Type of electrodes: The potential of an electrode changes with change in the activity of

the electrolyte.Potentiometric method

Direct Potentiometric method Potentiomtric titrationsA Calibration Curve by varyi- change in EMF of an electrolytic ing the conc. of an active species cell measured through the

addition may be derived and from of a titrantthat curve an unknown conc. E=Eo – 0.0591\n log [Red]\[ox]of the active sp. is deterimned. Ex. Agno3 vs Nacl titration ph meter is a potentiometer in A species of interest is titrated which potential is measured against a suitable titrant andin terms of ph. equivalence point is obtained.

Equipment required for potentiometric measurements:-

1. Reference electrode

2. Indicator electrode

3. Potential measuring device

Indicator Electrode:- two types

1. Metal Indicator Electrode

2. Membrane Electrode

(i) Metal Indicator Electrode of Ist Kind for cations:-

Electrode of this type are reversible with respect to the ions of the

metal phase. It consists of a metal in contact with a solution of its

own ion.

Examples:

Zinc rod in ZnSo4 solution

Cu rod in CuSo4 solution

Ag rod in AgNo3 solution

(ii) Metal Indicator Electrodes of IInd type for anions– These electrodes consists of a metal coated with a layer of its spraingly soluble salt dipped in a solution of a soluble salt having same anion.

Example:silver-silverchloride eletrode, calomel electrode

Silver – silverchloride electrode:- It is consists of a silver wire coated with a layer of silver chloride inserted in a solution of KCl of HCl of known concentration.

At the Ag/AgCl solution inter face the electrochemical equilibrium is AgCl(s) + e Ago +Cl- In addition there is chemical equilibrium----

AgCl(s) Ag+ + Cl-

Schematic representation of electrode:- Ag, AgCl(s) ; KCl (aq)

The electrode is reversible with respect to Cl- ion.Calemal Electrode :- It consists of mercury, solid mercurous chloride and a solution of KCl. Schematic representation:-

Hg, Hg2Cl2(s); KCl (solution)

If electrode reaction is reduction, Hg++ ions given by sparingly soluble Hg2Cl2 get dischanged at electrode. So more Hg2Cl2 would pass into solution and the conc. of Cl- ion increases.

Hg++(aq)+2e- 2Hg(l)

Hg2Cl2 (s) Hg2++ (aq) + 2Cl– (aq)

If electrode reaction is oxidation, then it liberate electrons and send Hg++ ions in solution. The Hg++ ions combine with Cl- ions (furnished by KCl) forming sparingly soluble Hg2Cl2. So conc. Of Cl- ions decreases in solution.

2Hg(l) Hg2 ++ (aq)+ 2e-

Hg2++ (aq) + 2Cl- (aq) Hg2Cl2(s)

Electrode reaction is--- Hg2Cl2(s)+2e- 2Hg (l)+2Cl- (aq) So electrode is reversible with respect to Cl- ion

Electrode of this kind can be used for direct determination of the activity of either the metal ions the anion in the coating, also used as indicator electrode to follow titration involving either.

Thind type:- This type of electrode consists of a metal, one of its insoluble salts, another insoluble salt of another metal having same anion and solution of any soluble salt of the latter having same cation

Pb PbC2O4(S) CaC2O4(S) CaCl2(aq)Lead Ist Dissolves to form Pb++ ions with the liberation of 2 electrons. The lead ions then Combine with C2O4

-- ions to form insoluble lead oxalate

Pb Pb++ + 2e-

Pb++ + C2O4- - PbC2O4 (s)

The removal of C2O4-- ions from solution results in dissolving of

Calcium oxalate which ionises to maintain the equilibrium CaC2O4(s) Ca++ + C2O4

-- Pb + CaC2O4 PbC2O4+ Ca++ + 2e-

The system behaves as an electrode reversible with respect to Ca++ ions.

Metal Indicator Electrode of third kind----

Pb PbC2O4 CaC2O4 CaCl2

Pb Pb+++ 2e-

Pb++ + C2O4-- PbC2O4

CaC2O4 Ca2+ + C2O4--

Pb +CaC2O4 Pb C2O4 + Ca2+ + 2e-

Metal Indicator Electrode of third kind----

Pb PbC2O4 CaC2O4 CaCl2

Pb Pb+++ 2e-

Pb++ + C2O4-- PbC2O4

CaC2O4 Ca2+ + C2O4--

Pb +CaC2O4 Pb C2O4 + Ca2+ + 2e-

Electrode of the 4th kind or oxidation Reduction electrode(Inert electrode):-• The term oxidation-reduction electrode is used for those electrodes in which the potential is developed due to the presence of ions of same substance in two different oxidation states. • It is set up by inserting an unattackable metal (Platinum or gold) in an appropriate solution. When platinum wire is inserted in a solution containing Fe++ and Fe+++ ions or Sn++ and Sn++++ ions or Ce+++ and Ce++++ ions, the wire acquires potential. The potential at electrode arises due to tendency of ions in one oxidation state to change in to more stable oxidation state.Electrode reactions--- Fe+++ (aq) + e- Fe++ (aq) Sn++++(aq) + 2e- Sn++ (aq) Ce++++ (aq) +e- Ce+++ (aq)

• The function of Pt wire is to ‘pick up’ the electrons and to provide electrical Contact to the electrode.

