announcements course evaluations final is wednesday afternoon on may 9 th homework 14-15, 14-26,...

Announcements

Course Evaluations

Final is Wednesday Afternoon on May 9th

Homework 14-15, 14-26, 15-6, 16-3, 16-6,

Course Evaluations

Request from ThelmoRequest from Thelmo “Teaching is a complex endeavor, capable of an almost

infinite variety of successful expressions, and thus, success as a teacher cannot be judged by any one criterion or through one single mechanism.”

Consider the many facets of the learning environment over the course of whole semester

How will your feedback be used?How will your feedback be used? Read by me to evaluate which aspects of the course most

contribute to student learning formative

Read by PSC as part of faculty’s permanent file to evaluate faculty for promotion and tenure

summative

Concentration Concentration dependency of Edependency of E

Concentration Dependency of E

Eo values are based on standard conditions. The E value will vary if any of the concentration vary

from standard conditions Theoretically

Predicted by the Nernst Equation

The Nernst Equation

The Nernst Equation

For aA + ne- bB

Qn

VEE o log

05916.0

aA

bBo

n

VEE

AA

log05916.0

Example Equilibrium constant and Eo

Find the equilibrium constant for the reaction

Cu (s) + 2Fe3+ 2Fe2+ + Cu2+

Example

Find the voltage of the cell Half reaction

Ag (s) into a solution of 0.50 M AgNO3 (aq)

The other half-reactionCd (s) is immersed into a 0.010 M Cd(NO3)2 (aq)

Metals are connected by wires Solution connected with salt bridge

0.50 M AgNO0.50 M AgNO330.010 M Cd(NO0.010 M Cd(NO33))22

Find the voltage of the cell

Potentiometric Methods

Potentiometric Methods

Basis of Method The difference b/w the E (not Eo) values for two

halves of a cell give rise to Eoverall.,

If one half reaction is known and held constant, we can measure the concentration of species on the other side!!!

ReferenceReference

Indicating Indicating electrode – electrode – The part of the cell The part of the cell that contains the that contains the solutions we are solutions we are interested in interested in measuringmeasuring

Electrodes The previous cell would

be difficult to use for many systems. We would like something

that can be placed in the solution we wish to measure

The electrodes in the following slides have that goal in mind but THEY STILL represent a

complete electrochemical cell when used

Reference Electrodes

Ag/AgCl


Calomel Electrode (SCE) Very Common Hg|Hg2Cl2 (sat), KCl|| Chloride is used to

maintain constant ionic strength


The SCE (Saturated Calomel Electrode) Different KCl concentrations

can (and are used) 0.1 M – least temperature

sensitive Saturated – easier to make

and maintain.

Eref = 0.244 V @STP

Reaction Eo (V)Hg2Cl2 + 2e- ->2Hg(l) + 2Cl- 0.244 V

HgHg22ClCl22 + 2e + 2e-- ->2Hg(l) + 2Cl ->2Hg(l) + 2Cl-- 0.241 0.241AgClAgCl (s) (s) + e + e-- ->Ag(s) + Cl ->Ag(s) + Cl-- 0.197 0.197

Sensing electrodes

Several types Simple Metal Solid State Electrodes Glass Membrane Etc.

Let’s look at some examples.

Simple metal electrodes

A bare metal in contact with a solution.

General Form: Mn+ + ne- -> M(s)

Simple Metal Electrodes A bare metal in contact with a

solution of its cation.

Ag+ + 1e- -> Ag(s) 0.799 V

General formMn+ + ne- -> M(s)

][

1log

0592.0

AgnEE o

ind

][

1log

0592.0n

oind Mn

EE

Example

A potential of 0.5000V was measured vs. SCE. What is the concentration of Ag+?

Hg2Cl2 + 2e- 2Hg(l) + 2 Cl- Eo = 0.241V

Ag+ + e- Ag(s) Eo = 0.799VUsing a simple metal electrode (Ag) and a reference electrode (Calomel), the voltage determined from this potentiometric set-up provides us with a direct measure of concentration

no calibration plot required!!no calibration plot required!!

Simple Metal Electrodes

Example Silver sensing

electrode

][

1log

1

0592.0799.0

AgVEE indred

])log[(0592.0799.0 AgVEind pAgVEind 0592.0799.0

redoxcell EEE redEVV 244.05000.0 pAgVVV 0592.0799.0241.05000.0

pAg0592.0058.0 pAg

0592.0

0580.0

Example (cont’d)

pAg

0592.0

0580.0

pAg9797.0

][101 9797.0 Agx

][104.0 7 AgM

Simple Metal Electrodes For some metals, a good electrode can’t be made or no metals

are involved – just ions or gas! An inert indicating electrode is used (graphite or Pt).

This type only measures the ratios of ions.

No quantitation but suitable for titrations!No quantitation but suitable for titrations!

Simple Metal Electrodes For some metals, a good

electrode can’t be made or no metals are involved – just ions or gas! An inert indicating electrode is used (graphite or Pt).


No quantitation but suitable for titrations!

