chapter 14: electrodes and potentiometrywemt.snu.ac.kr/lecture 2013_1/analytical chem/2013...

CHAPTER 14:

ELECTRODES AND POTENTIOMETRY

Chapter 14 Electrodes and Potentiometry

Potentiometry : The use of electrodes to measure voltages that provide chemical

information.

(The cell voltage tells us the activity of one unknown species ( g y p

if the activities of the other species are known).

I Reference Electrode : It maintains a fixed potential (a known fixed composition)I. Reference Electrode : It maintains a fixed potential (a known, fixed composition)

II. Indicator Electrode : It responds to analyte, an electroactive species

that can donate or accept electron at the electrode)

How to use potentiometry :

1) We connect two half-cells (indicator & reference electrodes) by a salt bridge.2) We measure the cell voltage which is the difference between the potential of

the indicator electrode and the constant potential of the reference electrode.3) We make an equation of first degree to get the unknown concentration of analyte.

14-1. Reference Electrodes ?1-15 Fig.in ][Fe][Fe measure toHow 3

2

][Fe

14-1. Reference Electrodes

Figure 14-2 Another view of Fig 15-1Figure 14-2. Another view of Fig. 15-1.The dashed box = reference electrode.

I. Common Reference Electrodes

1) Silver-Silver chloride Electrode

AgCl(s) + e- Ag(s) + Cl- Eo = +0.222

From Nernst EquationFrom Nernst Equation

E = Eo – 0.05916 log [Cl-]

(0.197V) (0.222V)

E ( d KCl) 25°C 0 197 VE (saturated KCl) at 25°C = + 0.197 V

(E = +0.222V when ACl = 1.0)

I. Reference Electrodes

The two half reactions :Right electrode: Fe3+ + e- Fe2+ E+0 = 0.771VLeft electrode: AgCl(s) + e- Ag(s) + Cl- E-o = 0.222V

E (Cell Voltage) = E+ - E-

)][Cl0.05916log()][Fe][Fe0.05916log( 3

2

oo EE

( g ) + -

][Fe

)][Cl0.05916log222.0()][Fe][Fe0.05916log771.0( 3

2

][Fe][Fe0.05916log][Cl0.05916log549.0 3

2-

][Fe

][i) [Cl-] = const., because of saturated KCl solution.ii) As the value of [Fe2+]/[Fe3+] changes, the cell voltage (E) changes.

I. Common Reference Electrodes

2) Calomel Electrode

(Saturated Calomel electrode = S.C.E)

1/2 Hg2Cl2(s) + e- Hg(l) + Cl-

Eo = +0 268V (A = 1)E +0.268V (ACl 1)

Mercury (I)Chloride (Calomel)

][Cllog05916.0 - oEE

(+0.241V at 25oC) (0.268V)

*Saturated KCl solution make the conc. of Cl- does

not change if some of liquid evaporates.

Voltage conversions between different reference scales

14-2. Indicator Electrodes

Indicator Electrodes

1) Metal Electrodei) It responds to a redox reaction at the metal surfacei) It responds to a redox reaction at the metal surface.

ii) It does not participate in many chemical reactions (Inert).

iii) Si l i f i i i l iiii) Simply transmit e- to or from a reactive species in solution.

iv) It works best when its surface is large and clean

(A brief dip in conc. HNO3,followed by rinsing with distilled water

is effective for cleaning).

v) Au, Pt, Ag, Cu, Zn, Cd and Hg can be used as indicator electrode (The

reaction of M Mn+ + ne- should be fast)

14-2. Indicator Electrodes

2) Ion Selective Electrode

i) It i t b d di) It is not based on redox processesii) Selective migration of one type of ion across a membrane

generates an electric potential.g p

14-3. What Is a Junction Potential ?

Junction Potential : Any time two dissimilar electrolyte solution are placedi l i l ( i i l)in contact, an electric voltage ( Junction Potential)develops at their interface.

There is an electric potential difference at the junction of NaCl and H O phases-There is an electric potential difference at the junction of NaCl and H2O phases.

- In Table 14-2, a high concentration of KCl in one solution reduces the magnitude of the potential.magnitude of the potential.

