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IB Chemistry Atomic Absorption Spectroscopy and Beer Lambert Law
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http://lawrencekok.blogspot.com
Prepared by Lawrence Kok
Tutorial on Atomic Absorption Spectroscopy and Beer Lambert Law
http://www.astrophys-assist.com/educate/orion/orion02.htm
Continuous Spectrum :Light spectrum with all wavelength/frequency
Emission Line Spectrum :• Spectrum with discrete wavelength/ frequency • Emitted when excited electrons drop from higher to
lower energy level Absorption Line Spectrum :• Spectrum with discrete wavelength/frequency • Absorbed when ground state electrons are excited
Continuous Spectrum Vs Line Spectrum
http://www.astrophys-assist.com/educate/orion/orion02.htm
Continuous Spectrum :Light spectrum with all wavelength/frequency
Emission Line Spectrum :• Spectrum with discrete wavelength/ frequency • Emitted when excited electrons drop from higher to
lower energy level Absorption Line Spectrum :• Spectrum with discrete wavelength/frequency • Absorbed when ground state electrons are excited
Atomic Emission Spectroscopy
Electrons from excited state
Emit radiation when drop to ground state
Radiation emitted
Emission Spectrum
Continuous Spectrum Vs Line Spectrum
http://www.astrophys-assist.com/educate/orion/orion02.htm
Continuous Spectrum :Light spectrum with all wavelength/frequency
Emission Line Spectrum :• Spectrum with discrete wavelength/ frequency • Emitted when excited electrons drop from higher to
lower energy level Absorption Line Spectrum :• Spectrum with discrete wavelength/frequency • Absorbed when ground state electrons are excited
Atomic Emission Spectroscopy Vs Atomic Absorption Spectroscopy
Electrons from excited state
Emit radiation when drop to ground state
Radiation emitted
Emission Spectrum
Electrons from ground state
Absorb radiationto excited state
Electrons in excited state
Radiation absorbed
Continuous Spectrum Vs Line Spectrum
http://www.astrophys-assist.com/educate/orion/orion02.htm
Continuous Spectrum :Light spectrum with all wavelength/frequency
Emission Line Spectrum :• Spectrum with discrete wavelength/ frequency • Emitted when excited electrons drop from higher to
lower energy level Absorption Line Spectrum :• Spectrum with discrete wavelength/frequency • Absorbed when ground state electrons are excited
Atomic Emission Spectroscopy Vs Atomic Absorption Spectroscopy
Ground state
Excited stateElectrons from excited state
Emit radiation when drop to ground state
Radiation emitted
Emission Spectrum
Electrons from ground state
Absorb radiationto excited state
Electrons in excited state
Radiation absorbed
Continuous Spectrum Vs Line Spectrum
Line Emission Spectra for Hydrogen
Energy supplied to atoms • Electrons are excited from ground to excited states• Electrons exist in fixed energy level (quantum)• Electrons drop from higher to lower, emitting energy of particular wavelength/frequency• Higher the energy level, smaller the difference in energy between successive energy level.• Spectrum will converge(get closer) with increasing frequency• Lines in spectrum converge- energy levels also converge• Ionisation energy can be determined (Limit of convergence)
N = 3-2, 656nm
N= 4-2486nm
N= 5-2434nm
N= 6-2410nm
Atomic Emission Spectroscopy
Visible region- Balmer Series
UV regionLyman Series n=∞ → n= 1
Visible regionBalmer Series n=∞ → n= 2
IR regionPaschen Series n=∞ → n= 3
Atomic Emission Spectra• Energy supplied• Electrons surround nucleus in allowed energy states (quantum)• Excited electron returning to lower energy level, photon of light with discrete energy/wavelength(colour) will be given out.• Light pass through a spectroscope, with a prism to separate out different colours• Line emission spectra is produced.
Line emission spectra for different elements can be found here
Atomic Emission Spectroscopy
Principle of Atomic Absorption Spectroscopy:• AAS spectroscopy uses absorption of light to measure the concentration of gas-phase atoms. • Sample in liquids/solids must be vaporized in a flame/graphite furnace to gaseous atomic vapour. • Atoms absorb UV / visible light causing electronic transitions to higher energy levels. • Analyte concentration determined from the amount of absorption.• Amount of light absorbed = (Ratio of light intensity transmitted/incident light) • Amount of light absorbed = directly proportional to concentration of analytes/ions/atoms.