Quinhydrone electrode – Oxidation reduction electrode •It consists of a platinum wire placed in a solution containing hydroquinone (QH2) and quinone(Q) in equimolar amounts. Electrode reaction--- O= =O + 2e- + 2H+ HO= =OH quinone HydroquinoneScematic representation of electrode ----- Pt, Q, QH2; H+ (aq) It is reversible with respect to H+ ions.

O

CONCENTRATION CELLS

• In galvanic cells electrical energy arises from chemical reactions

• In concentration cells EMF arises not due to any chemical • reaction but due to transfer of matter from one half cell to

other because of the difference in concentration of the species involved.

• Concentration cell one of two types - Electrode Concentration cells Electrolyte concentration cells Electrode Concentration cells - In these cells, two like

electrode at different concentrations are dipped in the same solution.

• ex. Two hydrogen electrodes at unequal gas pressures immersed in the same solution of H+ ions constitute an electrode-concentration cell.

Schematic Representation Pt; H2(P1) Solution of H+ ions, (ex. HCl) H2 (P2); pt

Reactions:- R.H.E : 2H+ + 2e- H2 (P2)

L.H.E: H2 (P1) 2H+ + 2e-

Overall: H2 (P1) H2(P2)

Reaction is independent of the conc. of electrolyte. At moderate pressures H2 considered as ideal gas.

So Ratio of activities = Ratio of gas pressures.

So nernst equation --

E=Eo – 0.591/2 log (P2/P1) at 25oC

By definition Eo=0

E= - 0.02955 log P2/P1 = 0.02955 log P1/P2

When P2 <P1, EMF is +ve and the process is spontaneous equivalent to expansion of H2 gas.

2ndexample- An amalgam with two different concentrations of the same metal.

Hg- Pb(C1), PbSO4 (soln.), Hg-Pb(C2)

Electrode Reactions:

R.H.E. Pb++ + 2e- Pb(C2)

L.H.E. Pb(C1) Pb++ + 2e-

Overall Pb(C1) Pb(C2)

EMF of the Cell – E = ER –EL

E = (E0Pb- 0.0591/2 log C2) – (E0

Pb - 0.0591/2 log C1)

= 0.0591/2 log C1/C2 = 0.02955 log C1/C2

If C2< C1, EMF is positive , process spontaneous i.e. lead will go spontaneously from high conc. to low conc. amalgam.

Electrolyte Concentration cells- In these cells two electrodes of the same metal are dipped in the solutions of metal ions of different concentrations and hence of different activities.

Zn, Zn++ (a+)1 Zn++ (a+)2, Zn

Both electrodes of zinc metal are in contact with solution of Zn++ ions, but the conc. and activities of ions are different.

Activities of zinc ions in two electrolytes – (a+)1 and (a+)2

The two electrolytes (ZnSO4) Solutions separated from salt bridgeReactions R.H.E. : Zn++ (a+)2 + 2e- Zn(s) Reduction L.H.E. : Zn (s) Zn++ (a+)1 + 2e- OxidationOverall: Zn++ (a+)2 Zn++ (a+)1

The net process involves transfer of 1 mole of Zn++ ions from the solution of activity (a+)2 to the solution of activity (a+)1.

Acc. to Nernst equation - Reduction potentials of RHE & LHE

ER = Eoel – RT/nF ln 1/(a+)2 = Eo

el + RT/nF ln (a+)2 ---(1)

EL= Eoel – RT/nF ln 1/(a+)1 = Eo

el + RT/nF ln (a+)1 ---(2)

ECell = ER – EL = RT/nF ln (a+)2/(a+)1 ----(3)

For the process to be feasible, EMF should be +ve. Hence

(a+)2 > (a+)1

a = ym

E Cell = RT/2F ln y2m2/y1m1

y1 & y2 - activity coefficients of the electrolytes

Other example of conc. Cell--- Pt, H2(g), H+(a+)1 ll H+(a+)2, H2(g),Pt 1atm 1atmBoth Hydrogen electrodes are in contact with H+ ions of different activities. The two solutions of HCl are separated by a salt bridge.Reactions--