- This is why saturated KCl is used in salt bridges.


- There is an electric potential difference at the junction of NaCl and H2O- There is an electric potential difference at the junction of NaCl and H2O phases.

- Steady-state junction potential : A balance between the unequal mobilitiesthat create a charge imbalance and the tendency of the resulting chargeimbalance to retard the movement of Cl-.

Junction Potential puts a fundamental limitation on the accuracy of direct- Junction Potential puts a fundamental limitation on the accuracy of directpotentiometric measurements, because we don’t know the contributionof the junction to the measured voltage:

E(measured) = E(cell) + E(junction)


- Junction Potential exists at the interface between the salt bridge and eachhalf-cell.

- The junction potentials at each end of a salt bridge often partially cancel each other.

- Although a salt bridge necessarily contributes some unknown net potentialto a galvanic cell, the contribution is fairly small (a few mV).

14-4. How Ion-Selective Electrode Work ?

Ion Selective Electrode

i) It is not based on redox processesii) It d l ti l t iii) It responds selectively to one ion.iii) Key feature of an ideal ion selective electrode:

- A thin membrane ideally capable of binding only the intended ion.A thin membrane ideally capable of binding only the intended ion.- Selective migration of one type of ion across a membrane generate

an electric potential.


For example, Liquid-based ion-selective electrode

- ion-selective electrode : a hydrophobic organic polymer impregnated with aviscous organic solution containing an ionophore (L, ligand)

hi h l ti l bi d th l t ti (C+)which selectively binds the analyte cation (C+).- Inside of the electrode : filling solution with C+ (aq) and B-(aq)- Outside of the electrode : it is immersed in analyte solution with C+ (aq) and A-(aq).-Two reference electrodes: to measure the electric potential difference (voltage)

across the membrane depending on [C+ ] in analyte.

Inside the membrane:

Membrane : polymer (polyvinyl chloride) impregnated with a nonpolar liquid that dissolves L, LC+, and R- (hydrophobic anion)

L : Ionophore. it has high affinity for C+. It binds only C+(ideal electrode).Howeve

it has some affinity for other cations (Real). L is soluble in the organic phase.

R- : hydrophobic anion for charge neutrality. R- cannot leave membrane

because it is soluble in membrane, but poorly soluble in water.

B ld l d iBold colored ions;excess charge in each phase

A- : it cannot enter the membrane because it’s not soluble in the organic phase.

C+ : it diffuses from the membrane to the aqueous solution because of the qfavorable solvation of the ion by water.

In the membrane, LC+ is in equilibrium with L + C+ (small amount).

- As soon as a few C+ ions diffuse from the membrane into the aqueous solution,there is excess positive charge in the aqueous phase.

-This imbalance creates an electric potential difference that opposes diffusion of more C+ into the aqueous phase.

First, Let’s see what happens when C+ diffuses from membrane to outer solution

-When C+ diffuses from membrane (activity in membrane, Am) to the outer aqueous solution (A0),

lnl ti RTGG Am

q ( 0),the free energy change is

lnsolvation RTGGAo

G due to change G due to changein acti itG due to change

in solventin activity

(concentration)

- G is always negative when a species diffuses from a region of high activity toG is always negative when a species diffuses from a region of high activity to one of lower activity.

- When C+ diffuses from membrane to the outer aqueous solution, there is a buildup of a positive charge in the water immediatelyp p g yadjacent to membrane.

- The charge separation creates an electric potential ( Eouter) across the membrane.The charge separation creates an electric potential ( Eouter) across the membrane.

- The free energy difference for C+ in the two phases is G = -nFEouter.

First, Let’s see what happens when C+ diffusesfrom membrane to outer solution

- At equilibrium the net change G from all processes

At equilibrium, the net change G from all processeswith diffusion of C+ across the membrane must be 0.

