Measurement of Lead Depends on: Chemistry of Lead for Separations

Measurement of Lead Depends on:Chemistry of Lead for SeparationsChemistry of Lead for Creating a signalChemistry of Lead for Creating BackgroundChemistry of Lead for the Stability of the Signal

Garbage In = Garbage Out

GIGO

-15000

-10000

-5000

0

5000

10000

15000

0 0.2 0.4 0.6 0.8 1 1.2

Time (s)

Am

plit

ud

e

Sample SamplePrep

Instrument Instrument Out put

Signal (Data)

Recall we mentioned that lead resides at surface of soils becauseof it’s insolubility

Beta

Recall from Quant

P b O H P bO H K f2

1

P bO H O H P b O H Kof

2 2

P b O H O H P b O H Kof2 3 3

P b O H O H P b O H K f3 42

4

P b P b P bO H P b O H P b O H P b O Hto ta lo 22 3 4

2

Mass balance

P bO H K P b O Hf

12

P b O H K P bO H O Hof2 2

P b O H K K P b O Hof f2 1 2

2 2

P b O H K P b O H O Hfo

3 3 2

P b O H K K K P b O Hf f f3 1 2 32 3

P b O H K P b O H O Hf42

4 3

P b O H K K K K P b O Hf f f f42

1 2 3 42 4

P b P b P bO H P b O H P b O H P b O Hto ta lo 22 3 4

2

Mass balance

P bO H K P b O Hf

12

P b O H K K P b O Hof f2 1 2

2 2

P b O H K K K P b O Hf f f3 1 2 32 3

P b O H K K K K P b O Hf f f f42

1 2 3 42 4

P b P b K P b O H K K P b O H

K K K P b O H K K K K P b O H

to ta l f f f

f f f f f f f

21

21 2

2 2

1 2 32 3

1 2 3 42 4

P b P b O H O H O H O Hto ta l 21 2

2

3

3

4

41

Make a definition to simplify the expression, and factor out terms

1 1

2 1 2

3 1 2 3

4 1 2 3 4

K

K K

K K K

K K K K

f

f f

f f f

f f f f

0

2 2

21 2

2

3

3

4

41

P b

P b

P b

P b O H O H O H O Hto ta l

P b P b O H O H O H O Hto ta l 21 2

2

3

3

4

41

0

1 2

2

3

3

4

4

1

1

1

O H O H O H O H D

1

21

21 2

2

3

3

4

41

P bO H

P b

P b O H

P b O H O H O H O Hto ta l

1

1

1 2

2

3

3

4

4

1

1

O H

O H O H O H O H

O H

D

D O H O H O H O H 1 1 2

2

3

3

4

4

2

2 2

2

1 2

2

3

3

4

4

2

2

1

P b O H

P b

O H

O H O H O H O H

O H

D

o

To ta l

3

3 3

3

1 2

2

3

3

4

4

3

3

1

P b O H

P b

O H

O H O H O H O H

O H

DTo ta l

4

4

2

4

4

1 2

2

3

3

4

4

4

4

1

P b O H

P b

O H

O H O H O H O H

O H

DTota l

0

1 2

2

3

3

4

4

1

1

1

O H O H O H O H D

1

1

1 2

2

3

3

4

4

1

1

O H

O H O H O H O H

O H

D

2. Set up a column for the pH value

3. Calculate [OH-]

1 4

1 4

1 0 1 4

pH pO H

pO H pHpH )

4. Calculate D

1. Calculate beta values

=1 + $J$5*B3+$K$5*(B3^2)+$L$5*(B3^3)+$M$5*(B3^4)

D O H O H O H O H 1 1 2

2

3

3

4

4

5. Calculate alpha 0 0

1

D

6. Calculate alpha 1

1

1

O H

D

This number should be 6.4

=1/C3

=$J$5*B3*D3

7. Calculate alpha 2

2

2

2

O H

D

=$K$5*B3^2*D3

8. Calculate alpha 3 and 4 in a similar fashion

=10^(-(14-A3))

0

0.2

0.4

0.6

0.8

1

1.2

0 2 4 6 8 10 12 14

pH

Fra

ctio

n

Pb2+

Pb(OH)+

Pb(OH)2

Pb(OH)3-

Pb(OH)42-

This graph indicates that if our instrument is measuring Pb2+ then when we Prepare the sample we need to have a pH of less than 6

Note that at pH < 6 all of the lead is present as Pb2+

Measurement of Lead Depends on:Chemistry of Lead for SeparationsChemistry of Lead for Creating a signalChemistry of Lead for Creating BackgroundChemistry of Lead for the Stability of the Signal

Garbage In = Garbage Out

GIGO

Measurements based on PbS

1820 Frederick AcumLondon

Measurements based on PbS

“1 part of acetate of leadMay be detected by means of it in 20000Parts of water.”

