Potentiometry Notes


potentiometry

Potentiometry is based exclusively upon the monitoring of potential at an appropriate reference electrode as a determinant critical function of the volume of titrant.

In potentiometric titration reference and indicator, electrodes are immersed in the solution of analyte and potential an indicator electrode is measured with relation to reference electrode.

The titrant is added to the analyte solution and the change in potential is measured. at  the endpoint, there is a sharp change in potential on the indicator electrode

The graph is plotted between indicator electrode potential and volume of titrant added, and the endpoint is determined.

EMF is calculated as,

Electrochemical cells

There are two types of electrochemical cells

  1. Electrolytic cell using electricity
  2. Galvanic cell producing electrical energy

Electrodes used in potentiometry

In potentiometry, the instrument consists of two electrodes

  • Reference electrode
  • Indicator electrode

Reference electrode

The electrode has a standard potential of its own potential value and it is stable when dipped into the sample solution. the salt bridge is used to prevent the interference of the analyte solution with that of the reference solution

Always treated as the left-hand electrode (anode)

Example of the reference electrode

  • Standard hydrogen electrode (SHE)
  • Saturated calomel electrode
  • Silver-silver chloride electrode

Standard hydrogen electrode (SHE)

  • Defined as the potential that is developed between H2 gas adsorbed on the pt metal and H+of the solution
  • It can be used as a primary reference electrode
  • It can be used as both reference and indicator electrode
  • Hydrogen electrode when dipped in the standard acid solution → Reference electrode
  • Hydrogen electrode when dipped in analyte solution → Indicator electrode

Construction

  • It consists of a glass tube having a hole at the bottom
  • This tube consists of one more tube having platinum wire with platinum foil, where the platinum foil is coated with platinum black
  • H2 gas is passed through a tube which can come out from the hole at the bottom of the tube
  • The whole tube is dipped in 1M HCl at 250c

Working

  • The potential of SHE is zero at all temperature
  • When the hydrogen electrode is connected to other electrodes (indicator electrode)through a salt bridge, the potential of the indicator electrode can be measured.
  • When pure & dry H2 gas is passed through the inlet tube, a part of the gas get adsorbed and the excess bubbles out Through the perforation between the H2 gas absorbed on the surface of the metal and H+ of the solution equilibrium is established and an electrical double layer of opposite charges is formed. the potential developed is called H2 electrode potential
  • In the above system, when the H2 gas at a pressure of 1 atm is bubbled through 1M HCl the electrode or formed is called standard hydrogen electrode (SHE) or normal H2 electrode (NHE) this is represented as pt, H2/H+
  • The standard electrode potential is defined as the potential that is developed between the H2 gas adsorbed on the pt metal and H+ of the solution

Advantages

  1. Used as both indicator & reference electrode
  2. Give reproducible result

Disadvantages

  1. Not used if strong oxidizing & reducing agent is present

Uses

  • Determination of pH of the solution
  • Determination of electrode potential of a metal electrode system

Limitation

  • If the solution contains an oxidizing agent, the H2 electrode cannot be used
  • It is rather difficult to regulate the pressure of the H2 gas to be at exactly 1 atm throughout the experiment Saturated calomel electrode (SCE)
  • SCE is a reference electrode based on the reaction between elemental mercury and mercury (I) chloride
  • The aqueous phase in contact with the mercury and the mercury (I) chloride (Calomel) is a saturated solution of potassium chloride in water
  • The electrode is normally linked via a porous fti to the solution in which the other electrode is immersed
  • Saturated calomel electrode

Construction

  • It consists of a glass tube having a sidearm tube
  • Pure mercury is placed at bottom of the tube and is covered by a paste of calomel & KCl
  • The platinum wire is placed in the center of a glass tube, which makes an electrical connection with mercury

 

Working

  • Calomel electrode when connected with indicator electrode by a salt bridge, the electromotive force of indicator electrode is calculated
  • Calomel electrode when connected to ‘H’electrode
  • It is used for measuring the pH of the solution

Advantages

  1. It can be used in various type of solvent
  2. It can be used over a wide pH range

Disadvantages

  1. Unstable at a temperature above 800c
  2. The calomel electrode contains mercury, which poses much greater health hazards than the silver metal used in Ag/AgCl electrode
Potentiometry