Calomel(Hg2Cl2)

Simple Metal Electrodes

Eoverall = Eox + Ered

Calomel(Hg2Cl2)

][

][log

1

0592.03

2

Fe

FeEE o

red

Constant = -0.241 V

Reduction at Platinum Electrode:Reduction at Platinum Electrode:

Reaction Eo

Fe3+ + 1e- -> Fe2+ 0.771 V

][

][log0592.0771.0

3

2

Fe

FeVEred

Simple Metal Electrodes For some metals, a good

electrode can’t be made or no metals are involved – just ions or gas!


No direct quantitation but suitable for titrations! Calomel

(Hg2Cl2)

CeCe4+4+

REDOX titrations

“Your titrant is commonly an oxidizing agent although reducing titrants can be used.”

Consider:Ce4+ + Fe2+ Ce3+ + Fe3+

General form:Aox + Bred Ared + Box

Determination of the Equivalence Point

The equivalence point is based on the concentration of the oxidized and reduced form of all species involved Use Nernst Equation to find Eeq.

Equivalence Point

][

][log

05916.0

ox

red

A

oAA A

A

n

VEE

][

][log

05916.0

ox

red

B

oBB B

B

n

VEE

Since at equilibrium, [Ared] = [Box] and [Bred] = [Aox] we massage the two general equations to yield:

BA

oBB

oAA

eq nn

EnEnE

Nernst Equation for ANernst Equation for A

Nernst Equation for BNernst Equation for B

Equivalence Point

BA

oBB

oAA

eq nn

EnEnE

Note: This expression only works for simple REDOX TITRATIONS:

Simple redox titrations:Only Aox, Box, Ared, Bred are involved in the reaction …

Two examples

Determine Eeq for the following reactions: Fe2+ + Ce4+ -> Fe3+ + Ce3+

Sn2+ + 2Ce4+ -> Sn4+ + 2Ce3+

Titration curves

What does a titration curve look like for an acid/base titration?

Typical pH titration

0

2

4

6

8

10

12

14

0 5 10 15 20

mL of HBr

pH

REDOX Titrations

Ecell

OverTitration

Just like Acid/Base Titrations

There are four significant regions, The StartThe Start The Buffer RegionThe Buffer Region The equivalence PointThe equivalence Point OvertitrationOvertitration

Let’s Use our simple example:Let’s Use our simple example: FeFe2+2+ + Ce + Ce4+4+ Fe Fe3+3+ + Ce + Ce3+3+

Our simple example

Let’s Use our simple example:Let’s Use our simple example: FeFe2+2+ + Ce + Ce4+4+ Fe Fe3+3+ + Ce + Ce3+3+

Titrate 50 mL of 0.05 M Fe2+ with 0.10 M Ce4+

0% Titration0% Titration

Unlike acid/base titrations, we can’t find this point exactly.

While some Fe3+ must be present, we can only guess what the concentration is.

No Ce4+ or Ce3+ present, so we don’t have a complete reaction

0% Titration0% Titration

][

][log

05916.03

2

Fe

Fe

n

VEE

A

oFeFe

0

05.0log

1

05916.0771.0

VVEFe

FeENO … some of the iron is oxidized by air to give some Fe3+ … how much ? We generally estimate that less Than one in 1000 are oxidized.

5105

05.0log

1

05916.0771.0

x

VVEFe

SHE vs.594.0177.0771.0 VVV

“Buffer Region”

FeFe2+2+ + Ce + Ce4+4+ Fe Fe3+3+ + Ce + Ce3+3+

10 ml of Ce4+ is added

Goes to completion … Excess Fe2+ pushes equilibrium to the right.

Thus E is not dependent on Ce3+/Ce4+, but only on Iron.

“Buffer Region”

][

][log

05916.03

2

Fe

Fe

n

VEE

A

oFeFe

2

2

107.1

105.2log

1

05916.0771.0

V

VEFe

VVEFe 761.0010.0771.0

Closer look at the “buffer” region

Fe2+/Fe3+ E

9 0.715

4 0.735

1.5 0.761

1 0.771

.25 0.807

.11 0.829

Equivalence PointEquivalence Point

From Before Eeq = 1.24 V

What volume?

25 ml

Excess Ce4+ (post titration) FeFe2+2+ + Ce + Ce4+4+ Fe Fe3+3+ + Ce + Ce3+3+

The predominate change is that CeThe predominate change is that Ce4+4+ is being added and is being added and diluteddiluted into a solution of Ceinto a solution of Ce3+3+..

All FeAll Fe2+2+ has been converted to Fe has been converted to Fe3+3+ and no longer figures into the and no longer figures into the calculationscalculations

We just need to keep track of the amounts of CeWe just need to keep track of the amounts of Ce3+3+ and Ce and Ce4+4+ as as well as the well as the VOLUME of the system.VOLUME of the system.