0)( ln outersolvation

nFERTG

Am

Ao

G due to transfer between G due to phase and activity difference charge imbalance

Electric potential

lsolvation RTGE Am

difference across phaseboundary betweenmembrane and analyte (outer solution):

lnsolvation

outer nFRT

nFE Am

Ao

membrane and analyte (outer solution):-Eouter is proportional to the concentration of C+ in outer analyte solution,

because Am is nearly constant.Ao

Why does is constant during the process?Am

Th h i lAmThe reason why is very nearly constant :

i) C+ + L LC+i) C + L LCIn this equilibrium in the membrane, C+ is very small, but LC+ is high concentration.

ii) R- ( hydrophobic) is poorly soluble in water and thus cannot leave the membrane.Very little C+ can diffuse out of the membrane because each C+ that entersthe aqueous phase leaves behind one R- .

iii) As soon as a tiny fraction of C+ diffuse from the membrane into solution, further diffusion is prevented by excess positive charge in the solution near the membrane.

Second, Let’s see what happens when C+ diffusesfrom membrane to inner filling solution

- At equilibrium the net change G from all processes

At equilibrium, the net change G from all processeswith diffusion of C+ across the membrane must be 0.

0)( ln innersolvation

nFERTG

Am

Ai

G due to transfer between G due to phase and activity difference charge imbalance

Electric potentialdifference across phaseboundary between

lnsolvation

inner nFRT

nFGE

Am

Aiymembrane and inner filling solution

Ai

- Einner is constant because and are constant. Ai Am


- The potential difference between the outer and inner solution :

innersolvation

innerouter ln EF

RTF

GEEE

Am

A

- The potential difference between the outer and inner solution :

innersolvation ln- ln ERTRTG

E

Ao Am

innerinnerouter nFnF

Ao

innernFnFnF

o m

C Constant ConstantConstant Constant Constant

- Combing the constant terms,

lnconstant

nFRTE Ao

Electric potential

)C25at (volts log05916.0constant

nE Ao

lect ic potentialdifference for ion-selective electrode:


lnconstant

RTE Alnconstant

nF

E Ao

)C25at (volts log05916.0constant E Ao

Electric potentialdifference for ion-selective electrode: )(g

nselective electrode:

Remarks:

i) This equation applies to any ion-selective electrode (including a glass pH electrode).

ii) If the analyte is anion, the sign n (charge of analyte ) is negative.

iii) A difference of 4.00 pH units would lead to a potential difference of ) p p4.00 x 59.16 = 237 mV.

iv) For every factor - of - 10 change in activity of Ca+2 would lead toiv) For every factor of 10 change in activity of Ca would lead to a potential difference of 59.16/2 = 29.58 mV.

Ion Selective Electrodes

Nitrate Chloride

Ion Selective Electrodes

14-6. Ion-Selective Electrodes

*Doctor needs blood chemistry informationquickly to help a critically ill patientquickly to help a critically ill patientmake a diagnosis and begin treatment.

Ion-selective electrodes are the method ofIon-selective electrodes are the method of choice for Na+, K +, Cl- , pH, and Pco2

.


Selectivity Coefficient : the relative response of the electrodes to different species with same charge.

* No electrode responds exclusively to one kind of ion.

Selectivity coefficient: (14-9)

A: analyte ion, X: interfering iony gPot: potentiometric


Response of ion-selective electrode:

(14-10)

th ti it f i i th ti it f i t f i iA A: the activity of primary ion, : the activity of interfering species,: the selectivity coefficient, + : A is a cation, - : A is an anion

AA AX

XA,k

Z : the magnitude of charge A

Changes in in the solution change the potential difference E across theA

Reminder : How Ion-Selective Electrode Work

ZA : the magnitude of charge A

- Changes in in the solution change the potential difference, E across the outer boundery of the ion selective electrode.

- By using the calibration curve, E is related to

AA

AA

- An ion-selective electrode responds to the activity of free analyte, not complexed analyte.


Classification of Ion-selective electrodes :

1) Gl b f H+ d t i l t ti1) Glass membranes for H+ and certain monovalent cations.

2) Solid-state electrodes2) Solid state electrodes

3) Liquid-based electrodes

4) Compound electrodes

2) Solid – State Electrodes


Sit t it t t ti f F i id th t lSite to site transportation of F – inside the crystal.

Filling solution ; 0.1 M NaF and 0.1 M NaCl


Response of F- electrode:

(14-12)

- The electrode is more responsive to F- than to most other ions by > 1,000.

