Spectrophotometry free essay sample

Potassium permanganate (KMnO_4) is commonly used as an oxidizing agent and in this experiment, it is used as both the titrant and indicator. A standard of potassium permanganate solution was prepared and computed to be 0. 02235 M and was used to titrate a sulfuric acid solution wherein the sample salt was dissolved. The solution was heated to 90 °C then titrated until a light pink colored solution was obtained. The experiment focused on determining the percentage of Sodium oxalate (Na2C2O4) in the sample and at the end of the experiment it was found to contain 49. 45%. Experiment 9 focuses on determining the different concentrations of potassium permanganate solutions by finding its absorbance through the use of spectrophotometer. Four known concentrations were prepared; 2. 5 x ? 10? ^(-3) M, 6. 25 x ? 10? ^(-4) M, 1. 25 x ? 10? ^(-4) M, 6. 25 x ? 10? ^(-4) M. The solutions were placed on the spectrophotometer to determine absorbance together with the unknown. Distilled water was placed before each trial to ensure the accuracy of results. For example, an orange solution appears orange because it is absorbing all of the colors except orange. A sample may also appear orange if all colors of light except blue are transmitted. This is because blue and orange are complimentary colors. (see figure below)The wavelength associated with the complimentary color is known as the wavelength of maximum absorbance. The wavelength of maximum absorbance is used when determining the concentration of a colored solution since at this wavelength a slight change in concentration allows for a significant change in the absorbance of light.Many compounds involving transition elements are colored. This is because the transition metals include d electrons in its atomic structure. The spacing of these d-orbitals allow for electronic transitions within the energy range of the visible region of the electromagnetic spectrum. Compounds containing the alkali and alkaline earth metals are white due to having only s electron transitions. More energy is required to cause this type of transition and, thus, light of shorter wavelength is involved.The amount of light absorbed by a solution is dependent on the ability of the compound to absorb light (molar absorptivity), the distance through which the light must pass through the sample (path length) and the molar concentration of the compound in the solution. This relationship is known as Beers Law and is represented by the equation:A = * l Cwhere A is the absorbance, * is the molar absorptivity, l is the path length and C is the molar concentration. If the same compound is being used and the path length is kept constant, then the absorbance is directly proportional to the concentration of the sample.A spectrophotometer is used to provide light of certain energy (wavelength) and to measure the absorbance of that light. The basic operation of the spectrophotometer includes a white light radiation source which passes through a monochromator. The monochromator is either a prism or a diffraction grating which separates the light into the colored components and allows only light of a particular wavelength to strike the sample. The sample is poured into a cuvette which is similar to a small test tube. It is marked so that it can be positioned in the light beam the same way each time to avoid variations due to the differences in the composition of the glass. The light passes through the sample and the unabsorbed portion strikes a photodetector which produces an electrical signal proportional to the intensity of light. The signal is converted to a readable output which is used in the analysis of the sample.OBJECTIVES†¢ To introduce the use of spectrophotometry for qualitative (what is it) and quantitative (how much is there of it) analysis of biological samples and molecules.†¢ To apply laboratory skills in the lab†¢ To determine absorbance spectrum of unknown compounds (qualitative analysis).†¢ To prepare and graph standard or calibration curves for quantitative analysis using colorimetry.