CM2007 Lecture 3

1. CM2007 Lecture 3

2. Background Correction • A baseline spectrum of the solvent must be obtained in order to subtract from the spectrum of the solvent + analyte. • The baseline spectrum is normally recorded by placing a cell, filled with the appropriate solvent (minus analyte) into the spectrophotometer. • Baseline spectrum is recorded before the analyte spectrum using a single beam instrument. • Double beam instruments record both spectra simultaneously. • However, the intensity of the dual beams must be the same, the cells must posses exactly the same absorbtivity, and the solvents must be exactly the same

3. Application of UV/Vis to Quantitative Analysis • Solvents need to be considered in UV/Vis Spectroscopy • Parameters to consider • Transparency • Solvent effect on absorbing species, e.g., polar solvents obliterate fine structure. • Compounds exhibit different absorption maxima in various solvents • Absorption maxima shift depending on the solvent • E.g., acetaldehyde absorbs most strongly at 287nm in heptane and at 278nm in water.

4. Solvent Effects on Acetaldehyde

5. The Origin and Position of Absorption Bands • Bathochromic (red) Shift = shift of max to longer wavelength • Hypsochromic (blue) Shift = shift of max to shorter wavelength • Hyperchromic Shift = intensity increase of the band. • Hypochromic Shift = intensity decrease of the band

6. Inorganic Spectra • UV/Vis can be used to quantitatively determine any absorbing species. • Also reagents can be used to react selectively with non-absorbing species to give products which absorb strongly in the UV/Vis. • Non-absorbing inorganic species can be determined using complexing agents. • E.g., thiocyanate ion for Fe, Co and Mo. Peroxide anion for Ti, V and Cr. Iodide for Bi, Pd and Te. • Also important are organic chelating agents that form stable, coloured complexes with cations. • E.g., o=phenanthroline for Fe, dimethylgloxime for Ni, diethyldithiocarbamate for Cu and diphenyldithiocarbazone for Pb

7. Experimental Considerations • Wavelength selection: make measurements at a wavelength corresponding to the absorption maxima. • Variables which influence absorption are solvent, pH, temperature, electrolyte concentration and interferences. • Cleaning and handling of cells • Materials used to make cells/cuvette. In order of preference try to use matched quartz cell, glass cells and as a last resort use plastic. • Prepare calibration curve to determine the relationship between absorbance and concentration.

8. Analysis of Mixtures • Total absorbance of a solution at a given wavelength is equal to the sum of absorbances of all the components present. • No wavelength exists at which the absorbance of the mixture is due to one of the components. • The absorbance of a mixture at two wavelngths ’ and ’’ may be expressed as: A’ = em’ cm l + en’ cnl A’’ = em’’ cm l + en’’ cnl • The molar absorbtivities em’, en’, em’’ and en’’ can be evaluated either from individual standards of M and N or from the slopes of the Beer-Lambert plots • The absorbances A’ and A’’ and the cell length l can be determined experimentally. Therefore the individual concentrations can be determined

9. Analysis of Mixtures

10. Isobestic Point • Often one absorbing species, X, is converted to another absorbing species, Y, during the course of a reaction. • This transformation leads to a very obvious and characteristic behaviour. • If the spectrum of pure X and pure Y cross each other at any wavelength, then any spectrum recorded during this reaction will cross at the same point • The observation of an isobestic point during a reaction is good evidence that only two principal species are present. • E.g., methyl red changes between red (Hin) and yellow (In-) near pH = 5.5

11. Isobestic Point

12. Infrared Spectroscopy • IR spectrum encompasses wavelengths 800 – 1,000,000nm or 0.8 - 1000μm • Analytical IR techniques normally only exploit radiation in the range 2500 – 16,000nm (2.5 - 16μm) • Molecules oscillate in a predictable manner around molecular bonds. (bending, stretching and vibrating) • IR is used for qualitative structural identification of compounds. • By historical convention IR spectra are displayed in a different manner to UV/Vis. • The y-axis is of an IR spectrum is plotted in terms of percentage transmittance.

13. IR Spectra

14. % Transmittance

15. IR Spectra • The x-axis is not displayed in terms of either wavelength of frequency but ‘wavenumber’. • Wavenumbers represent the reciprocal of wavelength (1/λ) and have units of cm-1. • It should be noted that increasing wavenumbers correspond to increasing frequency and, therefore, to progressively more energetic radiation. • The identification of absorption peaks can be further used to identify a class of molecule, e.g., alcohol, aldehyde, ketone, ether, ester. • Characteristic absorption bands for molecular vibration are tabulated in ‘correlation charts’ to aid structural identification of spectra

17. Interpretation of Spectra 3350 -- OH stretching vibrational frequency2950 -- CH aliphatic asymmetrical stretching vibrational band. The less intense band at 2860 is the symmetrical stretching vibrational band.1425 -- CH2 characteristic absorption1065 -- CO absorptionThe compound is cyclohexanol.

18. 3100 -- The broad intense absorption band seen here is characteristic of a carboxylic acid dimer. 2960 -- CH aliphatic assymmetric stretch2870 -- CH aliphatic symmetic stretching vibrational band.1415 -- Absorption in this region is due to CH3. Note the weak band just below 1400. This is the methyl bending vibrational band.1290 -- Due to coupling of the in-plane OH bending and CO stretching of the dimer.950 -- OH out-of-plane bending of the dimer.The compound is octanoic acid

19. CM2007 Tutorial

20. Tutorial Questions • Which wavelength range encompasses the UV/Vis spectrum? • Draw a schematic diagram of a spectrophotometer. • State the Beer Lambert law and define the parameters • What are the three possible deviation from the Beer Lambert law? Real, Instrumental and chemical. • What are the typical radiant sources used to provide broadband light in UV/Vis spectrophotometry? • How can the spectral range of a tungsten filament lamp be extended in the UV? • Describe how a monochromator works to provide monochromatic light.

21. Tutorial Questions • Describe the different categories of cells/cuvettes available as sample holders. • What is the basis and principles of IR spectroscopy? • What are the differences between IR and UV/Vis spectra? • Describe the different sample prep for the analysis of a solid using IR. Why is KBr used? • http://wwwchem.csustan.edu/Tutorials/INFRARED.HTM • What is the term used to describe a shift in λmax to a longer wavelength.

22. Tutorial Questions 16/ A mixture of zinc sulfate and cobalt tetrachloride yields an absorbance reading of 0.22 at a λ = 600nm. The concentration of cobalt tetrachloride is known to be 1.0x10-2 M in the mixture and has a molar absorbtivity coefficient of = 11 L mol-1 cm-1. What is the absorbance reading of cobalt tetra chloride if the path length of the cell is 1cm? What is the absorbance reading for zinc sulphate? Given the data for zinc sulphate below, plot the data on graph paper and determine the concentration of zinc sulphate in the mixture and calculate its molar absorbtivity coefficient. If 600nm = λmax for zinc sulphate, what is its likely colour?

23. Tutorial Questions

24. Tutorial Questions • Atotal = (ecl)x + (ecl)y • The absorbance reading of cobalt tetra chloride is calculated as follows • A = 11L mol-1 cm-1 x 1.0 x 10-2M x 1cm = 0.11 • Therefore the absorbance of copper sulfate = A = 0.22 – 0.11 = 0.11

25. Tutorial Questions

26. Tutorial Questions • Concentration of copper sulphate from the graph = 5.5 x 10-3 M and the molar absorbtivity coefficient = 20 L mol-1 cm-1. Likely colour = blue due to the absorbance of light in the red region of the spectrum.