8000 1000 1900 1990 2000 B.C.E. C.E.

ppm

ppb

ppt

LO

D1820 Sulphuretted water

cupellation

More PbS measurements

-log Ksp Ag2S=49

q

VC q

CV

Suppose we are using a lead ion selective electrode to measure Pb2+, canWe use any pH less than 6?

Pb2+

S2-Ag+

Pb2+

S2-Pb2+

Pb2+

S2-

S2-Ag+

S2-

S2-

S2-

Pb2+

Charge separation after motion ofAg+ leads to a potential across theMembrane = signal

Soln Pb

Ag+

controls

Which controls

Which controls

-log Ksp Ag2S=49

-log Ksp PbS=29

Inte

rnal

sol

utio

n fix

ed in

Ag+

Measurement of Lead Depends on:Chemistry of Lead for SeparationsChemistry of Lead for Creating a signalChemistry of Lead for Creating BackgroundChemistry of Lead for the Stability of the Signal

Garbage In = Garbage Out

GIGO

0

0.2

0.4

0.6

0.8

1

1.2

-5 -3 -1 1 3 5

pCl

Lea

d A

lph

a F

ract

ion

s

If we want to separate lead on an anion exchange column form the PbCl3- species.Which line would that be? And what conc. Cl would we want?

Other Alpha Plots are also useful

Measurement of Lead Depends on:Chemistry of Lead for SeparationsChemistry of Lead for Creating a signalChemistry of Lead for Creating BackgroundChemistry of Lead for the Stability of the Signal

Garbage In = Garbage Out

GIGO

Water is shown for comparison. What this means is if you get about 700 oCYou will have a large vapor pressure for PbCl2 which means you loseStuff from solution

Lead Chloride, while useful for an anion exchange separation is a problemBecause of it’s low vapor pressure

Measurement of Lead Depends on:Chemistry of Lead for SeparationsChemistry of preparing the sampleChemistry of Lead for Creating a signalChemistry of Lead for Creating BackgroundChemistry of Lead for the Stability of the Signal

Garbage In = Garbage Out

GIGO

UV-Vis monitors valence shell electrons

Need to convert Pb to something thata. Has UV-Vis activityb. That can be selective toward Pb bindingc. That can be separated from other binding metals

only instrument you have is a…..UV-Vis Spectrophotometer

Loss of a proton makes this a goodComplexing agent if mixed with Aqueous Pb2+

What problems can we run into?

Not water soluble

1. pH not high enough to remove proton2. pH too high and results in lead hydroxide

formation

0

0.2

0.4

0.6

0.8

1

1.2

0 2 4 6 8 10 12 14

pH

Fra

ctio

nPb2+

Pb(OH)+

Pb(OH)2

Pb(OH)3-

Pb(OH)42-

Need to consider this a separation

To get reproducible results you will need to Set a standard procedure for number of Shakes and total time.

D

HigherpH

Non-Water soluble

Also not water soluble

Other Considerations?

Any other metals (including Mg2+!!!) can cause a color change

False Positives

The chalk used to line the interior of yourProtective gloves can cause false positives

Solution?

Selectively complex other metals and leave behind the lead!!!!

Which complexing agent would you use?

Want low value for leadHigh value for others

CN might be goodBUT!!!!!

Iron ferricyanideServes as an Oxidizing/reductingreagent

Add CN to get ridOf Cd, Hg, Ni, Ag, and Zn,

But

Also add Citrate to pullThe iron from ferricyanideTo citrate form.

Mild oxidation of the un-reacted Dithizone results in a dimer linked by a S-S bond which absorbs at 420 (see spectra 2). More extended oxidation results in cyclization with a product that absorbs at ~610 to 620 nm.

Estimated molar absorptivity of the dimer is 30000 to 49000

Some “Data Considerations”

How will you choose a wavelength from which to make a calibration curve?

How will you determine if you still have unreacted dithizone contributing toYour signal?

How will you quantitate the absorbance at the wavelength you choose?

How will you choose a wavelength from which to make a calibration curve?

How will you determine if you still have unreacted dithizone contributing toYour signal?

How will you quantitate the absorbance at the wavelength you choose?

