GIGO
Measurement of Lead Depends on:
Chemistry of Lead for Separations
Chemistry of Lead for Creating a signal
Chemistry of Lead for Creating Background
Chemistry of Lead for the Stability of the Signal
Garbage In = Garbage Out
15000
10000
Amplitude
5000
0
0
0.2
0.4
0.6
0.8
-5000
-10000
-15000
Time (s)
Sample
Sample
Prep
Instrument
Instrument
Out put
Signal (Data)
1
1.2
Recall we mentioned that lead resides at surface of soils because
of it’s insolubility
Beta
Recall from Quant

Pb 2  OH  
 PbOH
Kf1
PbOH   OH  
 Pb OH  2
o
Kf 2
Pb OH  2  OH  
 Pb OH  3

o
Kf 3
Pb OH  3  OH  
 Pb OH  4

2
Kf 4
Mass balance

 
 
Pbtotal   Pb 2     PbOH    Pb OH  2  Pb OH  3  Pb OH  4


o

2

PbOH    K f 1  Pb 2  OH  
 Pb OH    K  PbOH  OH 
o
2



f2

Pb OH 2  K f 1 K f 2  Pb 2  OH  
o
2


Pb OH  3  K f 3 Pb OH  2  OH  

o


Pb OH  3  K f 1 K f 2 K f 3 Pb 2  OH  


3

Pb OH  4  K f 4 Pb OH  3  OH  
2



Pb OH  4  K f 1 K f 2 K f 3 K f 4 Pb 2  OH  
2
4
Mass balance

 
 
Pbtotal   Pb 2     PbOH    Pb OH  2  Pb OH  3  Pb OH  4


o

2

PbOH    K f 1  Pb 2  OH  
 Pb OH   K
o
2
2

f 1 K f 2  Pb  OH 
2


Pb OH  3  K f 1 K f 2 K f 3 Pb 2  OH  


3

Pb OH  4  K f 1 K f 2 K f 3 K f 4 Pb 2  OH  
2
Pbtotal   Pb 2    K f 1  Pb 2   OH    K f 1 K f 2  Pb 2   OH   
2
K f 1 K f 2 K f 3  Pb 2   OH    K f 1 K f 2 K f 3 K f 4  Pb 2   OH  
3
4
Make a definition to simplify the expression, and factor out terms
1  K f 1
2  K f 1 K f 2
3  K f 1 K f 2 K f 3
4  K f 1 K f 2 K f 3 K f 4

Pbtotal   Pb 2   1  1  OH    2  OH    3  OH    4  OH  
2
3
4

4
Pbtotal   Pb
2
1    OH     OH 
1


D  1  1 OH
0 
0 

Pb 2
Pbtotal

2

   OH 
 2
2

 3 OH

Pb 2
 Pb 1   OH    OH 
2

 3

 2

1
2


 3 OH
 3
2
PbOH 




1 
Pbtotal
 4
3
 Pb 1    OH

1   OH    OH
1
2
 2
 3
2
3
 3
3


 4

 4 OH

 4

 2

  OH 

 4
1
D
 Pb   OH 
    OH     OH 
1 OH 


1
1



 3
4
2
2
 4  OH
 4 OH
1   OH    OH    OH    OH  
 2


 3
1
1
1
 3  OH
 2


 4 OH

 4

 4  OH

1 OH 
D


 4

0 
1
1   OH    OH    OH    OH  
 2

1
1 
2
3
4


3
1   OH    OH





 4


4

  OH 
1 OH 
1
2
 3
2
PbOH  2
o
PbTotal
PbOH  3

PbTotal
PbOH  4

1   OH
1

2
PbTotal
 2

 4 OH

   OH 
2 OH 
 2

   OH 

1  1 OH 

 2
2

   OH 



1 OH 
D
2


 3 OH 
 2
2


1
D

 3

 4 OH


3

 4 OH 


 4
 3
4 OH
1  1 OH 


 4
 3 OH
2
3 OH


 3
3



4



3

 4 OH 

4


D

3 OH
 4
 3 OH 

2 OH 

 3
D

4 OH
D

 4
2
1. Calculate beta values
This number should be
6.4
2. Set up a column for the pH value
14  pH  pOH
pOH  14  pH
3. Calculate [OH-]
10  14  pH ) 
=10^(-(14-A3))
4. Calculate D


