BÁO CÁO ANH VĂN CHUYÊN NGÀNH 3 NGÀNH CÔNG NGHỆ HÓA HỌC

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GROUP : 4

 Before the beginning of the 20th century most

quantitative chemical analyses used titrimetry as the analytical method analysts achieved

highly accurate result

 But limited Other methods developed

during this period extended quantitative analysis

to include trace level analytes Colorimetry

 One example of an early colorimetric analysis is

Nessler’s method

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Nessler’s method.

 The Nessler’s for ammonia

 It was first proposed in 1856

 Nessler’s found that adding an alkaline solution

of HgI2 and KI to a dilute solution of ammonia

produced a yellow to reddish brown colloid with the color determined by the concentration of

 Infrared electromagnetic radiation.

 During the 20 th , spectroscopy has been extended to include other form of electromagnetic radiation (photon spectroscopy)

Introductions. SPECTROSCOPY Group 4 – DHHC6B

 Spectroscopy is used to qualitatively or

quantitatively study the atoms or molecules, or to study physical processes

 The interaction of radiation with matter can

cause redirection of the radiation and/or

transitions between the energy levels of the

atoms or molecule

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 A transition from a lower level to a higher level

absorption ( transfer energy)

 A transition from a higher level to a lower level

 emission (transfer energy)

 Redirection of light due to its interaction with

matter scattering (may or may not occur with transfer of energy)

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Absorption SPECTROSCOPY Group 4 – DHHC6B

Type of excitation depend on the wavelength of the light UV/Visible promoted electrons to higher orbital

Infared excited vibrations

Atoms or molecules absorb light  a higher energy level

Microwaves excited rolations

Measuring the concentration of absorbing species

in a sample is accomplished by Beer-Lambert Law

An absorption spectrum

Useful for indentifying

of compounds

Depend on its energy level structure

Emission SPECTROSCOPY Group 4 – DHHC6B

1.How is the emitting radiation?

2 Atomic – emission spectroscopy and Atomic – fluorescence spectroscopy

3 How is the flourescence of molecules and the phosphorescence of molecules?

Emission SPECTROSCOPY Group 4 – DHHC6B

Emission Group 4 – DHHC6B

 For molecules it is called :

• Fluorescence if the transition is between states of the same spin.

• Phosphorescence if the transition occur between

states of different spin

The emission intensity of an emitting substance is linearly proportional to analytes concentration at low concentration, and is useful for quantitating emitting species.

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UV/VIS and infrared

determined by:

• Using Nessler tubes.

• Using an instrument called a colorimeter.

3

IR was discovered

in 1800, their uses in optical molecular

absorption spectroscopy

Group 4 – DHHC6B SPECTROSCOPY

UV radiation was discovered in

1801, was limited

by the lack convenient for detecting the radiation.

4

Introduction UV/VIS

spectrophotometer

 The UV/VIS spectrophotometer uses two light sources:

 A deuterium (D2) lamp for ultraviolet light.

 A tungsten (W) lamp for visible light.

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Introduction UV/VIS

spectrophotometer

 Principles of machine operation:

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Single-Beam UV/VIS

Spectrophotometer

 Single-Beam spectrophotometer are often

sufficient for making quantitative absorption

measurements in the UV/VIS spectral region

 The concentration of analyte in solution can be determined by:

- Measuring the absorbance at a single

wavelength

- Applying the Beer-Lambert Law

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A photodiode dectector

A sample container.

The simplest instruments use a single-wavelength light source.

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Single-Beam UV/VIS

Spectrophotometer

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Dual-Beam UV/VIS

Spectrophotometer

 In UV absorption spectroscopy, obtaining a

spectrum requires manually measuring the

transmittance of the sample and solvent at each wavelenght

 The double-beam design greatly simplifies this process by measuring the transmittance of the sample and solvent simultaneously

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Mono-SPECTROSCOPY Group 4 – DHHC6B

ultraviolet or visible radiation find

widespread application for the qualitative and quantitative determination of molecular species

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Applications to absorbing species

Application

Applications to nonabsorbing species

Applications to nonabsorbing species

 UV/VIS spectrophotometry have somewhat

limited application for qualitative analysis

Unambiguous identification is impossible

 Confirmation of the presence of an aromatic

amine or a phenolic structure may be obtained

by comparing the effects of pH on the spectra of solutions containing the sample with those

Application of absorption

measurement to qualitative

analysis

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Quantitative analysis by

absorption measurements

 Absorption spectroscopy is one of the most useful and widely used tools available to the chemist for quantitative analysis