1. Want a region where the signal does not change rapidly (the top of a peak)2. Want a region where the analyte signal has the least contribution from the

background (peak of Pb-complex)

1. Monitor wavelength of peak in the 600 region or deconvolute the data

A A Am easured som e ana lyte som e background som e, , ,

A b Cbackground som e background cm pound a t som e cell pa th leng th background cm pound, , , 1 1

A b Cbackground som e B B, , 1

Method 1

A A Aana lyte som e m easured som e background som e, , ,

Monitor B at wavelength where only B absorbs and at the wavelength of interest

Make a calibration curve at those wavelengths with standards for The background (unreacted dithizone); determine molar absorptivities

Calculate concentration of unreacted dithizone for the measurement of Pb byUse of the calibration curve for unreacted dithizone

Calculate the absorbance due to unreacted dithizone

A b F ree d ith izonefree d ith izozne6 8 0 6 8 0 ,

A b F ree d ith izonefree d ith izone free d ith izozne5 5 5 5 5 5, ,

Ab F ree d ith izone

A b F ree d ith izone

AA

free d ith izozne

free d ith izone free d ith izozne

free d ith izone free d ith izozne

free d ith izozne

6 8 0

6 8 0

5 5 5 5 5 5

5 5 5 5 5 56 8 0

6 8 0

,

, ,

, ,

,

A A Alead d ith izone m easured unreacted free d ith izone5 5 5 5 5 5 5 5 5, , , ,

Set a baseline across the bottom of the peak

Background A

Measurement

The difference in absorbance between the two is the background corrected signal

Use this lab to introduce another data manipulation

A A Aana lyte som e m easured som e background som e, , , Baseline estimation

Method 2Much easierAnd makes noAssumptions aboutWhat is contributingTo the background

Use this lab to introduce another data manipulation

Method 3 – assess contribution by assuming Gaussians

f xA

ex

2

1

2

2

Assume absorbance peak is Gaussian in the energy spread

Energy ofThe absorption bands

Energy of light absorbed

Frequency= absorption

photonse

Energy levels are randomly populatedBy Temperature E h

c

Ehc

hc

Std~(first guess) width at ½ peak ht

AA

s

or peakv v

s

m ax

ex pm ax

2

1

2

2

Deconvolution

0

0.2

0.4

0.6

0.8

1

1.2

300 350 400 450 500 550 600 650 700 750

Wavelength (nm)

Ab

sorb

ance

Series1

1. Get the absorption spectra

Deconvolution

1.Assume absorbance peak is Gaussian in the energy spread

2.Convert data from A vs wavelength to A vs energy

0

0.2

0.4

0.6

0.8

1

1.2

0 5 10 15 20 25 30 35

cm-1

A Series1

0

0.2

0.4

0.6

0.8

1

1.2

300 350 400 450 500 550 600 650 700 750

Wavelength (nm)

Ab

sorb

ance

Series1

Notice the 2 curves lookDifferent!

Deconvolution

3.Using your data estimate: center of peak (mean) standard deviation amplitude

0

0.2

0.4

0.6

0.8

1

1.2

0 5 10 15 20 25 30 35

cm-1

A Series1A

std

mean

f xA

ex

2

1

2

2

0

0.2

0.4

0.6

0.8

1

1.2

300 350 400 450 500 550 600 650 700 750

Wavelength (nm)

Ab

sorb

ance

Series1

0

0.2

0.4

0.6

0.8

1

1.2

0 5 10 15 20 25 30 35

cm-1

A Series1

Wavelength, nm

Frequency, Cm-1

E hhc

Conversion to energy Guess four absorbance bands

Calculated bands based on a Gaussian eq

Sum of all the bands

=(H10-B10)^2

Value to be minimzed=10000/A10

MinimizeTarget cell

energy

wavelength

Prevent solver from giving You non-plausible (negative)numbers

Plot the wavelength based absorption dataAnd superimpose the data generated by solver

0

0.2

0.4

0.6

0.8

1

1.2

250 350 450 550 650 750 850

nm

A

0

0.2

0.4

0.6

0.8

1

1.2

250 350 450 550 650 750 850

nm

A

0

0.2

0.4

0.6

0.8

1

1.2

250 350 450 550 650 750 850

nm

A

0

0.2

0.4

0.6

0.8

1

1.2

250 350 450 550 650 750 850

nm

A

0

0.2

0.4

0.6

0.8

1

1.2

250 350 450 550 650 750 850

nm

A

Now sum all the individual bands and see if you get a low sum of sq differences

0

0.2

0.4

0.6

0.8

1

1.2

250 350 450 550 650 750 850

nm

A

sum sqrd0.034034

Some deviation here But generally pretty darn good

At the wavelength you are interested in

0

0.2

0.4

0.6

0.8

1

1.2

250 350 450 550 650 750 850

nm

A

Our “signal”Go to 550 and use the A from this bandOnly!!! (all other absorbances representBackground contributions)

