral-carrier
membranes with a neutral complexing agent (a neutral carrier)
such as crown ether, which is dissolved in a highly water
insoluble organic solvent.
 The neutral carrier complexes with the analyte at membrane-
sample interface to form a charged complex which is extracted
from the aqueous solution into the organic solvent in the
membrane.
 The selectivity of the membrane for a particular ion depends
upon the ability to extract the ion into the membrane, which in
turn depends upon the ability of the ion to form a complex with
the neutral carrier.

 After complexation and extraction, the species in the neutral-carrier
membrane has the same charge as the extracted ion.
 The solvent in which the neutral carrier is dissolved is usually a high
boiling organic compound such as nitrobenzene (used in Ba2+
selective electrodes), dibutylsebacate (used in K+selective electrode)
and onitrophenyl-n-octylether (used in a Ca2+selective electrode).
 The physical support for the neutral carrier and solvent is usually a
cellulose membrane, more commonly, a PVC membrane. In addition
to K+, Ca2+, Ba2+, neutral-carrier membrane electrodes are also
selective for Li+, H+, Mg2+, NH4+, Sr2+.

 They are considerably smaller than
other forms of ion selective electrodes
because the internal filling solution is
eliminated and the ion selective
membrane is coated directly on the
internal electrode wire.
 The ion selective membranes
utilized in coated wire electrodes
consists of either an ion exchanger or
neutral carrier immobilized in a
polymeric material that is coated on
the electrode.
 They are more sturdy than other
ISEs and can be constructed with
small tips.

 First the metal on the interior of the electrode is sealed into a glass or
some other suitable material so that several mm or less of the wire is
exposed.
 The exposed wire is successively dipped into a solution of the polymeric
material and then into a solution of the ion exchanger or neutral carrier.
 After the electrode has air-dried, the dipping procedure is repeated, if
necessary, until the membrane coating on the wire is the desired
thickness.
 Alternatively, the wire can be dipped into a single solution containing
both the membrane material and the polymerizer.
 The polymeric matrix can be any of materials including PVC, polymethyl
acrylate(PMM) or epoxy.
 The internal electrode can be constructed from metals like platinum,
copper, silver wire and graphite rods.

 The electrode consists of an ion
selective membrane deposited or
coated on the gate of a field effect
transistor (FET). The membrane can
be a sparingly soluble compound such
as silver bromide (solid state
membrane) or some other type of
membrane such as an ion exchanger or
neutral carrier in a PVC matrix.
 Often membranes in a PVC matrix
are used. Membranes in a PVC matrix
can be forced to adhere to the gate of
the FET by placing a polyimide mesh
over the gate prior to coating it with
the membrane.

 The potential at the membrane is partially determined by the
activity of the analyte in solution.
 That potential determines the flow of current through the
drain of the FET.
 The drain current consequently varies with the activity of the
analyte and is the monitored factor.

 It is an ion selective electrode which is coated with an enzyme-
containing acrylamide gel.
 The gel and enzyme are held in place on the surface of the ion
selective electrode by an inert physical support.
 The design is same as gas-sensing electrode.
 The support is a sheet of cellophane or a piece of gauze made
from dacron or nylon.
 The physical support is wrapped around the electrode membrane
and tied in place.
 The acrylamide gel containing the enzyme is coagulated on the
support-electrode combination.
 Enzymes are highly selective biochemical catalysts.
 The selectivity of Biomembrane electrode is due to the selectivity
of the enzymes that are used in electrodes.

 Here the enzyme-catalyzed reaction of the analyte yields an ionic
reaction product which is monitored by the internal ion-selective
electrode.
 The operation of the urea-selective electrode will serve to illustrate
the operation of Biomembrane electrodes.
 The glass membrane of an ammonium-sensitive glass electrode is
coated with an acrylamide gel layer containing the enzyme urease.
 When the electrode is dipped into a solution containing urea, the
following reaction occurs to yield NH4+ :
CO(NH2)2+ H2O 2NH4
++ CO2 .
 The NH4
+formed during the reaction is measured at the ammonium-
selective electrode.
 A working curve is prepared by plotting the potential of the electrode
in standard urea solutions as a function of the logarithm of urea
concentration.

 The urea concentration in the sample is obtained from the
working curve.
 Unfortunately the enzymes used in Biomembrane electrodes
gradually decay and the enzyme containing gel must be
periodically replaced.
 The Biomembrane of urea electrode lasts about 2 weeks.
 Biomembrane electrodes have long response time of 5 or more
minutes.