Excess Ce4+ (post titration)

At 30.0 mL Ce4+ Vt = 30.0 mL+ 50.0 mL

][

][log

05916.04

3

Ce

Ce

n

VEE oCeCe

][

][log

1

05916.070.1

4

3

Ce

CeVECe


FeFe2+2+ + Ce + Ce4+4+ Fe Fe3+3+ + Ce + Ce3+3+

Ce3+/Ce4+

?][ 3 Ce

?][ 4 Ce


][

][log

1

05916.070.1

4

3

Ce

CeVECe

3

2

102.6

101.3log

1

05916.070.1

x

xVECe

SHE vs.66.1041.070.1 VECe

Redox IndicatorsRedox Indicators

General

Specific

General Redox Indicators

Varies as a function of Ecell

Rely on a color change with Indox and Indred being different colors.

Indox + ne- Indred

][

][log

05916.0

ox

redoind Ind

Ind

n

VEE


In order to see a color change, you typically need approximately a 10% conversion from one form to another.

1

10

10

1

][

][ or

Ind

Ind

ox

red

n

VEE oind

05916.0


Examples Consider 1,10 phenanthrolene-Fe

C

C

N N

Fe(III)

C

C

N N

Fe(II)

+ e-

BLUEBLUE REDRED

Eo = 1.06 V


NH

S

O

O

OH

Examples Consider Diphenylamine sulphonic acid

Used with the iron in the dichromate method

Eo = 0.80V

SPECIFIC INDICATORS

Example from lab Starch

Starch + I3- blue complex

It is easy to detect and color change is rapid!! This interaction explains why we use iodine as a titrant even though it is a very weak oxidant.

Common Titrants

Usually oxidizing agents. Cr2O7

2- - need an indicator Very stable E=1.44 V

MnO4-

Solutions must be standardized Reagent slowly degrades No indicator needed excess reagent is pink - E=1.51 V

Ce4+ - Example in Class

Common titrants

Reducing Titrants Fe2+

Usually Fe(NH4)2(SO4)2.6H2O in 1M H2SO4

Solution must be standardized each day I-

Indirect methodYour lab was an excellent example

Sensing electrodes

Several types Simple Metal Glass Membrane Solid State Electrodes Etc.


Membrane Electrodes A potential difference is

created across a membrane that can be measured.

THERE IS NO CHANGE IN THE SOLUTIONS

“These electrodes are fundamentally different from metal electrodes in that they DO NOT involve redox reactions!!

These Electrodes ‘selectively bind’ the ion of interest

Membrane electrodes

pH Electrode First Discovered in early 1900’s! Refined through

the 1950’s Probably the most important

Relies on a Glass Membrane H3O+ selectively binds to glass membrane Na+ sluggishly transported across Potential is measured across the membrane!

pH membrane

Special Glass (72% SiO2) Doped with Na2O (22%)

and CaO (6%)

SiOCation

Key

Membrane Electrodes In order to work the Glass

must be hydrated To allow for diffusion of H+

and Na+ H3O+ populates BOTH side

of the electrode BUT DOES NOT cross the membrane

To perform an electrical measurement - Must be a complete circuit! But Na+ ‘sluggishly’ crosses

the membrane. Na+ transport ~ salt bridge Membranes resistance ~

1x108-9

Membrane Electrodes

While H3O+ causes a response, other ions also ‘interfere’. Alkali Error Many alkali metals (Li+, Na+, K+)

Severe interference result when Alkali ion is in greater concentration than H3O+

This false response is called Alkaline Error b/c of error associated when measuring solutions of sodium hydroxide. (NaOH)

Note – the electrode shows little interference with OH-. Why?

Membrane Electrodes

Acid Error Too many of the Si-O- sites are saturated with

H3O+ and no more sites are available for protonation.

The response of real glass electrodes is described by the following equation:

)(log)0592.0(constant outsideAE H B is the electromotive efficiency (ideally =1, usually > 0.98)

Sensing electrodes

Several types Simple Metal Glass Membrane Solid State Electrodes Etc.


Solid State Electrodes

The F- ISE The original solid state

electrode Works due to defects in a

LaF3 crystal.

Other Solid state electrodes work based on the presence of primary absorbed ion.

LaF3/Eu

F- Inorganic CrystalThe solid state electrode is a very popular type of ISE. As easy to maintain as a pH electrode (sometimes easier).


Our TISAB the pH was 5-6 and the ionic strength was held constant. Why?

F- and OH- are about the same size AND same CHARGE!! LaF3/Eu2+ doped crystal selects for size and charge Thus, OH- will cause a response.


Our TISAB the pH was 5-6 and the ionic strength was held constant. Why?

)(log)0592.0(constant outsideAE F

)]([log)0592.0(constant outsideFE F

)](log[)0592.0(log)0592.0(constant outsideFE F

Constant

)](log[)0592.0('constant outsideFE

Conclusions Several types

Simple Metal – Using Metal associated with Ion

Direct Quantitation Using Inert Electrode

Yields Information on Ratio of concentrations Glass Membrane

pH Electrode Two reference electrodes (Constant Potential) Measuring the ‘junction’ potential Alkali Error

Solid State Electrodes Flouride ISE Measuring junction potential Acid and Base Error

Interferences are based on similar size and charge for Membrane electrodes and SS Electrodes

Detection limits are between 10-6 M and 10-8 M

announcements course evaluations final is wednesday afternoon on may 9 th homework 14-15, 14-26,...

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