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Lecture 2. Introduction to Spectroscopic Methods of Analysis (part 2). This lecture will cover:. Molecular absorption spectroscopy Terms employed in absorption spectroscopy: Absorbance & Transmittance Beer’s Law. - PowerPoint PPT Presentation
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Introduction to Spectroscopic Introduction to Spectroscopic Methods of AnalysisMethods of Analysis
(part 2)(part 2)
Lecture 2Lecture 2
This lecture will cover:
• Molecular absorption spectroscopy• Terms employed in absorption
spectroscopy: Absorbance & Transmittance
• Beer’s Law
- is based on measurement of the transmittance, T or the absorbance, A of solutions contained in transparent cells
Molecular Absorption Spectroscopy
Molecular absorption spectroscopy
Absorption
is a measure of the decrease in radiant power
P0 P
Absorbing solution
TERMS EMPLOYED IN ABSORPTION
SPECTROSCOPY
T = P P0
A = -log T
Transmittance - The fraction of incident radiation transmitted through the sample medium.
P0 P
Absorbing solution
of concentration, c
Power of transmitted radiation
Power of incident radiation
b
Commonly expressed as a percentage:
%T = P x 100P0
Absorbance - A measurement of the amount of radiant power absorbed by the sample defined as the negative log of transmittance.
P0 P
b
A = -log T
A = log P0
P
Power of incident radiation
Power of transmitted radiation
Absorbing solution
of concentration, c
• Absorbance has a linear relationship with sample concentration defined by Beer’s Law.
Questions:
1. Convert the following percent transmittance data into absorbance:
i) 33.6 ii) 92.1 iii) 1.75
Answers: i) 0.474ii) 0.0357iii) 1.76
i) %T = 33.6
T = 33.6 / 100
= 0.336
A = - log T
= log 1/T
= log 1/0.336
= 0.474
Answer:
Questions:
2. Convert the following absorbance data into percent transmittance:
i) 0.375 ii) 1.325 iii) 0.012
Answers: i) 42.2% ii) 4.73%
iii) 97.3%
Beer’s Law
A = bc
ε = molar absorptivity, liter mol-1 cm-
1
b = sample path length, cm
c = concentration, mol per liter
- shows linear relationship between absorbance, concentration of the species measured, sample path length and the absorptivity of the species.
A = abc
The term “a” is a proportionality constant called absorptivity.
Absorptivity is a constant for a given chemical species at a specific wavelength.
absorbance
Pathlength (cm)
Absorptivity (Lg-1cm-1)
Concentration (g/liter)
Beer’s Law
Examples:Examples:
E.g. 1 What is the concentration of an absorbing species if its molar absorptivity is 1500 L/mol cm and the measured absorbance in a 1.00 cm cuvette is 0.742?
Answer:
A = εbc c = A / εb
c = 0.742
(1.00cm) (1500L/mol.cm)
= 4.95 x 10-4 M
E.g. 2 The measured absorbance of a sample in a 1.00cm cuvette is 0.544. If the concentration is 1.40 x 10-3
M, what is the molar absorptivity for the species?
Answer:
A = εbc ε = A / b c
ε = 0.544
(1.00cm)(1.40x10-3 mol/L)
= 389 L/mol.cm
E.g. 3 A sample in a 1.0cm cell is determined with a spectrometer to transmit 80% light at a certain wavelength. If the absorptivity of this substance at this wavelength is 2.0, what is the concentration of the substance?
Answer:
The percent transmittance is 80%. So, T = 0.80
A = abc
log 1/T = 2.0 L/g.cm x 1 cm x c
log 1/0.80 = 2.0 L/g x c
c = 0.10
2.0 L/g
= 0.050 g/L
A = abc
• Concentration• Width of cuvette• Inherent ability for the absorbing species to
absorb light
PARAMETERS THAT AFFECT ABSORBANCE
• Width of cuvette
b
b
Wider cuvette more absorbing species present in the path of the light, hence absorbance is greater.
• Inherent ability for the absorbing species to absorb light
- Chemical species vary with respect to this inherent ability since absorption depends on individual electronic and vibrational transitions available in a given species
• Applying Beer’s Law to Mixtures
- Beer’s law also applies to solutions containing more than one kind of absorbing substance, provided there is no interaction among the various species
- Total absorbance for a multicomponent system at a single wavelength is the sum of the individual absorbances.
APPLICATION OF BEER’S LAW
Atotal = A1 + A2 + ………+ An
= ε1bc1 + ε2bc2 + ……… + εnbcn
- Deviations are frequently observed from the direct proportionality btw absorbance, A and concentration, c when pathlength, b is constant.
LIMITATIONS TO THE APPLICABILITY OF BEER’S LAW
Deviations may be due to:
1. Fundamental2. Instrumental3. Chemical deviations
1. Fundamental deviations- Real limitation to the law- At high concentration (0.01M) each
particle affects the charge distribution of its neighbours.
- Therefore, this interaction alter the ability of analyte species to absorb a given wavelength of radiation.
Causing deviation from the linear relationship between absorbance and concentration.
2. Instrumental deviations - due to polychromatic radiation- Beer’s Law strictly applies when
measurements are made with monochromatic source radiation.
- In practice, polychromatic sources that have a continuous distribution of wavelengths are being used.
- Deviations occur if the radiation is polychromatic since the relationship btwn A and c is no longer linear when is differ.
2. Instrumental deviations - due to presence of stray radiation
- Due to instrument imperfections.- This stray radiation is the result of
scattering and reflection off the surfaces of gratings, lenses or mirrors, filters and windows.
- The wavelength of stray radiation differs greatly from the principal radiation & may not have passed thru’ the sample.
- When measurements are made in the presence of stray radiation,
A’ = log P0 + Ps
P + Ps
Ps – power of nonabsorbed stray radiation
% stray radiation = Ps x 100
P0
3. Chemical deviations
- Occur when the analyte undergo dissociation, association or reaction with the solvent to give products that absorb differently than the analyte.