Electrons in excited state
Electrons from ground state
Absorb radiationto excited state
Radiation absorbed
http://csep10.phys.utk.edu/astr162/lect/light/absorption.html
1) Detection of metal ions in small quantities (ppm/ppb)2) Detection of Al, Ca, Cr, Fe in various sample• Al - Blood serum• Ca – Blood serum• Cu – Copper based alloys• Cr – Sea water• Fe - Plants
Uses of Atomic Absorption Spectroscopy (AAS)
http://health-hazard-mercury.blogspot.kr/
Parts of Atomic Absorption Spectroscopy
Fuel• Burning mixture of ethyne with O2
(combustion mix)• Sample dehydrated/solvent removed
Atomiser• Atomise sample to atomic
state • Atom vapour (gas phase)
produced
Monochromator• Select light with specific frequency/wavelength of interest• Provide the specific light which will be absorb by metal
Detector• Measure the decrease in intensity of light absorb by analyte• Convert the decrease in light intensity into electrical signal using photomultiplier
Hollow Cathode Lamp• Emits light of specific wavelength to be absorbed by element
http://health-hazard-mercury.blogspot.kr/
Hollow Cathode Copper Lamp• Emits light of specific wavelength to be absorbed by copper element
How AAS works to measure the Copper concentration in a sample ?
1
http://health-hazard-mercury.blogspot.kr/
Hollow Cathode Copper Lamp• Emits light of specific wavelength to be absorbed by copper element
How AAS works to measure the Copper concentration in a sample ?
Hollow cathode lamp (Cu) • Made of element (Cu) and use to determine Cu conc in sample• Light source emits light of specific wavelength/frequency which will be absorb by
the sample/element
• Light source of 325nm used, Cu atoms in sample will absorb that particular wavelength to be excited.
Light , λ = 325nm emitted
Atomic Cu vapour absorb light, λ = 325nm
1
http://health-hazard-mercury.blogspot.kr/
How AAS works to measure the Copper concentration in a sample ?
Fuel/ Atomizer• Fuel – Air/acetylene flame, Temp of 2300°C• Desolvation (drying) – Solvent evaporated and dry vapour sample produced• Atomization – Sample molecules dissociate into free atoms /ions and converted finally to atomic vapour• Copper atoms vapour state produced .
Light , λ = 325nm emitted
Atomic Cu vapour absorb light, λ = 325nm
2
http://health-hazard-mercury.blogspot.kr/
How AAS works to measure the Copper concentration in a sample ?
Detector• Atoms absorb UV/visible light, exciting its electrons (electronic transition) to
higher energy levels. • Measure the decrease in light intensity (emitted by Cu lamp) after being absorbed
by copper atoms• Amount light absorbed is directly proportional to the concentration of the atoms.
3
http://health-hazard-mercury.blogspot.kr/
How AAS works to measure the Copper concentration in a sample ?
Light , λ = 325nm emitted
Atomic Cu vapour absorb light, λ = 325nm Decease in Light Intensity due to Absorption
Detector• Atoms absorb UV/visible light, exciting its electrons (electronic transition) to
higher energy levels. • Measure the decrease in light intensity (emitted by Cu lamp) after being absorbed
by copper atoms• Amount light absorbed is directly proportional to the concentration of the atoms.
3
Transmitted Light Intensity
Initial Light Intensity, Io
Beer’s Lambert Law• Apply for diluted solution• Absorbance, A α Concentration• Absorbance, A = log10 (Io/I) = έlc• Transmittance, T = Fraction of incident radiation (I) transmitted by the solution, T= I/Io
Beer’s- Lambert Law and Standard Calibration Curve
Beer’s Lambert Law• Apply for diluted solution• Absorbance, A α Concentration• Absorbance, A = log10 (Io/I) = έlc• Transmittance, T = Fraction of incident radiation (I) transmitted by the solution, T= I/Io
Beer’s- Lambert Law and Standard Calibration Curve
How Absorbance equation is derived ?
