A Seminar on Potentiometric and Diazotization Titrations

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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

• Aromatic primary amines react with sodium nitrite
  in acidic solutions to form diazonium salts.
• C6H5NH2 NaNO2HCl 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,
• 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.

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Applications

• Used in the determination of primary aromatic
  amines. May be used for the analysis of drugs
  such as benzocaine, dapsone, primaquine etc.

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Potentiometry

• 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

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Theory Nernst Equation

• 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 Eox,red (RT/zF).ln(aox/ared)

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Types of Potentiometric Titrations

• Acid-base titrations
• Complexometric Titrations
• Oxidation-Reduction Titrations
• Precipitation Titrations
• Non-Aqueous solvents

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Acid Base Titrations

• 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 25C. 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.

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Complexometric Titrations

• 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.

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Oxidation Reduction Titrations

• 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.

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Precipitation Titrations

• 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.

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Non Aqueous Solvents

• 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 non-aqueous
  systems exceed the pH scale.

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Reference Electrodes

• Calomel Electrodes potential of 250, 286 and
  338 mV in saturated, 1 M and 0.1M KCl
  respectively at 20C
• Silver Silver chloride electrode potential of
  200, 235.5 and 288 mV at 25C
• Mercury (I) Sulphate electrode potential of 682
  mV

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Salt Bridge

• 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.

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Indicator Electrodes

• Hydrogen Electrodes
• Glass Electrodes
• Ion Selective Electrodes (ISE)

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Hydrogen Electrode

• 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.

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Glass Electrode

• 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.

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Ion Selective Electrode

• 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.

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ISE Characteristics and Usage

• Response
• Limits of Detection
• Interference
• pH effects
• Electrode lifetime

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Measurement of pH

• 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 H3OOH-
• Or H2O HOH-
• By law of mass action, for pure water,
• HOH-Kw
• Taking log on both sides and rearranging, pH pOH
  pKw 14
• Applying this to Nernst equation,

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• E EH,H2 - 0.0592 pH
• Since EH,H2 is defined as zero,
• E - 0.0592 pH

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Applications

• 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.

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THANK YOU