Silver-silver chloride (Ag-AgCl) electrode

It is commonly used in electrochemical measurement

For environmental reasons it has widely replaced the saturated calomel electrode

Construction

  • The electrode assembly consists of a silver metal electrode in contact with solid silver chloride (usually as a coating on the silver metal) immersed in an aqueous chloride salt solution saturated with AgCl
  • All these are contained in a small vessel typically made up of a glass tube
  • The internal electrolyte of  the reference electrode assembly & external electrolyte into which the whole assembly is immersed are in ionic contact through the separator
  • A typical separator is a small porous ceramic plug sealed into the end of the glass tubing

Working

The standard potential of this electrode is determined by combining it with hydrogen electrode

Advantages

  1. Simple construction
  2. Inexpensive
  3. Stable potential
  4. Non-toxic components

Disadvantages

  1. UV light decomposes AgCl to give silver
  2. NH3 will complex silver and will dissolve AgCl
  3. Silver sulfide is quite insoluble

Indicator Electrode

  • It is used to measure the potential of the analyte solution comparing with that of the reference electrode
  • It is potential is directly proportional to the ion concentration of the analyte

There are two classes of the indicator electrode

  1. Metal indicator electrode
  2. Ion-selective electrode

Example

a) Glass electrode

b) Metal electrode

Glass electrode

  • It is a type of ion-selective electrode
  • It is made up of a doped glass membrane that is sensitive to specific ion
  • It is commonly used for the measurement of pH
  • This electrode is used in instrumentation for chemical analysis & Physico-chemical studies

Construction

  • It is made up of a combination of the electrode (combine both glass & reference electrode into one body)
  • The bottom of the pH electrode balloons out into a  round thin glass bulb
  • The inner tube contains an unchanging 1×10-7mol/L HCl solution
  • Inside the inner tube is the cathode terminus of the reference probe
  • The anodic terminus wraps itself around the outside of the inner tube
  • The inner tube is filled with a reference solution of KCl and his contact with the solution on the outside of the pH

Working

  • The glass bulb should be placed in a small quantity of water for the electrical conductivity of the glass bulb
  • If the glass bulb undergoes dehydration then there will not be any electrical conductivity, which in turn affect the determination of pH of the sample
  • The glass bulb is composed of soda-lime glass & the test solution consists of H+ ions

Si – O -Na + H+ → Si – O -H + Na+

  • During the reaction, all cations from the glass bulb undergo exchange with H+ ions of the test solution
  • The glass membrane acts as a highly semipermeable membrane for the exchange & absorption of H+ ions

The potential of glass electrode depends upon following

♦ Composition of a glass bulb

♦ The thickness of the glass bulb

♦ The surface area of a glass bulb

Application

Used for determination of pH

Used for determination of the concentration of Lithium, Na, Ammonia, etc

For analysis of food, cosmetics

Used for research purpose

Advantages

  1. Useful over wide pH range
  2. Simple to operate
  3. Used in viscous/Coloured solutions
  4. Not affected by an oxidizing agent, reducing agent
  5. Obey Nernst equation

Disadvantages

  1. Glass is fragile
  2. Minute damage/ scratches on the glass membrane, make it useless
  3. The glass bulb may be hydrated all time
  4. Chance of development of internal resistance

 

Potentiometry

Metal electrode

These develop electric potential in response to redox reaction  on the metal surface

These electrodes are classified into 3 types

a)First kind electrode

b)Second kind of electrode

c)Third kind electrode

 

First kind electrode

They are composed of the metal rod immersed in its metal solution

Electrode respond to the ionic activity

eg. Silver electrode dipped into AgNo3 solution

Ag+ + e → Ag

Copper electrode immersed into copper sulfate solution

Cu2+ 2e→ Cu

Potentiometry

Second kind electrode

Composed of metal wire coated with salt precipitates

Electrode respond to change in ionic activity due to the formation of complex

Mainly used to measure the activity of chloride ion in a solution

eg. Ag electrode & AgCl as its salt precipitate

Ag/AgCl/KCl

 

Third kind electrodes (Inert electrodes or Redox electrode)

Composed of an inert metal electrode immersed in a redox solution hence it is also known as an inert electrode / Redox electrode

The potential of such an inert electrode is determined by the ratio of the reduced & oxidized species in half-reaction

Mainly used platinum as the inert material

eg. Pt- H2 electrode

Potentiometry

Method for determining the endpoint in potentiometry

Normal titration curve method

  • The curve is obtained by plotting the successive value of cell EMF on ordinate & volume of titrant added, this gives an ‘S’ shaped curve
  • When there is a small potential change at the endpoint like in the titration of weak acid & strong base, titration is a very dilute solution, etc. hence it is difficult to locate the endpoint.

