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
