UNIT-II

ELECTROCHEMISTRY

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24-Aug-21

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Dr.A.Geetha

Associate Professor

Department of Chemistry

Kongu Engineering College

• Introduction – electrochemical cells - applications of electrochemical series - reference electrode - standard calomel electrode - ion selective electrode - glass electrode - concentration cells - electrode and electrolyte concentration cells (simple problems) -applications- potentiometric titrations - acid-base, redox, precipitation titrations - advantages- conductometric titrations - strong acid vs strong base, weak acid vs strong base, mixture of weak and strong acid vs strong base- advantages of conductometric titrations.

24-Aug-21

Potentiometric Titration

• According to Nernst equation, the potential of an electrode depends upon the concentration of the ions in the solution.
• In a titration, there is a change in concentration of ions, which can be noted by measuring the potential of a suitable electrode.

Thus potentiometric titrations are titrations which involve the measurement of electrode potential with the addition of the titrant.

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• It needs two electrodes.

Reference electrode and Indicator electrode

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Potentiometric titrations depend on measurement of emf between reference electrode and an indicator electrode.

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Indicator or working electrode:

The electrode in which the potential changes with change in concentration is known as Indicator electrode.

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Theory

• Potential of an Electrode dipping in solution of eletrolyte depends upon the concentration of active ions.

E= E⁰ + (RT/nF) log C

• Small Change in active ion concentration in the solution changes the electrode potential correspondingly
• Concentration of Active ion decreases electrode potential of indicator electrode decreases
• The potential of Indicator electrode is measured potentiometrically by connecting with a reference electrode (Saturated Calomel Electrode)

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Determination of End point

• The emf of a cell changes by the addition of a small amount of titrant. So concentration of reversible ion in contact with indicator electrode changes.
• Record the change in emf with every small addition of titrant.
• The changes of potential will be slow at first, but at equivalence, the point change will be sharp.
• The values are plotted against corresponding volume changes.
• Change in emf with addition of titrant (⧍E/⧍V) is plotted against volume (V)
• The maximum of the curve gives the end point.

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• Fig (a)– Volume of Titrant Vs Emf
• Fig(b) Volume of titrant Vs (⧍E/⧍V)

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Classification of Potentiometric Titration

• Acid –Base titration (HCl vs NaOH)

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• Redox titration(Reduction- Oxidation)

(FeSO₄ vs K₂Cr₂O₇)

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• Precipitation titration (AgNO₃ vs KCl)

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Acid –Base Titration

• Hydrogen or Glass electrode is indicator electrode.
• Reference electrode-Calomel Electrode
• Burette solution-Alkali solution

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Redox Titration

• Oxidizing Agent--- K₂Cr₂O₇ or KMnO₄
• Reducing Agent--- Ferrous Salt
• Indicator----- Platinum Indicator

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K₂Cr₂O₇ +6 FeSO₄+7 H₂SO₄------------K₂SO₄+ Cr₂(SO₄)₃+3Fe₂(SO₄)₃ +7H₂O

• The initial concentration of an ion Fe²⁺ can be found by titrating with a strong Oxidizing agent such as Cr⁶⁺ . That reaction is carried out half side other half is reference electrode

^Pt_(S)^/Fe2+/Fe3+ /^{Reference electrode(Calomel)}

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on adding K₂Cr₂O₇ from the burette, emf of the cell will increase first slowly, but at the equivalence point,Fe²⁺ will have been totally consumed and the potential will then be controlled by the concentration ratio of Cr⁶⁺/Cr³⁺.

• emf value is recorded for every one mL of Oxidizing agent.
• Plot the graph between Volume and emf. From the graph find the end point.

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3Fe²⁺ +Cr⁶⁺ 3Fe³⁺ +Cr³⁺

Precipitation Titration

• Silver electrode dipping in silver nitrate solution is used as the working or indicator electrode
• The electrode is connected with saturated calomel electrode through a salt bridge of ammonium nitrate.
• Indicator electrode-Ag Electrode dipping in AgNO₃ solution.