R.H.E. : H+ (a+)2 + e- ½ H2 (g) ReductionL.H.E. : ½ H2(g) H+(a+)1 + e- OxidationOverall: H+ (a+)2 H+(a+)1

ECell = ER – EL

= [Eoel – RT/F ln 1/(a+)2] – [Eo

el – RT/F ln 1/(a+)1] = RT/F ln (a+)2/(a+)1

For E to be +ve (a+)2 Should be greater than (a+)1

Types of Electrolyte concentration Cells---

Electrolyte conc. Cells in which solutions of the same electrolyte

of different conc. are used are of two types —

1. Concentration cells without transference

The two electrolytic solutions are not in direct contact with

each other. So transference of ions does not take place

directly.

solutions ore separated by salt bridge.

2. Conc. Cell with transference.

The two solutions are in direct contact.

Transference of ions take place directly.

Concentration Cells Without Transference.

Concentration cells already discussed( Zn and H) separated by

salt bridge are concentration cells without transference.

But some other means can also be used to keep the solutions apart

and bringing the transference of ions indirectly.

Ex. Pt, H2 (g), HCl (a1) AgCl(s), Ag 1atmSuppose : activity of H+ ions ---- (a+)1

activity of Cl- ions ----(a-)1

R.H.E. --- Reduction half Cell reaction AgCl (s ) + e- Cl- (a-)1 + Ag (s)----------- (1)L.H.E.--- Oxidation Half Cell reaction ½ H2(g) H+ (a+)1 + e- -------------- (2)

Net reaction – (1) + (2)

½ H2(g) + AgCl (s) H+ (a+)1 + Cl- (a-)1 + Ag (s ) -----(3)

Same cell with activity of HCl solution a2---Pt, H2(g), HCl (a2) AgCl (s), Ag 1atm

Net Cell reaction

½ H2 (g) + AgCl (s) H+(a+)2 + Cl- (a-)2 + Ag (s) -----(4)

Now the two cells are connected in such a way that they send

current in opposite derection

Pt, H2(g),HCl (a1), AgCl (s), Ag Ag, AgCl (s), HCl (a2), H2(g),pt. 1atm 1atm

for the passage of 1 faraday of electricity, overall reaction of combined Cell ------ 4 - 3 H+ (a+)2 + Cl- (a-)2 H+(a+)1+ Cl- (a-)1

So, for the passage of 1 faraday of electricity, the overall reaction is the transfer of 1 mole of H+ and Cl- ions or 1 mole of HCl from the solution of activity a2 to that of activity a1

EMF of Cell –

Ew.o.t. = RT/F ln (a+)2/(a+)1 + RT/F ln (a-)2/(a-)1

= RT/F ln (a+)22/(a+)2

1

= 2RT/F ln (a+)2/(a+)1

(a+ )1 & (a+ )2 -- Mean ionic activities of the electrolyte in 2 solutions Ew.o.t. = (RT/F) ln (a2/a1) a1 & a2 activities of HCl in two solutions Transfer of electrolyte from one to another solution takes place indirectly so the cell is cellw.o.t

If middle electrode Ag, AgCl(s) is withdrawn then two solutions of HCl will be in direct contact and cell becomes conc. Cell with transference. So conc. cell with transference in which the electrodes one reversible with respect to cations (H+ ions) can be converted in conc. Cell without transference by interposing another electrode which is reversible with respect to anions (Cl- ions in this case).

Another Example---

(1)Zn, ZnSO4(a1), PbSO4(s), Pb,PbSO4(s), ZnSO4(a2),Zn

Electrodes are reversible with respect to cation.

(2) Ag, AgCl(s), HCl(a1), H2(g), HCl(a2), AgCl(s), Ag

The end electrodes are reversible with respect to Cl-ions while the intermediate electrode is reversible with respect to H+ions.

Concentration Cell with Transference.-----

Concentration cell formed by combining two hydrogen gas

electrodes in contact with HCl solutions of different

concentrations. The two solutions are in direct contact with

each other :Pt, H2(g), HCl(a1) : HCl(a2), H2(g), Pt

1atm H+ : 1atm

Cl-

The reaction on the left involves oxidation and that on the

right involves reduction.

The following changes are involved for the flow of one faraday

of electricity:

Left hand electrode : 1/2H2(g) H+(a+)1 + e- -------(1)

Right hand electrode : H+ (a+)2 + e- 1/2H2(g) --------(2)

H+ ions are generated at LHE and consumed at RHE as the

current follows.

Solutions are in direct contact with each other, ions are free to

move from one solution to another when current flows through

the cell.