2)Analytical/ Derivative method

There are two types of curve, using this curve we can locate the endpoint

a)First derivative curve

  • Here plot of the slope of the curve against the volume of titrant added
  • At the endpoint, there is the maximum change in EMF/pH
  • The endpoint is calculated by drawing the perpendicular peak of a graph on the volume axis

b)Second derivative curve

  • Here we plot the slope of 1st derivative curve against the volume of titrant
  • A point on the volume axis where the curve cuts through zero on ordinate gives the endpoint

3)Gran’s Plot/ Method

  • This method is developed by G.Gran in 1952
  • This method does the numerical manipulation of titration curve into linear straight line intersecting at the equivalence point
  • An endpoint can be obtained by either taking the points before the endpoint/ making the points after the endpoint
  • Result obtained from the linear curve would be more accurate than from non-linear ones
  • The linear straight lines can be extrapolated to the volume axis to locate the endpoint
  • Later on, computer & using ion-selective electrode use of Gran plot is increasing

Advantages of Grans plot

  1. Simple to measure & calculate
  2. Versatility & Precision

Disadvantages of Grans plot

  1. Required skilled person &  special graph paper
  2. Difficult to plot
  3. Time-consuming

Electrochemical cells

There are two types of the electrochemical cell

1)Electrolytic cell → Using electricity

2)Galvanic cell → Producing electrical energy

Potentiometry

Electrolytic cell

It is an electrochemical cell that drives a non-spontaneous redox titration using electrical energy

They are also used to decompose chemical compounds, in a process called electrolysis

e,g. Decomposition of H20 into H & O

Electroplating is done using an electrolytic cell

The electrolysis process involves DC (Direct+current)

The electrolytic cell contains 3 components

a) An electrolyte

b) Cathode

c) Anode

The electrolyte is a solution usually water or other solvents in which ions are dissolved eg. molten salt

When the external voltage applied to the electrodes, the ions in the electrolyte are attracted to the electrode with opposite charge, where a charge transferring reaction (eg. redox) takes place.

Only with an external electrical potential (i.e. voltage) on electrolytic cell decompose a normally stable/ inert chemical compound in solution

2)Galvanic cell

It is also known as voltaic cell

It is an electrochemical cell that derives electrical energy from spontaneous redox reactions taking place within a cell

It consists of two different metals connected by a salt bridge/ individual half cell separated by a porous membrane

The half cell consists of the electrode, that submerged in solution contains cations(+) of the electrode metal & anion(-)to balance the charge of the cation

A full cell consists of two half cells, usually separated by a semi-permeable membrane or by a salt bridge

Ex. Daniell cell → Where salt bridge, which tubes filled with a saturated solution like KCl, NaCl, etc is used as a separator

Potentiometry

  • Through salt bridge movement of ions between solution take place
  • The zinc half cell has a solution of ZnSO4 & the copper half cell has a solution of CuSO4
  • External electrical conductor connect Cu & Zn electrodes
  • In Zn half cell, Zn from the Zn electrode dissolves into the solution as Zn2+ ions (oxidation) releasing electrons that enter the external conductor
  • In Ca half cell, the Cu plate onto Cu electrode(reduction) taking up electrons that leaves the external conductor, since ions (cations) plate onto the Cu electrode, the latter is called as cathode & Zn electrode is anode

Zn  + Cu2+ → Zn2+ + Cu

Application of Potentiometry

  • Analysis of pollutants in water
  • Drug analysis in the pharmaceutical industry
  • Food industry for analysis of quality
  • Biochemical & biological assay or analysis to check the quality of cosmetics
  • Also used as an analytical tool in textile, paper, points,k explosive energy, and more.
  • For measurement of pH of the solution

 

REDOX TITRATION -TYPES, PRINCIPLE & APPLICATION