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UV / visible Spectroscopy

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Presentation on theme: "UV / visible Spectroscopy"— Presentation transcript:

1 UV / visible Spectroscopy
Colorimetric Analysis Quantitative Analysis Inorganic species 3 20 2

2 UV / visible Spectroscopy
Two types of electronic transition Transisitions involving atomic orbitals Charge transfer transitions 3 21 2

3 UV / visible Spectroscopy
The ions and complexes of elements of the first two transition series absorb broad bands of visible radiation in at least one of their oxidation states and are, as a consequence, coloured. 3 22 2

4 UV / visible Spectroscopy
3 31 2

5 UV / visible Spectroscopy
Absorption involves transitions between filled and unfilled d-orbitals. The energy differences between the d-orbitals (and thus the position of the corresponding absorption peak) depend upon the the position of the element in the periodic table, its oxidation state, and the nature of the ligand bonded to it. 3 23 2

6 UV / visible Spectroscopy
3 31 2

7 UV / visible Spectroscopy
Charge transfer absorption is of particular interest to analytical chemists because molar absorptivities are usually large (max > 10,000). Methods based upon this type of absorption are highly sensitive. 3 25 2

8 UV / visible Spectroscopy
Many organic and inorganic complexes exhibit charge transfer absorption and are known as charge transfer complexes. Thiocyanate and phenolic complexes of iron(III). 3 26 2

9 UV / visible Spectroscopy
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10 UV / visible Spectroscopy
Charge transfer complexes generally contain both an electron donor group and an electron acceptor group. Absorption of radiation involves the transfer of an electron from the donor group to an orbital associated with the acceptor group or vice versa. 3 27 2

11 UV / visible Spectroscopy
Six criteria for a successful analysis Specificity of the colour reaction Proportionality between colour and concentration Stability of the colour Reproducibility Clarity of the solution High sensitivity. 3 28 2

12 UV / visible Spectroscopy
Specificity of the colour reaction Very few reactions are specific for a particular substance. However, many only give colours for a small group of related substances. Therefore they can be considered to be selective. Alteration of oxidation states and pH enhances selectivity. 3 29 2

13 UV / visible Spectroscopy
Proportionality between colour and concentration It is desirable that the system under investigation follows the Beer-Lambert law. Stability of the colour Not all complexes are stable with respect to time. The stability of the complex should be sufficient to allow precise measurements to be made. 3 2 30

14 UV / visible Spectroscopy
Reproducibility The colorimetric procedure must give reproducible results under the experimental conditions. The reaction need not represent a stoichiometrically quantitiative change. 3 32 2

15 UV / visible Spectroscopy
Clarity of the solution The solution must be free of precipitates. Turbidity scatters and absorbs light. High sensitivity It is desirable that the colour reaction be highly sensitive. i.e.  is very large. 3 33 2

16 UV / visible Spectroscopy
Steps in carrying out a colorimetric analysis. Choose the wavelength of maximum absorbance. Prepare a calibration curve using known quantities of the complex measured at this wavelength. Measure the absorbance of your unknown sample. Calculate the concentration from the equation of the best fit line. 3 34 2

17 UV / visible Spectroscopy
Calibration Curves 1 2 3 4 Nickel Concentration (mg/l) Abs y = x R2 = 3 34 39

18 UV / visible Spectroscopy
Diethyl dithiocarbamate Diphenyl carbazone 3 2 31

19 UV / visible Spectroscopy
Advantages of colorimetric analysis. Better at low concentrations than titrimetric or gravimetric analysis. Can be applied under conditions where there are no satisfactory titrimetric or gravimetric procedures. Very rapid once a calibration curve as been obtained. 3 35 2

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