H+ ions move from the solution on left hand side to that on the

right hand side.

Since anions move in direction opposite to that of cations move,

Cl- ions migrate from right to left. Pt, H2(g), HCl(a1) : HCl(a2), H2(g), Pt

1atm : 1atm H+ Cl-

• If t- is the transport number of Cl- ion and t+ (= 1- t-) for H+

ion in HCl

• For one faraday of electricity passing through, t–

faraday will be carried by Cl- ions and t+ faraday by H+ ions.

• According to Faraday’s second law, t- equivalent of Cl- ions

will be transferred from the solution of activity a2 to the

solution of activity a1.

t- Cl- (a-)2 t- Cl- (a-)1 -------------- (3)

At the same time, t+ equivalent of H+ will be transferred from

the solution of activity a1 to that of activity a2

t+ H+ (a+)1 t+ H+ (a+)2 -------------- (4)

The net result for the flow of one faraday of electricity is -

Left hand electrode

Gain of 1 gram equivalent of H+ ions by process (1)

1/2H2(g) H+(a+)1 + e-

Loss of t+ gram equivalent of H+ ions by process (4)

t- H+ (a+)1 t- H+ (a+)2

Therefore, Net gain of H+ ions = (1 - t+) gram equivalent = t- gram equivalent

At the same time,Net gain of Cl- ions = t- grams equivalent by process (3)

Right hand electrode

Loss of 1 gram equivalent of H+ ions by process (2)

Gain of t+ gram equivalent of H+ ions by process (4)

Therefore, Net loss of H+ ions = (1 - t+) gram equivalent

= t- gram equivalent At the same time,Net gain of Cl- ions = t- grams equivalent by process (3)

Thus, for every one faraday of electricity ,

there is net transfer of t- gm equivalent of H+ ions and t- gm

equivalent of Cl- ions from right to left i.e. from the solution in

which activity of HCl is a2 to that in which activity of HCl is a1.

t- H+ (a+)2 t- H+ (a+)1

t- Cl- (a-)2 t- Cl- (a-)1

The EMF of concentration cell with transference---

Ew.t.= t- RT/F ln (a+)2/(a+)1 + t- RT/F ln (a-)2/(a-)1 -------(5)

(a+)1 and (a+)2 --- mean ionic activities of 2 HCl solutions.

(a +)12 = (a+)1 (a-)1 and (a+)2

2 = (a+)2 (a-)2

So eq. (5) may be written as

Ew.t. = t- RT/F ln (a+)22/(a+)1

2 ---------------------(6)

Activity of uni-univalent electrolyte is given by-------

a = (a+)2

Ew.t. = t- RT/F ln a2/a1

a2 and a1 - activities of HCl solutions of R.H.E and L.H.E.

Reference Electrodes ----- The electrodes of standard potential with which we can compare the potentials of an other electrode, is called a reference electrode. Ex.(1). SHE--- whose electrode pot at all temp. is taken as zero.(2) Calomel electrode – Mercury-mercurous chloride electrode. The potential of the calomel electrode on the Hydrogen scale has been found to very with the conc. Of the KCl solution. The reduction potentials for the various KCl concentration at 25oC are—

Electrode Reaction – Hg Cl2(s) + 2e- Hg(l) + 2Cl-(Oa)The pot of this electrode (due to reduction of Mercurous ion to mercury) is eanal to end of cell.Pt H2(atm) H+(1m) KCl (o.1N ) Hg2 Cl2 Hg

KCl conc. 0.1N( 1N(none) Saturated

EP in volt. 0.3335 0.2810 0.2422

Liquid Junction Potential

• When two salt solutions of different concentration are placed

in contact with one another,the ions from the concentrated

solution will tend to diffuse in to dilute one.

• The rate of diffusion of each ion is approximately proportional

to the speed of the ion in electric field.

• Suppose positive ion moves with a greater speed than the

negative ion. It means that the positive ion from the conc.

solution will diffuse ahead of the –ve ion in the dilute solution.

• Thus dilute solution becomes positively changed with respect to

concentrated solution. .

• Suppose the negative ion moves faster. It means that negative

ion will diffuse rapidly in dilute solution than the positive ion

and the dilute solution gets a negative change.

• In both cases, an electrical double layer is set up at the

junction of the two solutions and thus a potential difference is

setup at this junction.

• This potential difference developed at the junction of two

solutions of different concentration is termed as liquid liquid

junction potential (LJP).

(1) If two liquid one moving with the same speed, there will not

be any liquid –liquid junction potential.

(2) The liquid-liquid junction potential is due to the different

migration velocities of the two ions.

(3) Magnitude of liquid-liquid junction potential depends on the

relative speed of the ions.