 Ion-selective electrodes are used in a wide variety of applications for
determining the concentrations of various ions in aqueous solutions.
 The electrodes can be used as an end point detector in a titration.
 Pollution Monitoring: CN, F, S, Cl, NO3 etc., in effluents, and natural
waters.
 Agriculture: NO3, Cl, NH4, K, Ca, I, CN in soils, plant material,
fertilizers and feedstuffs.
 Food Processing: NO3, NO2 in meat preservatives.
 Salt content of meat, fish, dairy products, fruit juices, brewing
solutions.
 F in drinking water and other drinks.
 Detergent Manufacture: Ca, Ba, F for studying effects on water
quality.
 Determination of equilibrium constants.

 Determination of ionic product of water.
 Determination of Dissociation constant of acids.
 Biochemistry: Analysis of Ca, K, Cl in body fluids like blood,
plasma, sweat and serum.
 Paper Manufacture: S and Cl in pulping and recovery cycle
liquors.
 F in skeletal and dental studies.
 Ca in dairy products and beer .
 Explosives: F, Cl, NO3 in explosive materials and combustion
products.

 When a material is heated its structural and chemical composition
can undergo changes such as fusion, melting, crystallization,
oxidation, decomposition, transition, expansion and sintering.
 Using Thermal Analysis such changes can be monitored in every
atmosphere of interest. The obtained information is very useful in
both quality control and problem solving.
 Monitoring a property of the sample as a function of temperature
or Monitors the temperature change associated with a chemical
Reactions.
 Thermogravimetry.
 Differential thermal Analysis.
 Differential scanning colorimetry.
 Dynamic Mechanical Analysis.
 Thermo Mechanical Analysis
 Direct Injection Enthalpimetry.

 Analytical technique In which the difference between the analyte
sample and a non reactive reference material as a function of
furnace temperature or time , while they are subjected to identical
controlled temperature program.
 Any transformation – change in specific heat or an enthaply of
transition can be detected by DTA.
 If zero temperature difference b/w sample & reference material –
sample does not undergo any chemical or physical change.
 The thermal curve is a plot of the temperature difference as a
function of the temperature of one of the two substances.
 In DTA both test sample & an inert reference material (alumina) –
controlled heating or cooling programming.
 The thermal effect may either be endothermic or exothermic and are
caused by physical phenomena such as fusion, crystalline structure
inversion, boiling, vapourisation, sublimation or others. Some
enthalpic effect are also caused by chemical reaction. In this
manner, endo- and exothermal bands and peaks appearing on
thermogram give info, regarding the detection of enthalpic changes.

 Sample holder : Thermocouples, containers,
ceramic or metallic block.
 Furnace
 Temperature Programmer
 Recording System

 The sample and reference are held
in separate containers within the
same furnace. Sample cups/holders
are made of platinum and are fairly
close to each other to ensure
identical heating.
 Temperatures are monitored with
thermocouples placed in the
pedestals that support the sample
cups.(this arrangement varies in
some instruments)
 Reference substance is thermally inert, alumina is often
used.(silicon carbide, glass beads-phase change,not used
throughout the temperature interval)

• This is constructed
with an appropriate
material (wire or
ribbon) wound on a
refractory tube.
• Furnaces possess
the desired
characteristics
for good temperature
regulation and programming.
• These are fairly inexpensive. Generally, the choice of the
resistance material is decided from the intended maximum
temperature of operation and gaseous environments.

 Temperature control unit controls the furnace temperature.
 The combination of the furnace and sensor enables the various
types of the measurement techniques. The computer can be
connected to several instruments which has the other type of
measurement techniques, enables the simultaneous
measurement and analysis.
 In order to control temperature, the three basic elements are
required.-Those are sensor, control element and heater.
 Fluctuations of temperature around the set value can be
minimized if the heat input to the system is progressively
reduced as the temperature approaches the desired value. Such a
controller that anticipates the approach to the set value is known
as a proportional controller.

 In thermo analytical studies, the signal obtained from the
sensors can be recorded in which the signal trace is produced
on paper or film, by ink, heating stylus, electric writing or
optical beam.
 There are two types of recording devices similar to the TG.-
one is deflection type and other is null type.