Go to the column of data representing thatSingle absorbance band here I use550 and find the max=max(data range)This will be your absorbance of the band Without the contribution from the otherbands

8000 1000 1900 1990 2000 B.C.E. C.E.

ppm

ppb

ppt

LO

D

cuppellation

Suphuretted water

titrimetric

dithizone

Chemistry

This method resulted in the first public healthAwareness of lead as an issue for childrenBaltimore, Department of Public Health

Measurement of Lead Depends on:Chemistry of Lead for SeparationsChemistry of Lead for Creating a signalChemistry of Lead for Creating BackgroundChemistry of Lead for the Stability of the Signal

Garbage In = Garbage Out

GIGO

Convert lead to some compound which canBe measured by some instrument (what ever happens to Be available in your lab)

Suppose you only have a fluorimeter!

OOH O

CH3

N+

O-O O

-

O

H

Chromophore – part of molecule sensitive to light

“Selectivity” arm – complexes the metal ion and turnsOn and off fluorescence

Calcein Blue

Carboxyl groups only deprotonated

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

200 250 300 350 400 450 500 550 600

wavelength, nm

Ab

sorb

ance

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

Rel

ativ

e F

luo

resc

ence

In

ten

sity

Absorbancespectra

EmissionSpectra, excitation at 320

OOH O

CH3

N+

O-O O

-

O

H

OO OH

CH3

N+

O-O O

-

O

H

ExcitedStateProtontransfer

320 nm excitation

480-490 nm emission18-33 nsduration

OOH O

CH3

N+

O-O O

-

O

H

pH 6-8

Note role of resonance here

Ground state phenolic deprotonation

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

200 250 300 350 400 450 500 550 600

Wavelength, nmA

bso

rba

nc

e

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Re

lati

ve

Flu

ore

sc

en

ce

In

ten

sit

y

440-460

OO- O

CH3

N+

O-O O

-

O

H

OO O-

CH3

N+

O-O O

-

O

H

OOH O

CH3

N+

O-O O

-

O

H

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

270 320 370 420 470

Wavelength

AU

350-360

H-N

pH 8-11

OO- O

CH3

N+

O-O O

-

O

H

ExcitedStateElectrontransfer

Key point so far – Excitation is pH dependentTherefore the emission location and intensity is also pH dependent

If the solution is fluctuating in pH will not get a linear working curve.

Since you have to control pH for the chemical signal, need to also considerThe role of pH in the form of the lead that is present.

Lead quenchesemission

Why might Pb quench the emission?

Structures as determined from NMR

Measurement of Lead Depends on:Chemistry of Lead for SeparationsChemistry of Lead for Creating a signalChemistry of Lead for Creating BackgroundChemistry of Lead for the Stability of the Signal

Garbage In = Garbage Out

GIGO

Convert lead to some compound which canBe measured by some instrument (what ever happens to Be available in your lab)

Suppose you only have an IR!

Need to convert Pb to some form that is amenable toIR and/or Raman spectroscopies.

1. React lead with some reagent

This data canBe found inThe appendixTo “SublimeLead” webpage

Need to convert Pb to some form that is amenable toIR and/or Raman spectroscopies.

O

OH

N

O

OHN OH

O OH

O Change in bond length

photon

Pb2+

Key Data Manipulation Concepts from the Lab

IR instrument allows you to set the number of waveforms that youWill average.

You will need to enhance the sensitivity near the base of one peak soThat you can see the background fluctuations in a single scan

Repeat for 4 scans

Repeat for 9 summed scans

Repeat for 16 summed scans, etc.

What do you think you will be asked to observe?

A Case of Forensic Chemistry: Art and Forgeries

Lead Tin II, Paolo Veronese,Allegory of Love

Peter Rubens, The Dying SenecaLead Tin I

Lead Antimonate

Forensic Art Chemistry

Lead Tin I

Lead Tin II

Two Sb octahedraLinked via vertices to a) eight pointed polyhedra Of Pb &b) Hexagonal bipyramid

Chains ofSn octahedraJoined by PyramidallyCoordinated Pb(II)

Lead Antimonate