Transcript Potentiometric and Diazotization Titrations

A Seminar on
Potentiometric and Diazotization
Titrations
By Rojison Koshy,
Dept. of Pharmaceutical Analysis
The Erode College of Pharmacy
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Diazotization Titrations
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Aromatic primary amines react with sodium
nitrite in acidic solutions to form diazonium
salts.
C6H5NH2 + NaNO2+HCl
C6H5N2Cl+
NaCl + 2H2O
End point is indicated by the presence of
small amounts of nitrous acid.
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End point detection by two methods,
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Visual end point
Amperometrically
Visual end point is indicated using starch
iodide paper according to the formula
KI + HCl
HI + KCl
2HI + 2HNO2
I2 + 2NO + 2H2O
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Amperometric method is using bright
platinum electrodes. At the end point,
permanent deflection of the galvanometer is
observed. Usually 30 – 50mV of potential is
applied.
Applications
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Used in the determination of primary
aromatic amines. May be used for the
analysis of drugs such as benzocaine,
dapsone, primaquine etc.
Potentiometry
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In potentiometric titrations the change in the
electrode potential upon the addition of titrant
are noted against the volume of titrant
added.
Ecell = Eref + Eindicator + Ejunction
Theory – Nernst Equation
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Nernst equation is the basis for the
relationship between the voltage generated
by an electrochemical cells a result of the two
half cell reactions and the relevant
concentration at each electrode.
Nernst equation for a redox electrode may be
given as
E = E°ox,red + (RT/zF).ln(aox/ared)
Types of Potentiometric Titrations
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Acid-base titrations
Complexometric Titrations
Oxidation-Reduction Titrations
Precipitation Titrations
Non-Aqueous solvents
Acid – Base Titrations
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Here hydrogen electrode may be employed
and the reference electrode may be N –
Calomel electrode
The potential of any hydrogen electrode may
be given by the equation,
E = E° - 0.0591 log aH+ at 25°C.
Where E° is the standard electrode potential.
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It can be concluded
from the graph that the
change in the electrode
potential or EMF is
proportional to the
change in pH during
titration. The point
where the EMF
increases rapidly gives
the end point.
Complexometric Titrations
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Complexometric titrations can be followed
with an electrode of the metal ion whose ion
is involved in complex formation.
After the end point, addition of further ions
does not affect the concentration of the
complex so that the titration curve has almost
horizontal portion after the equivalence point.
Oxidation – Reduction Titrations
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These type of reactions can be followed by
inert indicator electrodes. The electrode
assumes a potential proportional to the
logarithm of concentration ratio of the two
oxidation states of the reactant or the titrant
which ever is capable of properly poising the
electrodes.
Equivalence point is indicated by a marked
deflection in the titration curve.
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These may be used in monitoring procedures
such as monitoring cyanide wastes from
metal plating industries or chlorine
compounds in bleach compound
manufacturing, etc.
Precipitation Titrations
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Here, the solubility product of the almost
insoluble material formed during a
precipitation reaction determine the ionic
concentration at the equivalence point. The
indicator electrodes must readily come into
equilibrium with one of the ions.
Non – Aqueous Solvents
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Here, the ordinary glass calomel electrode
system can be used.
In non-aqueous titrations, usually, the milli
volt scale of potentiometer is used rather
than pH scale since the potential of the nonaqueous systems exceed the pH scale.
Reference Electrodes
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Calomel Electrodes – potential of 250, 286
and 338 mV in saturated, 1 M and 0.1M KCl
respectively at 20°C
Silver – Silver chloride electrode – potential
of 200, 235.5 and 288 mV at 25°C
Mercury (I) Sulphate electrode – potential of
682 mV
Salt Bridge
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Salt bridge of potassium chloride, potassium
nitrate or ammonium nitrate is used to
prevent the possible contamination of the
reference electrode with test solution.
Usually the salts are solidified with 3% agar.
Indicator Electrodes
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Hydrogen Electrodes
Glass Electrodes
Ion Selective Electrodes (ISE)
Hydrogen Electrode
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Consists of a small piece of Pt foil coated
with Pt black, over which hydrogen gas is
passing.
Thus the electrode will act as if it were an
electrode of metallic hydrogen.
Glass Electrode
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Advantage of rapid response, unaffected by
the presence of oxidizing and reducing
agents, or salts in moderate concentrations.
Disadvantage of fragility, imperfections in the
bulb may cause error.
Rejuvenation required over a period of time
to avoid any errors.
Ion Selective Electrode
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Generally consists of a thin layer of an
electrically conducting material called the
membrane across which a potential
develops.
Classified as solid state, heterogeneous,
liquid ion exchanger and glass type.
ISE – Characteristics and Usage
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Response
Limits of Detection
Interference
pH effects
Electrode lifetime
Measurement of pH
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The activity of hydrogen ions in solution is
variable in the Nernst equation for an
electrode reversible to these ions, and
therefore such an electrode can produce an
EMF related to solution pH as of definitions,
pH = -log10aH3O+
pOH = -log10aOH-
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2H2O
H3O++OHOr H2O
H++OHBy law of mass action, for pure water,
[H+][OH-]=Kw
Taking log on both sides and rearranging,
pH+ pOH = pKw =14
Applying this to Nernst equation,
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E = E°H+,H2 - 0.0592 pH
Since E°H+,H2 is defined as zero,
E = - 0.0592 pH
Applications
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For the measurement of the endpoint of the
titrations which may not be feasible for visual
end point detection using indicators.
For the measurement of pH
In Non-aqueous titrations
In complexometric and precipitation titrations.
In redox titrations
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For the determination of ferrous ammonium
sulphate (redox titrations), titration of
potassium bromide with silver nitrate (
precipitation titration), back titrations of
reagents such as pyridine, glycine, PABA
with HCl followed by NaOH etc.
THANK YOU
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