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Measurement of Lead Depends on: Chemistry of Lead for Separations Chemistry of Lead for Creating a signal Chemistry of Lead for Creating Background Chemistry.

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Presentation on theme: "Measurement of Lead Depends on: Chemistry of Lead for Separations Chemistry of Lead for Creating a signal Chemistry of Lead for Creating Background Chemistry."— Presentation transcript:

1 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 GIGO Sample Prep Instrument Out put Signal (Data)

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

3 Beta

4 Recall from Quant Mass balance

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

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8 2. Set up a column for the pH value 3. Calculate [OH-] 4. Calculate D 1. Calculate beta values =1 + $J$5*B3+$K$5*(B3^2)+$L$5*(B3^3)+$M$5*(B3^4) 5. Calculate alpha 0 6. Calculate alpha 1 This number should be 6.4 =1/C3 =$J$5*B3*D3 7. Calculate alpha 2 =$K$5*B3^2*D3 8. Calculate alpha 3 and 4 in a similar fashion =10^(-(14-A3))

9 This graph indicates that if our instrument is measuring Pb 2+ 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 Pb 2+

10 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 GIGO

11 Measurements based on PbS

12 1820 Frederick Acum London Measurements based on PbS

13 “1 part of acetate of lead May be detected by means of it in Parts of water.”

14 B.C.E. C.E. ppm ppb ppt LOD 1820 Sulphuretted water cupellation

15 More PbS measurements -log Ksp Ag 2 S=49

16 Suppose we are using a lead ion selective electrode to measure Pb 2+, can We use any pH less than 6? Pb 2+ S 2- Ag + Pb 2+ S 2- Pb 2+ S 2- Ag + S 2- Pb 2+ Charge separation after motion of Ag+ leads to a potential across the Membrane = signal Soln Pb Ag + controls Which controls -log K sp Ag 2 S=49 -log K sp PbS=29 Internal solution fixed in Ag +

17 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 GIGO

18 If we want to separate lead on an anion exchange column form the PbCl 3 - species. Which line would that be? And what conc. Cl would we want? Other Alpha Plots are also useful

19 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 GIGO

20 Water is shown for comparison. What this means is if you get about 700 o C You will have a large vapor pressure for PbCl 2 which means you lose Stuff from solution Lead Chloride, while useful for an anion exchange separation is a problem Because of it’s low vapor pressure

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

22 Loss of a proton makes this a good Complexing agent if mixed with Aqueous Pb 2+ What problems can we run into? Not water soluble 1.pH not high enough to remove proton 2.pH too high and results in lead hydroxide formation

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24 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 Higher pH

25 Non-Water soluble Also not water soluble

26 Other Considerations? Any other metals (including Mg 2+ !!!) can cause a color change False Positives 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!!!!

27 Which complexing agent would you use? Want low value for lead High value for others CN might be good BUT!!!!!

28 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.

29 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 to 49000

30 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?

31 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? 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

32 Method 1 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

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34 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 Baseline estimation Method 2 Much easier And makes no Assumptions about What is contributing To the background

35 Use this lab to introduce another data manipulation Method 3 – assess contribution by assuming Gaussians

36 Assume absorbance peak is Gaussian in the energy spread Energy of The absorption bands Energy of light absorbed Frequency = absorption photons e Energy levels are randomly populated By Temperature Std~(first guess) width at ½ peak ht

37 Deconvolution 1. Get the absorption spectra

38 Deconvolution 1.Assume absorbance peak is Gaussian in the energy spread 2.Convert data from A vs wavelength to A vs energy Notice the 2 curves look Different!

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

40 Wavelength, nm Frequency, Cm -1

41 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

42 Minimize Target cell energy wavelength Prevent solver from giving You non-plausible (negative) numbers

43 Plot the wavelength based absorption data And superimpose the data generated by solver

44 Now sum all the individual bands and see if you get a low sum of sq differences Some deviation here But generally pretty darn good

45 At the wavelength you are interested in Our “signal” Go to 550 and use the A from this band Only!!! (all other absorbances represent Background contributions) 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

46 B.C.E. C.E. ppm ppb ppt LOD cuppellation Suphuretted water titrimetric dithizone Chemistry This method resulted in the first public health Awareness of lead as an issue for children Baltimore, Department of Public Health

47 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 GIGO 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!

48 Chromophore – part of molecule sensitive to light “Selectivity” arm – complexes the metal ion and turns On and off fluorescence Calcein Blue

49 Carboxyl groups only deprotonated Absorbance spectra Emission Spectra, excitation at 320 Excited State Proton transfer 320 nm excitation nm emission ns duration pH 6-8 Note role of resonance here

50 Ground state phenolic deprotonation H-N pH 8-11 Excited State Electron transfer

51 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.

52 Lead quenches emission Why might Pb quench the emission?

53 Structures as determined from NMR

54 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 GIGO 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!

55 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

56 Need to convert Pb to some form that is amenable to IR and/or Raman spectroscopies. Change in bond length photon Pb 2+

57 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?

58 A Case of Forensic Chemistry: Art and Forgeries Lead Tin II, Paolo Veronese, Allegory of Love Peter Rubens, The Dying Seneca Lead Tin I Lead Antimonate

59 Forensic Art Chemistry

60 Lead Tin I Lead Tin II Two Sb octahedra Linked via vertices to a) eight pointed polyhedra Of Pb & b) Hexagonal bipyramid Chains of Sn octahedra Joined by Pyramidally Coordinated Pb(II) Lead Antimonate


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