 Insert a very thin thermocouple into a disposable sample tube 2mm
in diameter and containing 0.1-10 mg of sample.
 Another identical tube is either kept empty or filled with reference
substance, such as quartz, sand, alumina or alundum powder.
 The two tubes are simultaneously inserted into a sample block and
subsequently heated(or cooled) at uniform predetermined
programmed rate.
 DTA- being a dynamic process, it is extremely imp that all aspects of
the technique must be thoroughly standardized so as to obtain
reproducible results. Such aspects are,-pretreatment of specimen, -
particle size & packing of specimen, -dilution of the specimen, -
nature of inert diluent, -crystalline substances must be powdered
and sieved thru 100 mesh sieve, -micelle size is critical for colloidal
particle and – adequate control of atmosphere.

 The furnace temperature is increased at a linear rate. The
temperature between the sample and reference ΔT is
continuously measured and a thermogram is recorded.
 (The plot of ΔT v/s T temperature of the reference)
 A break in the curve indicates that there is either a physical or a
chemical change in the sample.
 If ΔT is (TS-TR) then, the trough in the curve indicates
endothermic process and a crest indicates exothermic process.
 To avoid Oxidation of the sample, the furnace is purged with
inert Gas such as nitrogen to displace air in the furnace.

Sharp Endothermic– changes in crystallanity or fusion
Broad endotherms- dehydration reaction
Physical changes usually result in endothermic curves
Chemical reactions leads to exothermic curves.

Advantages:
 instruments can be used at very high
temperatures
 instruments are highly sensitive
 characteristic transition or reaction
temperatures can be accurately determined
Disadvantages:
 uncertainty of heats of fusion, transition, or
reaction estimations is 20-50%

 Identification of substances – We know that the DTA curve for two
substances is not identical. Therefore, these serve as finger prints for
various substances. Particularly, DTA has become an established
technique for the identification of clays.
 Identification of products – When a substance reacts with another
substance, the products is identified by their specific DTA curves.
Therefore, this technique has been termed ‘reaction DTA’.
 Melting points – As melting points can be easily determined by DTA,
it means that this technique can be used as a direct check of the
purity of the compound.
 Quantitative Analysis – We know that the area of DTA peak is
proportional to the total heat of reaction and hence to the weight of
the sample. Therefore, the quantitative analysis is possible with the
help of standard curves of peak area vs. weight.
 Quality control – DTA technique has been widely used for the quality
control of a large number of substances like cement, glass, soil,
catalysts, textiles, explosives, resins etc

 Thermogravimetry is the measurement of change in weight(mass) of
a sample as a function of temperature (while it is being heated)
 The apparatus used is thermobalance.
 This method is useful for determining sample purity and water,
carbonate, and organic content; and for studying decomposition
reactions.
 Dynamic Thermogravimetry: Sample is subjected to conditions of
continuous increase in temperature, which is linear with time.
 Static/ isothermal TGA: Sample is maintained at constant
temperature for a period of time during which any change in weight
is noted.

 Thermobalance
 Sample Holder
 Furnace
 Precision balance
 Recorder

 Sample Holder: Depending upon the Nature of sample, its weight
and quantity to be handled, different size and shape of sample
holders known as crucibles/shallow dish are employed . They are
made of Glass, quartz, aluminium, Stainless steel, Platinum etc.
 2 types:
1.Shallow Dish/pan: Holding samples which eliminates
Gas, vapours or volatile matter by diffusion during heating.
2.Deep crucibles for general purposes.
 Furnace: Made of high quality metal and varies in shape and size.
Sample can be easily introduces. Temperature Is maintained between
1000o-2000o C By temperature control panel.
 Both linear or fixed temperature heating is maintained by the use of
thermocouples like Chromel-Alumel, Copper-Platinum, Platinum-
Rhodium.
 For Higher temperature Tungsten or rhenium thermocouples are
used.

 Furnace temp. programme : They provide gradual rise of
temperature at a fixed rate . It has a fine control knobs through
which desired temperature with respect to time/rate can be
obtained. The controlling is done by increasing voltage through
the heated element by motor driven variable transformer or by
different thermocouples.
 Recorder: These are of common type having strip and pen
mechanism. Electrical supply duly amplified is fed to recorder
chart. The speed of recorder is variable and adjusted according
to the use.

 Most imp part of the Instrument.
 Accuracy, reproducibility, sensitivity, toughness, toughness etc. are
some important features of thermobalance.
 It should have some ideal properties,
1. it should cover wide range of temperature.
2. it should have high degree of mechanical rigidity and
electronic stability.
3.it should be capable of recording changes in weight
rapidly, accurately and continuously as a function of
temperature and time.
4. The heating rate should be linear in temperature range.
 Recording thermobalances are of two types.
1.Deflection type
2.Null type

In deflection type they are,
1. Beam type: conversion of beam deflection is recorded as a
signal on photographic recorder or measuring device.
2. Helical type: use of spring which contracts or elongates as a
result of weight changes during heating
 Null point balances are more common, A sensor is employed to
detect deviation of beam from a null point. Opposite force of
electrical or mechanical is supplied to the balance beam to
restore null position. The opposing force is calibrated to the
weight change which is recorded directly.

