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Chapter Eleven

Ultraviolet-Visible Spectrophotometry

• Spectrophotometry is one of the most widely used methods of analysis in clinical chemistry, hygiene analysis, medicine analysis.

• It is used to measure the absorption capabilities of

certain compounds with reference to wavelengths

of light on the ultraviolet and visible light ranges.

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• High sensitive, can used microanalysis

10-3 ～ 10-6mol/L

e.g.

There is 10-8g Fe in 10ml solution, if titrate it using KMnO4 standard solution

(c=2.0×10-4 mol/L), only consumed 0.02ml KMnO4 standard solution.

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11-1 Properties of Light

11-2 Absorption of Light *

11-3 The Spectrophotometer

11-4 Typical Analysis Procedures

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11-1 Properties of Light

• Light is electromagnetic wave• The dual nature of particles and waves *Wavelength λ unit: nm *Frequencyν (Hz) unit of frequency is s-1

Hertz: One oscillations per second is called 1Hz

106 s-1 is therefore said to be 106 Hz, or one megahertz (MHz).

*Wavenumber unit: nm-1, m-1, cm-1

1

v

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The relationship between wavelength and frequency

c

c

c is the speed of 1ight in vacuum. c=2.99792458 ×108 m s-1

c

vv

1

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• E=hv = c

h vhc

• The light with certain wavelength

or frequency have definite energy

• Photons

Planck's constant h=6.626176×10-34J·s

One mole of photons is called one einstein.

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• Complement Color: The observed color is said

to be the complement of the absorbed color.

红橙

黄

绿青

青蓝

蓝

紫

白光

• Any substance that

absorbs visible light

will appear coloredwhite light

yellow

green

orangeredpurple

blue

greenish blue

Bluish green

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Table 11-1 Colors of visible light and complementary hue Wavelength Color Color Absorbed/nm absorbed observed 380-420 Violet Green-yellow 420-440 Violet-blue Yellow 440-470 Blue Orange 470-500 Blue-green Red 500-520 Green Purple 520-550 Yellow-green Violet 550-580 Yellow Violet-blue 580-620 Orange Blue 620-680 Red Blue-green 680-780 Purple Green 

 

 

 

 

 

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11-2 Absorption of Light

Ia

c

b

= abcI

Iolg

Absorbance

Lambert – Beer,s law

The absorbance (A) of a sample

is the logarithm of the ratio of Io / I .

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= abcI

IA olg Lambert–Beer,s law

A- absorbancea - absorptivity (L·g-1·cm-1 )b - path length (cm)c - mass concentration (g/L)

c ： mass concentration(g/L)→ concentration (mol·L-1) a ： absorptivity (L/g·cm ) → ε (L·mol-1·cm-1 ) ε ： the molar absorptivity

bcA

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Transmittance (T) is the ratio of I/Io .

I

IT

TTI

IA o lg

1lglg

bcI

I

TA o lg

1lg

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• T: 　 range 0 ~ 1

• l00·T : (the percent transmittance) varies

between 0 and 100%.

• When no light is absorbed

T=1=100% , P=P0 and A=0

I= I0 and A=0

kcbTp

pA loglog 0

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• Lambert – Beer,s law, commonly called Beer's law:

Absorbance is directly proportional to the co

ncentration of light-absorbing species in the s

ample and the pathlength of the solution.

Lambert–Beer,s law is strictly valid for purely

monochromatic radiation; that is, for radiation

consisting of only one wavelength.

Monochromatic light

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Absorption spectrum of iron( )Ⅱ-o-phenanthroline complex

Absorption spectrum depicts what wavelengths of light are absorbed by a sample. One can readily see what wavelengths of light are absorbed (peaks).

The maximum absorption at 508nm was characteristic of the complex solution. When the concentration of complex increased, the absorption spectrum increased.

……

……

……

λmax

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11-3 The Spectrophotometer • Major Components

Light source

Wavelength Selector

monochromator

Samplecell

Lightdetectorindicator

tungsten lamp a grating

phototube

cuvette

scale

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A1=εbc1

A2=εbc2

11-4 Typical Analytical Procedures

Direct comparison method

Methods and Applications of

Spectrophotometry

Ⅰ. Direct comparison method

2

1

2

1

2

1

c

c

bc

bc

A

A

11

22 c

A

Ac

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Standard curve

··

·

·

·

Ax ··········

···········

cx

abcA• c0 c1 c2 c3 c4 c5

A0 A1 A2 A3 A4 A5

Ⅱ. Standard Curve Method

The graph should be a straight line.

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• Analytical Procedures

1) Selection of monochromatic light

(Choice of Wavelength)

2) Prepare a reagent blank containing all reagents, but with analyte replaced by distilled water.

3) Prepare a series of standards solution and

measure their absorbance 4) Establish a calibration curve

5) the unknowns be prepared using the same procedure as for standards solution

6) Calculate the content of unknown

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Analytical

Procedures

Choice of λmax

Measure their absorbance

Prepare solutions

Calculate the content of unknown

Establish a calibration curve

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Example:

A sample in a 1.0 cm 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,

1) what is the concentration of the substance(g/L).

2) what is the absorbance of the substance

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Solution:The percent transmittance is 80%, and so T=0.80 ， a=2.0 ， b=1.0

Lgc /05.00.2

10.0

c 0.10.280.0

1log

A=abc = - logT = -log0.80 = 0.0969

abcT

A 1

lg

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Problems:1. The transmittance of a solution is found to be

35.0%. What is the transmittance if the solution

is diluted in half?

2.The transmittance of a solution is found to be 85.0% when measured in a cell whose path length is 1.00cm. What is the transmittance if the path length is increased to 4.00 cm ?

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3. A colored substance M has an absorption maximum at 520 nm. A solution containing 2.00 mg M per liter has an absorbance of 0.840 using a 2.00 cm cell. The formula weight of M is 150.

(a) Calculate the molar absorptivity of M at 520 nm.(b) How many milligrams of M are contained in 25.00 mL of a solution giving an absorbance of 0.250 at 520 nm when measured with a 1.00 cm cell ?

4. The molar absorptivity of benzoic acid (M=122.1g·mol-

1) in methanol at 275 nm is about 1950 L·mol-1·cm-1. If it is desired to use an absorbance not exceeding 1.25, what is the maximum allowable concentration in g·L-1 that can be used in a 2.00 cm cell.