D  1  1 OH

   OH 
 2
2

 3 OH

 3

 4 OH

 4
=1 + $J$5*B3+$K$5*(B3^2)+$L$5*(B3^3)+$M$5*(B3^4)
5. Calculate alpha 0   1 =1/C3
0
D
6. Calculate alpha 1
7. Calculate alpha 2
1 

1 OH 
2 

=$J$5*B3*D3
D

2 OH

 2
=$K$5*B3^2*D3
D
8. Calculate alpha 3 and 4 in a similar fashion
Note that at pH < 6 all of the lead is present as Pb2+
1.2
Pb2+
Pb(OH)42-
1
Pb(OH)+
Pb(OH)2
Fraction
0.8
0.6
0.4
Pb(OH)3-
0.2
0
0
2
4
6
8
10
12
pH
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
14
GIGO
Measurement of Lead Depends on:
Chemistry of Lead for Separations
Chemistry of Lead for Creating a signal
Chemistry of Lead for Creating Background
Chemistry of Lead for the Stability of the Signal
Garbage In = Garbage Out
Measurements based on PbS
Measurements based on PbS
1820
Frederick Acum
London
“1 part of acetate of lead
May be detected by means
of it in 20000
Parts of water.”
1820 Sulphuretted water
cupellation
ppm
ppb
ppt
8000
B.C.E.
1000
C.E.
1900
1990
2000
More PbS measurements
-log Ksp Ag2S=49
q
 C
V
q
V
C
Internal solution fixed in Ag+
Suppose we are using a lead ion selective electrode to measure Pb2+, can
We use any pH less than 6?
Soln Pb
-log Ksp PbS=29
Pb2+
Pb2+
S2Ag+
Pb2+
S2S2controls
S2S2S2- Pb2+
Charge separation after motion of
Ag+ leads to a potential across the
Membrane = signal
Ag+
Pb2+
S2Which controls
-log Ksp Ag2S=49
Which controls
Ag+
GIGO
Measurement of Lead Depends on:
Chemistry of Lead for Separations
Chemistry of Lead for Creating a signal
Chemistry of Lead for Creating Background
Chemistry of Lead for the Stability of the Signal
Garbage In = Garbage Out
Other Alpha Plots are also useful
1.2
Lead Alpha Fractions
1
0.8
0.6
0.4
0.2
0
-5
-3
-1
1
3
5
pCl
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?
GIGO
Measurement of Lead Depends on:
Chemistry of Lead for Separations
Chemistry of Lead for Creating a signal
Chemistry of Lead for Creating Background
Chemistry of Lead for the Stability of the Signal
Garbage In = Garbage Out
Lead Chloride, while useful for an anion exchange separation is a problem
Because of it’s low vapor pressure
Water is shown for comparison. What this means is if you get about 700 oC
You will have a large vapor pressure for PbCl2 which means you lose
Stuff from solution
GIGO
Measurement of Lead Depends on:
Chemistry of Lead for Separations
Chemistry of preparing the sample
Chemistry of Lead for Creating a signal
Chemistry of Lead for Creating Background
Chemistry of Lead for the Stability of the Signal
Garbage In = Garbage Out
only instrument you have is a…..
UV-Vis Spectrophotometer
UV-Vis monitors valence shell electrons
Need to convert Pb to something that
a.
Has UV-Vis activity
b.
That can be selective toward Pb binding
c.
That can be separated from other binding metals
Not water soluble
Loss of a proton makes this a good
Complexing agent if mixed with
Aqueous Pb2+
What problems can we run into?
1. pH not high enough to remove proton
2. pH too high and results in lead hydroxide
formation
1.2
Pb2+
Pb(OH)42-
1
Pb(OH)+
Pb(OH)2
Fraction
0.8
0.6
0.4
Pb(OH)3-
0.2
0
0
2
4
6
8
pH
10
12
14
Need to consider this a separation
D
Higher
pH
To get reproducible results you will need to
Set a standard procedure for number of
Shakes and total time.
Also not water soluble
Non-Water soluble
Other Considerations?
False Positives
Any other metals (including Mg2+!!!) can cause a color change
The chalk used to line the interior of your
Protective 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 lead
High value for others
CN might be good
BUT!!!!!
Iron ferricyanide
Serves as an
Oxidizing/reducting
reagent
Add CN to get rid
Of Cd, Hg, Ni, Ag, and Zn,
But
Also add Citrate to pull
The iron from ferricyanide
To citrate form.
Mild oxidation of the unreacted 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 to
Your 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?
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)
How will you determine if you still have unreacted dithizone contributing to
Your signal?
1. Monitor wavelength of peak in the 600 region or deconvolute the data
How will you quantitate the absorbance at the wavelength you choose?
Ameasured ,some   Aanalyte,some   Abackground , some
Aanalyte,some   Ameasured ,some   Abackground , some
Method 1