- OH- is the only interfering species ( kF-, OH- = 0.1 )

- At low pH, F- is converted to HF (pKa = 3.17), to which the electrode is insensitive.

-At pH is 5.5, no interference by OH- and little conversion of F- to HF.

- Citrate complexes Fe 3+ and Al 3+, which would otherwise bind F-p ,and interfere with the analyte, F -.


E = constant - (0.05916) log AF (outside) (14-12)Response of F- electrode:

E constant (0.05916) log AF- (outside) (14 12)

A routine procedure is to dilute the unknown in a high ionic strength buffer containing- A routine procedure is to dilute the unknown in a high ionic strength buffer containing

acetic acid, sodium citrate, NaCl, and NaOH to adjust the pH to 5.5.

Th b ff k ll t d d d k t t t i i t th th- The buffer keeps all standards and unknowns at a constant ionic strength, so the fluoride activity coefficient is constant in all solutions (and can therefore be ignored).

E = constant - (0.05916) log [F-]F-

= constant - (0.05916) logF- - (0.05916) log [F-]

This expression is constant becausepF- is constant at constant ionic strength

3) Liquid–Based Ion–Selective Electrodes

-A liquid-based ion-selective electrode is similar to the solid-state electrode (Fig 14 19) except that the liquid based electrode has a hydrophobic membrane(Fig. 14-19), except that the liquid-based electrode has a hydrophobic membrane

impregnated with a hydrophobic ion exchanger (ionophore) that is selective for analyte ion (Fig. 14-23),

(Fig 14 19) (Fig. 14-23)(Fig. 14-19)


L; Ionophore (neutral)i fR- ; anion for charge

neutralization


- A liquid-based ion-selective electrode is similar to the solid-state electrode

(14-13)

Response of Ca2+ electrode:

(14 13)

β is close to 1.00

Breakthrough in Ion-Selective Electrode Detection Limits

Black curve: the electrode detects changes in the Pb+2 concentration above 10-6 M, but not below 10 -6 M when PbCl2 = 0.5 mM in the internal solution.

Blue Curve: the elctrode responds to change in Pb+2 concentration down to ~10-11M.when PbCl2 =10 -12 M in the internal solution by a metal ion buffer.2 y go to 14-7 metal ion buffers

Breakthrough in Ion-Selective Electrode Detection Limits

- The sensitivity of liquid-based ion selective electrodes has been limited by the l k f th i i (Pb+2) f th i t l filli l ti th h th ileakage of the primary ion (Pb+2) from the internal filling solution through the ion selective membrane.

- Leakage provides a substantial concentration of primary ion at the external of the membrane (10- 6 M) and thus the electrode always responds to 10- 6 M although the real analyte concentration is far below 10- 6 M.y

4) Compound Electrodes

- Compounds electrodes contain- Compounds electrodes contain a conventional electrode surrounded by a membrane that isolates (or generates)th l t t hi h th l t d dthe analyte to which the electrode responds.

-CO2 diffuses through the semi-permeable bb brubber membrane,

it lowers the pH in the electrolyte.

-The response of the glass pH electrode to the change in pH is a measure of the CO2 concentration outside the electrode.2

14-7. Using Ion – Selective Electrodesg

Advantages of ion-selective electrodes:

1) Linear response to log over a wide range (4 to 6 orders of magnitude).A

2) Nondestructive ( they don’t consume unknown ).

3) Noncontaminating (they introduce negligible contamination.)* They can be used inside living cells.

4) Short response time (seconds or minutes)

5) Unaffected by color or turbidity (unlike spectrophotometry and titrations)5) Unaffected by color or turbidity (unlike spectrophotometry and titrations).

14-7. Using Ion – Selective Electrodes

Di d t f i l ti l t dDisadvantages of ion-selective electrodes:

1) They respond to the activity of analytes .1) They respond to the activity of analytes .*we usually want concentrations, not activities

2) They respond to the activity of uncomplexed ions (advantage or disadvantage ?)2) They respond to the activity of uncomplexed ions (advantage or disadvantage ?)

