Applications of UV-Vis Spectroscopy Lecture 27

Absorption Involving d and f Orbitals   Many transition metals have colored solutions and are also colored in the solid state. The transition metals have some of their d orbitals empty where a d-d transition can occur. The d-d transitions require excitation energy in the UV-Vis region. The direct interaction of the d electrons with ligands around the transition metal results in a spectrum of broad band nature. On the other hand, inner transition elements show transitions by absorption of UV-Vis radiation (f-f transitions). Since the electrons in the f orbitals are far inside the metal orbitals and are screened by electrons in orbitals of higher principal quantum numbers, f-f electronic transitions will not be affected by the nature of ligands or solvent around the inner transition metals. Therefore, the spectra of inner transition metals have narrow bands.

The crystal field theory is usually used to explain splitting of the d orbital energy so that a transition from a lower energy d orbital electron can be excited to a higher energy d orbital. The theory will be described for a transition metal with six ligands or molecules of water around it. An octahedral (only this case will be discussed) arrangement of these ligands is most appropriate where ligands will be located at the z axis and at the x and y axis (will repel electronic cloud in the dz2 and dx2-y2 orbitals and thus will make these two orbitals to have higher energies). The other four ligands will be arranged in between axis (dxy, dxz, and dyz) which will increase their energies but to a lower extent. The net result is the splitting of the degenerate d orbital into two groups of d orbitals of different energies (the energy difference is referred to as D).

Transition metal ions

Charge Transfer Absorption   When a ligand permanently donates an electron to a metal ion, a charge transfer is said to take place. The net outcome of the process is an oxidation reduction phenomenon occurring within the complex. An example is the reaction of Fe3+ with thiocyanate where the product is an excited species with neutral thiocyanate and Fe3+. In less common situations, the transfer of electrons can take place from the electron deficient metal ion to the ligand. An example is the Fe2+ or Cu+ complexes with 1,10-phenanthroline where Fe2+ and Cu+ metal ions donate electrons to 1,10-phenanthroline. The complex will then have Fe3+ and Cu2+ ions. Charge transfer complexes are of special interest their molar absorptivities are unusually high; allowing very sensitive determinations.

Applications of Absorbance Measurement to Qualitative Analysis   As seen earlier, the broad band absorption spectra obtained in UV-Vis absorption spectroscopy is usually featureless and lacks details that can be used in qualitative analysis. Therefore, this technique is mainly a quantitative technique. Plotting Spectral Data A plot of either the absorbance or %transmittance against wavelength can be made. However, the most common practice is to plot the absorbance versus wavelength.

Solvents   We have seen earlier that solvent polarity affects the absorptivity of the analyte molecules; due to change in transition energies. Usually, polar solvents are used when possible. However, polar solvents like water or alcohol tend to remove the fine spectral details. Therefore, in cases where the fine spectral details are really needed (as in qualitative analysis) a non polar solvent like hexane should be used. In addition, the solvent must be optically clear (does not absorb incident radiation), well dissolve the sample, and chemically pure.

Quantitative Analysis   The basis for quantitative analysis in the UV-Vis relies on Beer’s law. Several characteristics of quantitative measurements using UV-Vis absorption spectroscopy can be rationalized: 1. Applicability to all types of analytes as far as they absorb in the UV-Vis region. 2. Moderate sensitivities in the range from 10-4 to 10-6 with possibility to extend this range under certain conditions. 3. The relative standard deviation occurs within 1-3% which reflects good precision. 4. Easy to perform and convenient. 5. Can be used for quantitative analysis in liquid chromatographic separations. 6. Non absorbing species can also be determined if they are derivatized with an absorbing species as the case of metal ions when complexed to ligands.

Procedural Details Selection of Wavelength   The first step in a successful determination is to find the suitable wavelength for the analysis. This is accomplished by plotting the absorbance/wavelength curve. However, the following points should also be considered: If more than an absorption maximum is available, the wavelength far from the instrument extremes should be preferred A wavelength at the maximum of a broad peak should be preferred to another of a sharp peak The peak with a maximum peak height is preferred If interferences are present, the wavelength that is far away from interferences should be selected Working in the visible region should be preferred

2. Cleaning and Handling the Cell   First, one should appreciate the use of good quality matched cells that are free from wearing, etching, and scratches. In addition, cleaning procedures of external and internal cell surfaces are also important. A suggested cleaning procedure involves moistening a lens paper with methanol and wiping the external surface, then leaving the cell to evaporate. The interior of the cell is first washed with water followed by methanol and the solvent is also allowed to evaporate. Disposable polypropylene cuvettes are incompatible with non polar solvents and formulations having these solvents should be avoided; or large errors will be encountered.

3. Calibration Curves   Usually, a plot of the absorbance of a series of standards is plotted versus the concentration. The absorbance of the unknown is then determined and the prepared calibration plot is used for the determination of the analyte concentration. If the absorbance of the analyte was located outside the calibration plot, more standars should be made or the analyte concentration must be adjusted to occur on the calibration plot. We have seen earlier that it is not allowed to theoretically extrapolate or interpolate a calibration plot. It should also be appreciated that the composition of standard solutions must approximate that of the sample solution. In cases where the sample composition is not clear, the method of standard addition should be used. The slope of the linear calibration plot is the molar absorptivity when the path length is 1.00 cm. Larger slopes mean higher sensitivities.

4. Standard Addition method   The use of this method provides better correlations between absorbances of standards and sample due to constant matrix effects. The method involves addition of the same sample volume to a set of tubes or containers. Variable volumes of a standard are added to the tube set followed by completion to a specific volume. Now, all tubes contain the same amount of sample but different concentrations of analyte. A plot is then made for the volume of standard and absorbance. This plot will have an intercept (b) with the y axis and a slope equals m. The concentration of the analyte can be determined by the relation: Cx = bCs/mVx Where, Cx and Vx are concentration and volume of analyte and Cs is the concentration of standard.

One can only use two points to get the analyte concentration using the relation: Cx = AxCsVs/(At –Ax)Vx Where, Ax is the absorbance of the analyte solution brought to a final volume Vt and At is the total absorbance of the solution (final volume is Vt) containing same amount of analyte and a volume of the standard (Vs). However, this procedure assumes that both points occur at the linear portion of the calibration plot. In case where this is not true, an error will be encountered.