Abs = έlc
Beer’s Lambert Law• Apply for diluted solution• Absorbance, A α Concentration• Absorbance, A = log10 (Io/I) = έlc• Transmittance, T = Fraction of incident radiation (I) transmitted by the solution, T= I/Io
Beer’s- Lambert Law and Standard Calibration Curve
Amount light absorb depend on • έ = Molar extinction of
compound• c= Concentration• l = path length of absorbing
solution
How Absorbance equation is derived ?
Molar extinction of compound, έ :• Measure the strength of absorption of
compound• Higher έ ↑ = Higher ↑ Absorbance• Compound with high έ = effective at absorbing light even when low conc is used.Path Length, l:• Longer path length ↑ – Higher ↑ AbsorbanceConcentration, c:• Higher conc of analyte – Higher ↑ Absorbance
If έ and l = constant• Abs α Conc analyte• Abs ↑= Conc ↑
Abs α Conc, c
Abs = έlc
Preparing a Standard Calibration Curve for Lead ions concentration
Determine the unknown concentration of Pb2+ using Beer-Lambert Law
1
Preparing a Standard Calibration Curve for Lead ions concentration
Determine the unknown concentration of Pb2+ using Beer-Lambert Law
Plot a Standard calibration curve of Abs vs Conc
Conc
Abs
1
2
Preparing a Standard Calibration Curve for Lead ions concentration
Determine the unknown concentration of Pb2+ using Beer-Lambert Law
Plot a Standard calibration curve of Abs vs Conc
Measure Absorbance of unknown (Pb2+) sample
Conc
Conc
Abs
Abs
Abs = 0.340
Conc = 0.310
1
2
3
Preparing a Standard Calibration Curve for Lead ions concentration
Determine the unknown concentration of Pb2+ using Beer-Lambert Law
Plot a Standard calibration curve of Abs vs Conc
Measure Absorbance of unknown (Pb2+) sample
Determine Conc of unknown (Pb2+) by interpolation
Conc
Conc
Abs
Abs
Abs = 0.340
Conc = 0.310
1
2
3 4
Amount of light absorb α Conc of Pb vapour in flameAbsorbance α Conc of Pb atomsAbsorbance, A = έlc έ = molar absorptivity
l = path length constantc = conc of sample
Since έ and l are constantA = c ( Absorbance directly proportional to Conc)
Determine the concentration of unknown using Beer-Lambert Law
7.25 x 10-5M solution of X has a transmittance of 44.1% when measured in a 2.10cm cell at wavelength of 525nm. Calculate the a) Absorbance, Ab) Molar absorptivity, έ
100dm3 of contaminated water was reduced by boiling to 7.50dm3. The reduced volume was tested, it had absorbance of 0.55. Calculate the conc of Pb2+ ions (mgdm3) in original sample.
Question, Q1
Question, Q2
a) Absorbance = -log10
T or Absorbance = έlc b) Molar absorptivity,
έ Abs = - log
10 T έ = A/bc =
0.3554 / ((2.10 x 7.25 x 10-5) = - log10 0.441 = 2.33 x 103 L mol-1cm-1
= -(-0.3554) = 0.355
Determine the concentration of unknown using Beer-Lambert Law
7.25 x 10-5M solution of X has a transmittance of 44.1% when measured in a 2.10cm cell at wavelength of 525nm. Calculate the a) Absorbance, Ab) Molar absorptivity, έ
100dm3 of contaminated water was reduced by boiling to 7.50dm3. The reduced volume was tested, it had absorbance of 0.55. Calculate the conc of Pb2+ ions (mgdm3) in original sample.
V= 100dm3
M= ?V= 7.50dm3
M= 1.15mg/dm3
Amount /moles before heating = Amount/moles after heatingMoles before = Moles afterM x V = M x VM x 100 = 1.15 x 7.50M = (1.15 x 7.50)/100M = 8.63 x 10-2 mgdm-3
Question, Q1
Answer to Q1
Question, Q2
Answer to Q2
Conc of Pb2+ from graph – 1.15mgdm-3
Acknowledgements
Thanks to source of pictures and video used in this presentation
Thanks to Creative Commons for excellent contribution on licenseshttp://creativecommons.org/licenses/
Prepared by Lawrence Kok
Check out more video tutorials from my site and hope you enjoy this tutorialhttp://lawrencekok.blogspot.com