3) Precision is rarely better than 1%.* I V 4% i l t i ti it* I mV error 4% error in monovalent ion activity.

It doubles for divalent ions and tripled for trivalent ions.

4) They can be fouled by organic solutes like proteins which leads to sluggish, drifting response.

5) They are fragile and have limited shelf-life.

2) They respond to the activity of uncomplexed ions (advantage or disadvantage ?)

14-7. Using Ion – Selective Electrodes

Disadvantages of ion-selective electrodes:Disadvantages of ion selective electrodes:

1) They respond to the activity of analytes .* ll i i i i*we usually want concentrations, not activities.

How to solve this problem:

An inert salt is added to all the standards and samples to bring them to constant and high ionic strength. If the activity coefficients remain constant, g g y ,the electrode potential gives concentrations directly because we know the concentrations of standards.

Standard Addition with Ion - Selective Electrodes

Matrix: the medium in which the analyte exists.yStandard Addition Method:

- It is used when the matrix of unknown sample is different from that of standard.- The electrode immersed in the unknown and then a small volume of standardThe electrode immersed in the unknown and then a small volume of standard

solution is added intermittently until the concentration of the analyte increases to 1.5 to 3 times its original concentration.

- The graphical procedure based on the equation for the response of the ion-selective electrode.

X]log[10lnβ

nFRTkE (15-9)

n

E : meter reading in volts

0.05916 V at 25 0C

g[X] : concentration of analyte

: constants depending on particular ion-selective electrodeβ,k

15-7. Standard Addition with Ion - Selective Electrodes

X]log[10lnβ

RTkE (14-14)X]log[β

nF

kE (14 14)

Let V0 : the initial volume of unknownC : the initial concentration of analyteCx : the initial concentration of analyteVs : the volume of added standardCs : the concentration of standard

Then the concentration of analyte after standard is added:(V0 C + V C ) / (V0 + V )(V0 Cx + Vs Cs ) / (V0 + Vs )

Substituting this expression for [X] in equation 15-9 gives 15-10

(V0 + VS)10E/S = 10k/SV0cX + 10k/ScSVS (14-15)y b m

xWhere S =

nFRT 10lnβ

(V0 + VS)10E/S = 10k/SV0cX + 10k/ScSVS (14 15)y b m

x

(14-15)

S

X0

S/

X0/

1010

interceptccV

ccV

mbx Sk

Sk

(14-16)

From equation (15-12), Cx is obtained from x-intercept, Cs and V0.

Metal Ion Buffers

It i i tl t dil t C Cl t 10 6 M f t d di i i l ti l t d- It is pointless to dilute CaCl2 to 10-6 M for standardizing an ion-selective electrode.At this low concentration, Ca +2 will be lost by adsorption on glass or reaction withimpurities.

* Glass vessels are not used for very dilute solutions, because ions are adsorbed on the glass. Plastic bottles are better than glass for dilute solutions.

* Adding strong acid (0.1 – 1M) to any solution helps minimize adsorption of cations on the wall of container. It is because H+ competes with other cationson the wall of container. It is because H competes with other cations for ion-exchange sites.

* An alternative is to prepare a metal ion buffer from the metal and a suitable ligand An alternative is to prepare a metal ion buffer from the metal and a suitable ligand.

Metal Ion Buffers is used to keep metal ion concentration so low !

Metal Ion Buffers

M t l I B ff i d t k t l i t ti lMetal Ion Buffers is used to keep metal ion concentration so low.

- How to keep Ca+2 concentration less than 10-6 ?

Ca2+ + NTA3- CaNTA- ( at high pH)

p C

(15-14)346.6

32f KNO M 0.1in 10]][NTA[Ca

][CaNTA

K

]][NTA[Ca

If you add NTA at high pH into the high concentration of Ca+2 solution 3 2+

][CaNTA

and make [CaNTA-] = [NTA3-] , then you obtain so low Ca2+ concentrationwhich does not vary substantially due to the metal buffer action.

M 10][NTA][CaNTA]Ca[ 46.6

3f

2

K

chapter 14: electrodes and potentiometrywemt.snu.ac.kr/lecture 2013_1/analytical chem/2013...

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