Abackground , some  background cmpound 1, at some  bcell pathlength Cbackground cmpound 1 ,


Abackground , some   B ,  1 b CB
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 by
Use of the calibration curve for unreacted dithizone
Calculate the absorbance due to unreacted dithizone



A680   freedithizozne,680 b Free dithizone



A555, free dithizone   freedithizozne,555 b Free dithizone

A680
 freedithizozne ,680



 b Free dithizone

 

 

A555, freedithizone   freedithizozne ,555 b Free dithizone
A555, freedithizone
freedithizozne ,555
A680
freedithizozne , 680

A555,lead  dithizone  Ameasured ,555  A555,unreacted , freedithizone
Measurement
Background A
Method 2
Much easier
And makes no
Assumptions about
What is contributing
To the background
Set a baseline across the bottom of the peak
The difference in absorbance between the two is the background corrected signal
Aanalyte,some   Ameasured ,some   Abackground , some
Baseline estimation
Use this lab to introduce another data manipulation
Use this lab to introduce another data manipulation
Method 3 – assess contribution by assuming Gaussians
A
f  x 
e
 2
1 x   2
 

2  
Assume absorbance peak is Gaussian in the energy spread
Energy of
The absorption
bands
photons
e
Energy levels are randomly populated
By Temperature E  h

Frequency
= absorption
E
c

hc

Std~(first guess) width at ½ peak ht
A
Amax or peak
s 2
2
exp
 1  v  vmax 
 
 
s  
 2 
hc

Energy of light absorbed
Deconvolution
1.2
1. Get the absorption spectra
1
Absorbance
0.8
0.6
Series1
0.4
0.2
0
300
350
400
450
500
550
Wavelength (nm)
600
650
700
750
Deconvolution
1.Assume absorbance peak is Gaussian in the energy spread
2.Convert data from A vs wavelength to A vs energy
1.2
1
0.6
Series1
1.2
0.4
1
0.2
0.8
0
300
350
400
450
500
550
600
650
700
750
Wavelength (nm)
A
Absorbance
0.8
0.6
Series1
0.4
Notice the 2 curves look
Different!
0.2
0
0
5
10
15
20
cm-1
25
30
35
Deconvolution
3.Using your data estimate: center of peak (mean)
standard deviation
amplitude
1.2
1
0.8
A
A
A
f  x 
e
 2
1 x   2
 

2  
0.6
Series1
0.4
std
0.2
0
0
5
10
15
20
cm-1
25
30
35
1.2
1
Absorbance
0.8
0.6
Series1
0.4
0.2
0
300
350
400
450
500
550
600
650
700
Wavelength, nm
750
Wavelength (nm)
1.2
1
A
0.8
0.6
Series1
E  h 
0.4
0.2
hc

