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A Seminar on A Seminar on Potentiometric and Diazotization Potentiometric and Diazotization

TitrationsTitrations

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 + 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,– 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 = E°ox,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 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.

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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 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

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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 H3O++OH-

Or H2O H++OH-

By 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

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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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