 Important characteristics of spectrophotometric and photometric methods include:

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• Moderate to high selectivity

• Moderate to high selectivity

 Examples include nitrite, nitrate, and chromate ions;

osmium and ruthenium tetroxides; molecular iodine; and ozone

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Applications to

nonabsorbing species

 Numerous reagents react selectively with

nonabsorbing species to yield products that

absorb strongly in the ultraviolet or visible

regions

 The successful application of such:

 Reagents to quantitative analysis usually requires that the color

 Forming reaction be forced to near completion

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Applications to

nonabsorbing species

• Forming reagents are also frequently employed for the determination of absorbing species such as transition- metal ions

• The molar absorptivity of the product will frequently be orders of manitude greater than that of the uncombined psecies

Note

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Applications to

nonasorbing species.

determination of inorganic species.

agents that form stable, colored complexes with

cations.

The thiocyanate

ion for Fe,

Co, Mo

The thiocyanate

ion for Fe,

Co, Mo

The anion of H2O2 for Ti,

Va, Cr

The anion of H2O2 for Ti,

Cleaning and handing of cells

Selection of wavelength

Standard addition

method

Variables that influence absorbance

Determination of the

relationship between

absorbance and concentration

Procedural details

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The nature of the solvent

The presence of interfering subtances

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Procedural details Group 4 – DHHC6B

 Spectrophotometric absorbance measurements are ordinarily made at a wavelength corresponding to an absorption peak, because the change in absorbance per unit of concentration is greatest at this point; the maximum sensitivity is thus realized

 In addition, the absorption curve is often flat in the region; under these circumstances, good adherence

to Beer’s law can be expected Finally, the measurements are less sensitive to uncertainties arising from failure to reproduce precisely the wavelength setting of the instrument

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Procedural details Group 4 – DHHC6B

 After deciding upon the conditions for the analysis,

it is necessary to prepare a calibration curve from a series of standard solutions These standards should approximate the overall composition of the actual samples and should cover a reasonable concentration range of the analyte

 Seldom, if ever, is it safe to assume adherence to Beer’s law and use only a single standard to determine the molar absorptivity The results of an

analysis should never be based on a literature value for the molar absorptivity

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Procedural details Group 4 – DHHC6B

 It is apparent that accurate spectrophotometric analysis requires the use of good – quality, matched cells These should be regularly calibrated against one another to detect differences that can arise from scratches, etching, and wear

 Equally important is the use of proper cell cleaning and drying techniques

 Erickson and Suries recommend the following cleaning sequence for the outside windows of cell

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Procedural details Group 4 – DHHC6B

 Prior to measurement, the cell surfaces are cleaned with a lens paper soaked in spectrograde methanol

 The paper is held with a hemostal; after wiping, the methanol is allowed to evaporate, leaving the cell surfaces free of contaminants

 The authors showed that this method was far superior to the usual procedure of wiping the cell surfaces with a dry lens paper, which apparently leaves lint and films on the surface

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Procedural details Group 4 – DHHC6B

 Ideally, calibration standards should approximate the composition of the sample to be analyzed not only with respect to the analyte concentrations of the other species in the sample matrix, in order to

minimize the effect of various components of the sample on the measured absorbance

 For example, the absorbance of many colored complexes of metal ions is decreased to a varying degree in the presence of sulfate and phosphate ions

as a consequence of the tendency of these anions to form colorless complexes with metal ions

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Procedural details Group 4 – DHHC6B

 The color – formation reaction is often less complete

as a consequence, and lowered absorbances are the results The matrix effect of sulfate and phosphate can often be counteracted by introducing into the standards amounts of the two species that approximate the amounts found in the samples

 When complex materials as solid, minerals, plant ash are being analyzed, preparation of standards that match the samples is often impossible When this is the case, the standard addition method is often helpful

in counteracting matrix effects

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Procedural details Group 4 – DHHC6B

 The standard addition method can take several forms The one most often chosen for photometric or spectrophotometric analyses, and the one that will be discussed here, involves adding one or more increments of standard solution to the sample aliquots of the same size

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Procedural details Group 4 – DHHC6B

 Each solution is then diluted to a fixed volume before measuring its absorbance It should be noted that when the amount of sample is limited, standard additions can be carried out by successive introductions of increments of the standard to a single measured aliquot of the unknown Measurements are made on the original and after each addition This procedure is often more convenient for voltammetric and potentiometric

measurements and will be discussed in later sections

of the text

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