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• The potential of the Ag⁺/Ag half cell is measured by connecting it with Saturated Calomel electrode through salt bridge of NH₄NO₃
• As the titration proceeds, by addition of standard potassium chloride solution from the burette into the silver nitrate solution, the concentration of Ag⁺ ions decreases, due to the precipitation of Ag⁺ ions as AgCl.

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• The reduction potential of the indicator electrode(Ag) goes on decreasing progressively on the addition of KCl.
• At the end point, the concentration change Ag+ is quite rapid.
• End point the Ag+ concentration becomes very small on account of slight solubility of AgCl.
• If addition of KCl is continued, the Ag+ ion concentration is not affected, except for a very small decrease, due to decrease in solubility of AgCl on account of common ion (Cl^-) effect.

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Ag⁺NO^-₃ +K⁺Cl^- AgCl + K⁺ NO^-₃

• The addition of KCl beyond end point causes only a very small decrease in electrode potential or emf of the cell.
• On plotting (⧍E/⧍V) versus V, the point of maxima gives the end point

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Advantages of Potentiometric Titrations

• Potentiometric titrations can be carried out in colored solutions, where indicators cannot be used
• There is no need of prior information about the relative strength of titrant before the titration

CONDUCTOMETRIC TITRATION

• Volumetric method based on the measurement of conductance of the solution during the titration

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Process

• Taking a solution to be titrated in a beaker kept in a water bath at a constant temperature.
• Conductivity cell is dipped and connected to a conductivity bridge.
• The titrant is added from the burette(Fig)
• Conductance is measured each addition of solution.
• Recorded value is plotted
• From the graph end point is noted.

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Types of Conductometric Titrations

• Acid –Base titration
• Strong Acid Vs Strong Base
• Weak Acid Vs Strong Base
• Mixture of Weak and Strong Acid Vs Strong Base
• Precipitation titration
• Replacement titration
• Redox titration
• Complexometric titration

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Procedure

• Calibrate the instrument by releasing the calibration knob
• Standard Sodium Hydroxide is taken in the burette
• The given acids is made upto 100ml in the standard measuring flask (SMF)
• 20 ml of made up acids + 20 ml of conductivity water are added in 100 ml beaker
• Conductance is noted for addition of every 1ml of Standard Sodium Hydroxide
• Plot a graph between Volume of Standard Sodium Hydroxide Vs Conductance
• End Points I & II are noted from the graph
• Equivalent Weight of Hydrochloric Acid = 36.5
• Equivalent Weight of Acetic Acid = 60

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Strong Acid Vs Strong Base(HCl Vs NaOH)

• Solution of electrolytes conducts electricity due to the presence of ions. The specific conductance of solution is proportional to the concentration of ions in it. The reaction between HCl and NaOH may be represented as

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• When a solution of hydrochloric acid is titrated with NaOH, the fast moving hydrogen ions are progressively replaced by slow moving sodium ions. As a result conductance of the solution decreases. This decrease in conductance will take place until the end point is reached. Further addition of alkali raises the conductance sharply as there is an excess of hydroxide ions.
• A graph is drawn between volume of NaOH added and the conductance of solution. The exact end point is the point of intersection of the two straight lines.

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HCl + NaOH NaCl + H₂O

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Weak Acid Vs Strong Base�( CH₃COOH Vs NaOH)�

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CH₃COOH+NaOH 3COO‾ + Na⁺ +H2O

Mixture of Weak and Strong Acid Vs Strong Base�(HCl and CH₃COOH Vs NaOH)�

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HCl + NaOH NaCl +H₂O

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CH₃COOH+NaOH CH₃COO^-Na⁺ +H₂O

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Advantages

• This method can be used with very diluted Solution
• This method can be used with Coloured and Turbid solution in which the end point cannot be seen clearly
• This method can be used in which there is no suitable indicator
• Used for acid-base, redox, precipitation titration etc.,

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