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Forensic Science - Skills For The Laboratory - Experiment Title: The Absorption of Light (Calibration Curve).

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Forensic Science Skills For The Laboratory Experiment Title: The Absorption of Light (Calibration Curve) Aims: To establish good safe and organised working practice. To encourage consistent and meticulous keeping of data and records. To develop consistent and accurate laboratory measurement procedures. Background Info: Many substances absorb electromagnetic radiation in the visible or ultraviolet regions of the spectrum as a consequence of electronic transitions within a molecule. Studying such absorptions provides information about the electronic structure of the species. Where absorption occurs in the visible region of the spectrum, the substance is coloured. The colour arises from light of the complementary colour being absorbed, e.g. the purple colour of potassium permanganate is caused by the absorption of green light. At a particular wavelength, the fraction of light absorbed is proportional to the thickness of the absorbing medium (the path length l) and to the concentration of the absorbing species (c): -dI/dl = ?cI (1) Where ? is a constant called the absorption coefficient and I is the light intensity. Integration of equation (1) gives the Beer-Lambert law: ln(Io/I) = ?cl (2) Where Io is the incident light intensity. In practice this is more usually expressed as: A = ln(Io/I) = ?cl Where A is the absorbance (optical density) ? (=?/2.303) is the molar absorption coefficient (extinction coefficient) ...read more.


Results: Dilution Concentration of potassium permanganate Log Concentration Absorbance Undiluted 0.005 -2.301029996 unobtainable 1 0.0025 -2.602059991 unobtainable 2 0.00125 -2.903089987 unobtainable 3 0.000625 -3.204119983 unobtainable 4 0.0003125 -3.505149978 1.236 5 0.00015625 -3.806179974 0.611 6 0.000078125 -4.10720997 0.298 7 3.90625E-05 -4.408239965 0.142 8 1.95313E-05 -4.709269961 0.072 9 9.76563E-06 -5.010299957 0.038 Unknown ??? ??? 0.089 To look at the results we can see that the experiment has produced the results that we should have expected. By this I mean that as each concentration halves so does the absorbance. To confirm this, a graph of concentration against path length was plotted using Microsoft Excel: As you can see all points lie along the line of best fit indicating a good linearity and consistency in the results. Using this graph it is possible to calculate a value for the molar absorption coefficient for potassium permanganate, this is done as follows: At constant path length Since the path length is constant at 1 cm it may be ignored to give ? = m, and since b = the intercept = 0 as calculated by Microsoft Excel (-0.00032) it also may be ignored. Since calculating the gradient by hand can incur some human error a least squares fitting routine (Microsoft Excel) was used to calculate a molar absorption coefficient of 3965.288 L mol-1 cm-1 (3 d.p.).Using this it is now possible to calculate the concentration of the ...read more.


This is as was expected and so immediately we can be satisfied with the results obtained from an experiment that appears to have worked well. All result lie within a good error limit and aid a confident calculation of both the molar absorption coefficient and the concentration of the unknown sample (2.24�10-5 �2.1�10-6 mol dm-3). Evaluation: Although the experiment has ran accordingly well I would have been more confident if the solutions prepared would have been individually made-up using volumetric flasks and a burette to measure out the undiluted potassium permanganate. The use of hand pipettes introduces a problem in that they cannot be quite as well controlled as the burette could. Small leaks in the seal between the pipette and the syringe can result in a loss of the product resulting in a lower dilution than required. It would also have been useful to investigate the effect of path length with constant concentration on the molar absorption coefficient. The various path lengths that are available include 0.5cm 1cm 2cm and 4cm. If these could have been used it would have been used at the same wavelength is would have allowed for the calculation of a second molar absorption coefficient which could have given an average that may be closer to the actual than the one that has been presented. The spectrophotometer used was adequate for this experiment but more advanced equipment as shown below can provide better wavelength readability, better wavelength accuracy and greater absorbance reliability. ...read more.

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