 The sample is continuously weighed while being heated in an
inert atmosphere of nitrogen gas or in vacuum. The plot of
sample weight vs. temperature is called thermogram. The break
in the curve at various temperatures indicates the physical or
chemical change occurring in the sample.

 (1) Instrumental factors
(a) Furnace heating rate
(b) Furnace atmosphere
 (2) Sample characteristics includes
(a) Weight of the sample
(b) Sample particle size

Instrumental factors
 Furnace Heating rate: The temperature at which the compound (or
sample) decompose depends upon the heating rate. When the
heating rate is high, the decomposition temperature is also high. A
heating rate of 3.5°C per minute is usually recommended for reliable
and reproducible TGA.
 Furnace atmosphere: The atmosphere inside the furnace surrounding
the sample has a profound effect on the decomposition temperature
of the sample. A pure N2 gas from a cylinder passed through the
furnace which provides an inert atmosphere.

Sample characteristics
 Weight of the sample: A small weight of the sample is
recommended, using a small weight eliminates the existence of
temperature gradient through the sample.
 Particle size of the sample: The particle size of the sample should
be small and uniform. The use of large particle or crystal may
result in apparent, very rapid weight loss during heating.

 Determination of the purity and thermal stability of both primary and
secondary standard.
 Determination of the composition of complex mixture and
decomposition of complex.
 For studying the sublimation behavior of various substances.
 TGA is used to study the kinetics of the reaction rate constant.
 Used in the study of catalyst: The change in the chemical states of
the catalyst may be studied by TGA techniques. (Zn-ZnCrO4) Zinc-
Zinc chromate is used as the catalyst in the synthesis of methanol.

 Determining the composition of alloys and mixtures.
Thermogravimetry is a valuable technique for assessing the purity of
materials.
 Analytical reagents, especially those used in titrimetric analysis as
primary standards, e.g. Na2CO3, KHP have been examined. Many
primary standards absorb appreciable amounts of water when
exposed to moist atmospheres.TG data can show the extent of this
absorption, hence the most suitable drying temperature for a given
reagent may
 This method can also be extended to the analysis of a three
component mixtures.
 The most important applications of thermogravimetry is in
examining the thermal stability of polymers.be determined.

 This technique is very much similar to the DTA.
 Measurement of difference in heat flow into substance and reference
material as a function of sample temperature when both substances
are exposed to controlled temperature.
 In DSC the sample and the reference compound are gradually heated
in an inert atmosphere if the sample undergoes a physical or
chemical changes by absorption of heat then heat energy is added or
removed from the sample to maintain to maintain both sample and
reference at the same temperature.
 Thermogram in DSC is the plot of energy added to the sample vs
temperature.
 Two basic types of DSC instruments: power compensation DSC
and heat-flux DSC.

Heat flux DSC:
 Both sample and reference pans are heated by a single furnace
through heat sink and heat resistor. Heat flow is proportional to the
heat difference of heat sink and holders. The temperature versus
time profile through a phase transition in a heat flux instrument is
not linear.
 At a phase transition, there is a large change in the heat capacity of
the sample, which leads to a difference in temperatures between the
sample and reference pan.
 A set of mathematical equations convert the signal into heat flow
information. By calibrating the standard material, the unknown
sample quantitative measurement is achievable.
 Sample and reference holders: Al or Pt pans placed on constantan
disc and are connected by a low-resistance heat flow path. A metallic
disc made of constantan alloy is the primary means of heat transfer .
Sample and reference sit on raised constantan discs.

