Quantitative Analysis Of Uv-vis Spectros g6956

Quantitative Analysis of UVVis Spectroscopy

1

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. 2

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.

3

Quantitative Analysis The basis for quantitative analysis in the UVVis 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

4

3. The relative standard deviation occurs within 1-3% which reflects good precision. 4. Easy to perform and convenient. 5. 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.

5

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:

6

1. If an interferences are present, the wavelength that is far away from interferences should be selected 2. A wavelength at the maximum of a broad peak should be preferred to another of a sharp peak 3. The peak with a maximum peak height is preferred

7

An analyst should use his experience and knowledge to work for the best bargain of the abovementioned points. Several factors affect the location of the wavelength and the absorbance and thus must be considered. These include the nature of solvent, the pH of solution, electrolyte concentration, interferences, as well as temperature. 8

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. 9

Calibration Curves

10

Standard Addition method

11

Analysis of Mixtures of Absorbing Substances

12

Analysis of Mixtures of Absorbing Substances When the sample solution contains more than one absorbing species, the absorbance of the solution will be the sum of all absorbances: At = A1 + A2 + A3 + ….

13

When only two absorbing species are present, the solution is formidable and is executed by finding the absorbance of the solution at two wavelength (wavelength maximum for each analyte): A’ =  x’bcx +  y’bcy A” =  x”bcx +  y”bcy

(1) (2)

 x’,  x”,  y’,  y” can be determined from standards of analytes x and y at ’” * values obtained are inserted in equations 1 and 2 where two equations in two unknowns can be easily solved. 14

15