Frequency, Cm-1
0
0
5
10
15
20
cm-1
25
30
35
Sum of all the bands
Conversion to energy
=10000/A10
Guess four absorbance bands
Value to be
minimzed
=(H10-B10)^2
energy
wavelength
Minimize
Target cell
Prevent solver from giving
You non-plausible (negative)
numbers
Plot the wavelength based absorption data
And superimpose the data generated by solver
Now sum all the individual bands and see if you get a low sum of sq differences
1.2
1.2
sum sqrd
0.034034
11
A
A
0.8
0.8
0.6
0.6
0.4
0.4
0.2
0.2
00
250
250
350
350
450
450
550
550
650
650
nm
nm
Some deviation here
But generally pretty darn good
750
750
850
850
At the wavelength you are interested in
1.2
1
A
0.8
0.6
0.4
0.2
0
250
350
450
550
nm
650
750
850
Go to the column of data representing that
Single absorbance band here I use
550 and find the max
=max(data range)
This will be your absorbance of the band
Without the contribution from the other
bands
Our “signal”
Go to 550 and use the A from this band
Only!!! (all other absorbances represent
Background contributions)
Chemistry
titrimetric
ppm
cuppellation
ppb
Suphuretted water
dithizone
ppt
8000
B.C.E.
1000
C.E.
1900
1990
2000
This method resulted in the first public health
Awareness of lead as an issue for children
Baltimore, Department of Public Health
GIGO
Measurement of Lead Depends on:
Chemistry of Lead for Separations
Chemistry of Lead for Creating a signal
Chemistry of Lead for Creating Background
Chemistry of Lead for the Stability of the Signal
Garbage In = Garbage Out
Convert lead to some compound which can
Be measured by some instrument (what ever happens to
Be available in your lab)
Suppose you only have a fluorimeter!
Calcein Blue
H3C
Chromophore – part of molecule sensitive to light
HO
O
O
+
N
H
O
O
O
-
O
-
“Selectivity” arm – complexes the metal ion and turns
On and off fluorescence
H3C
H3C
O
O
+
N
H
O
O
O
-
-
Note role of resonance here
O
OH
O
N
H
O
O
O
-
-
O
O
Emission
Spectra, excitation at 320
Absorbance
spectra
+
1
0.4
0.9
0.35
0.8
0.3
Absorbance
0.7
0.25
0.6
0.5
480-490 nm emission
0.4
18-33 ns
0.3
duration
0.2
0.15
0.1
0.2
320 nm excitation
0
200
H3C
HO
O
O
N
H
O
O
-
250
300
350
400
wavelength, nm
+
O
0.05
0.1
-
O
pH 6-8
Carboxyl groups only deprotonated
450
500
550
0
600
Relative Fluorescence Intensity
HO
Excited
State
Proton
transfer
H3C
O
O
+
N
H
O
O
O
-
Excited
State
Electron
transfer
0.8
0.7
O
O
O
-
0.6
+
N
H
-
0.5
O
O
O
O
-
AU
O
-
H3C
-
O
0.4
0.3
0.2
0.1
0
270
320
370
Wavelength
420
470
1
0.7
0.9
0.5
0.7
Absorbance
350-360
440-460
0.6
0.4
0.5
0.3
0.4
0.3
0.2
0.2
0.1
0.1
0
H3C
H3C200
H-N
HO
O
O
+
pH 8-11
O
-
O
O
+
N
H
O
-
-
O
Ground state phenolic deprotonation
O
O
O
-
O
-
300
350
400
Wavelength, nm
O
O
N
H
250
450
500
550
0
600
Relative Fluorescence Intensity
0.6
0.8
Key point so far – Excitation is pH dependent
Therefore 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 consider
The role of pH in the form of the lead that is present.
Why might Pb quench the emission?
Lead quenches
emission
Structures as determined from NMR
GIGO
Measurement of Lead Depends on:
Chemistry of Lead for Separations
Chemistry of Lead for Creating a signal
Chemistry of Lead for Creating Background
Chemistry of Lead for the Stability of the Signal
Garbage In = Garbage Out
Convert lead to some compound which can
Be 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 to
IR and/or Raman spectroscopies.
1. React lead with some reagent
This data can
Be found in
The appendix
To “Sublime
Lead” web
page
Need to convert Pb to some form that is amenable to
IR and/or Raman spectroscopies.
photon
Change in bond length
O
O
OH
N
OH
HO
N
HO
O
O
Pb2+
Key Data Manipulation Concepts from the Lab
IR instrument allows you to set the number of waveforms that you
Will average.
You will need to enhance the sensitivity near the base of one peak so
That 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
Lead Antimonate
Peter Rubens, The Dying Seneca
Lead Tin I
Forensic Art Chemistry
Two Sb octahedra
Linked via vertices
to a) eight pointed
polyhedra Of Pb &
b) Hexagonal
bipyramid
Lead Antimonate
Lead Tin I
Lead Tin II
Chains of
Sn octahedra
Joined by
Pyramidally
Coordinated Pb(II)