 Sensors
• Chromel® (an alloy made of 90% nickel and 10% chromium)-
constantan area thermocouples (differential heat flow)
• Chromel®-alumel (an alloy consisting of approximately 95% nickel,
2% manganese, 2%
aluminium and 1% silicon) thermocouples (sample temperature)
 Furnace - same block is used for sample and reference.
 Temperature controller:
• The temperature difference between the sample and reference is
converted to differential thermal power, which is supplied to the
heaters to maintain the temperature of the sample and reference at the
program value

 Differential heat flow to sample and reference is measured by
thermocouples which are connected in series, located at the junction
of constantan disc and chromel wafers.
 With this, it is possible to achieve heating or cooling rates of
1000c /min to 00c/min (isothermal).
 It needs mathematical equations to get the heat flow .
 DSC Heat Flow Equation
 Dh/Dt - DSC Heat Flow Signal, Cp - Sample Heat Capacity = Sample
Specific Heat X Sample Weight, Dt/Dt - Heating Rate
 F(t,t) - Heat Flow That Is A Function Of Time At An Absolute
Temperature (Kinetic)
Dh/Dt = Cp Dt/Dt + F(t,t)

Power compensation DSC
 Both sample and reference pans are heated by a different furnaces.
When an event occurs in the sample, sensitive Platinum Resistance
Thermometer (PRT) detects the changes in the sample, and power
(energy) is applied to or removed from the sample furnace to
compensate for the change in heat flow to or from the sample. As a
result, the system is maintained at a “thermal null” state at all times.
The amount of power required to maintain system equilibrium is
directly proportional to the energy changes occurring in the sample.
No complex heat flux equations are necessary with a power
compensation DSC because the system directly measures energy flow
to and from the sample.
 In addition, PC type DSC has enhanced modulated temperature DSC
(StepScan) technique and fast scan DSC (Hyper DSC) for dramatic
improvements in productivity, as well as greater sensitivity.
 Furthermore, the heating and cooling rate of PC types DSC can be as
high as 500°C/min.

 Sample holder
• Aluminum or Platinum pans
 Sensors
• Platinum resistance thermocouples
• Separate sensors and heaters for the sample and reference
 Furnace
• Separate blocks for sample and reference cells
 Temperature controller
• Supply the differential thermal power to the heaters to maintain the
temperature of the sample and reference at the program value

 Accurately weigh samples (approx. 3 to 20 mg). encapsulate in the
metal pans of high thermal conductivity materials (aluminium,
platinum, stainless steel) are used.
 Pan configurations may be open, pinhole or hermetically sealed.
Same pan material and configuration for both sample and reference.
Material should entirely cover the bottom of the pan to ensure
thermal contact.
 Avoid overfilling of the pan to minimize the thermal lag from the
bulk of the material to the sensor . Small sample masses and low
heating rates improve resolution [Do not Decompose The Samples In
DSC Cell]
 Sample Shape: Cut the sample to uniform shape, do not crush the
sample. If the sample to be taken is pellet, cross section is to be
taken. If the sample material is powder then, it is spread uniformly
over the bottom of the sample pan.

 Using Sample Press: When using crimped pans, the pans should not
be over crimped. The bottom of the pans should remain flat, even
after crimping. When using hermetic pans, a little more pressure is
required to crimp the pans. Hermetic pans are sealed by forming a
cold wield on the aluminium pans.
 Sample Size: Smaller samples will increase the resolution but will
decrease the sensitivity.
Larger samples will decrease the resolution but will increase the
sensitivity.
Sample size depends on the type of material being measured.
If the sample is
Extremely reactive in nature - very small samples (<1 mg) are to be
taken.
Pure organics or pharmaceuticals - 1 to 5 mg
Polymers - approximately 10 mg
Composite materials -15 to 20 mg

 Reference Materials - An inert material like α-alumina is generally
used. Empty pan can also be used, if the sample weight is small. With
higher sample weights it is necessary to use a reference material,
because the total weight of the sample and its container should be
approximately the same as the total weight of the reference and its
container . The reference material should be selected so that it
possesses similar thermal characteristics to the sample.
 The most widely used reference material is α-alumina, which must
be of analytical reagent quality . Before use, α-alumina should be
recalcined and stored over magnesium perchlorate in a desiccator .
Kieselguhr is another reference material normally used when the
sample has a fibrous nature.
 If it doesn’t have similar thermal characteristics, dilutions are made.
Dilutions are done by thoroughly mixing suitable proportions of
sample and reference material.

 Purge Gases - Sample may react with air and may oxidize or burn.
The problem is overcome by using inert gases. Inert gases are used
to control moisture in the surrounding atmosphere.
 Commonly used inert gases are nitrogen, helium, argon etc.
 Nitrogen - It is the most commonly used inert gas. It increases the
sensitivity of the experiment. Typical flow rate is 50 ml/min.
 Helium - It has high thermal conductivity . It increases the resolution
of the peaks. The upper temperature limit for this gas is upto 3500c.
Flow rate is 25 ml/min
 Air or oxygen - Sometimes it is deliberately used to view oxidative
effects of the sample. Flow rate is 50 ml/min
 Heating Rate - Faster heating rate will increase the sensitivity but will
decrease the resolution. Slow heating rate will decrease the
sensitivity but will increase the resolution. Good starting point is
100c/min.

 The ultimate goal of calibration and adjustment is to provide a
measuring system that consistently delivers reproducible and
accurate results.
• Evaluation of the thermal resistance of the sample and reference
sensors
• Measurements over the temperature range of interest
 2-Step process
• The temperature difference of two empty crucibles is measured
• The thermal response is then acquired for a standard material,
usually sapphire, on both the sample and reference platforms
 Amplified DSC signal is automatically varied with the
temperature to maintain a constant calorimetric sensitivity with
temperature.

Temperature
 Goal is to match the melting onset temperatures indicated by
the furnace Thermocouple readouts to the known melting points of
standards analyzed by DSC.
 Should be calibrated as close as possible to the desired temperature
range.
 Celebrant's
• High purity
• Accurately known enthalpies
• Thermally stable
• Light stable
• Non-hygroscopic
• Un-reactive (pan, atmosphere)

 The result of a DSC experiment is a curve of heat flux versus
temperature or versus time. There are two different conventions:
exothermic reactions in the sample shown with a positive or
negative peak, depending on the kind of technology used in the
experiment.
 This curve can be used to calculate enthalpies of transitions.
 Area under the peak is directly proportional to heat absorbed or
evolved by the reaction.
 Height of the peak is directly proportional to rate of the
reaction.

Factors affecting DSC curve/ Thermogram
Instrumental factors Sample characteristics
Furnace heating rate Amount of sample
Recording or Chart speed Nature of sample
Furnace atmosphere Sample packing
Geometry of sample holders/
location of sensors
Particle size
Sensitivity of the recording
system
Solubility of evolved gases in
the sample
Composition of sample
containers
Heat of reaction
Thermal conductivity

1) Sample shape:
The shape of the sample has little effect on the quantitative aspect of
DSC but more effect on the qualitative aspects. samples in the form of
a disc film or powder spread on the pan are preferred. In the case of
polymeric sheets, a disc cut with a cork-borer gives good results.
2)Sample size:
About 0.5 to 10mg is usually sufficient. Smaller samples enable faster
scanning, give better shaped peaks with good resolution and provide
better contact with the gaseous environment. With larger samples,
smaller heats of transitions may be measured with greater precision.
3) Atmosphere and geometry of sample holders:
There are a number of variables that affect DSC results includes the
type of pan, heating rate, the nature and mass of the compound, the
particle size distribution, packaging and porosity, pre-treatment and
dilution of the sample. It is used for purity analysis of above 98% pure
compounds.

 Calibration
 Contamination
 Sample preparation – how sample is loaded into a pan
 Residual solvents and moisture.
 Thermal lag
•Heating/Cooling rates
•Sample mass
 Processing errors

 Determination of purity of the sample.
 Detection of polymorphism and quantification of Polymorph,
Detection of metastable polymorphs.
 Detection of isomerism.
 Stability and compatibility studies.
 Lyophilization studies.
 Estimation of amorphous content in excipients.
 Percentage crystallinity Determination.
 Characterization of Membranes, lipids, nucleic acids & micellar
systems.
 Assessment of the effects of structural change on a molecules
stability.
 Measurement of Ultra-light molecular interactions
 Assessment of biocompatibility during manufacturing.
 Protein Stability and Folding
 Liquid Biopharmaceutical Formulations and Process Development

 https://www.hitachi-
hightech.com/global/products/science/tech/ana/thermal/descriptio
ns/ta.html
 Instrumental Methods of Chemical Analysis B. K. Sharma
 Instrumental methods of analysis, seventh edition – Willard, Merritt,
Dean, Settle.
 Instrumental methods of analysis – M. Calvin, S.C. Bhatia .
 Basic Concepts of Analytical Chemistry – Third edition by S.M.
Khopkar .
 Introduction to Instrumental Analysis – Robert D. Braun.
 Instrumental methods in Pharmaceutical Analysis– Dr. U.B.Hadkar
 Pharmaceutical Analysis- volume II – A.V.Kasture, K.R.Mahdik,
S.G.Wadodkar, H.N.More.
 www.slideshare.com
 www.google.co.in
 https://nptel.ac.in/courses/115103030/module4/lec22/1.html
 www.wikipedia.org

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