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UV-Visible Spectrophotometry

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Introduction

Name: Chu Ka Ki Student No.: 50555542 Group: 1 Date: 17-02-2005 Experiment: 2 BCH 2004 Principle of Analytical Chemistry Lab Report (Report One) Title: UV-Visible Spectrophotometry Aim: To determine phosphate composition in cola beverages by UV-Visible spectrophotometry and pH titrimetry. Introduction: UV-Visible spectrophotometry is one of the most important methods for the chemical analyze. Ultraviolet-Visible spectroscopy or Ultraviolet-Visible spectrophotometry (UV/VIS) involves the spectroscopy of photons (spectrophotometry). It uses light in the visible and adjacent near ultraviolet (UV) and near infrared (NIR) ranges. In this region of energy space molecules undergo electronic transitions. The method is used in a quantitative way to determine concentrations of an absorbing species in solution, using the Beer-Lambert law: where A is the measured absorbance, I0 is the intensity of the incident light at a given wavelength, I is the transmitted intensity, L the length of the cell, and c the concentration of the absorbing species. For each species and wavelength, ? is a constant known as the extinction coefficient. The absorbance A and extinction ? ...read more.

Middle

9.23 8.5 4.21 16.7 6.66 29 9.27 8.8 4.27 17 6.7 29.5 9.31 9.1 4.43 17.3 6.75 30 9.34 9.4 4.8 17.6 6.8 30.5 9.38 9.7 5.04 17.9 6.84 31 9.41 10 5.27 18.1 6.88 31.5 9.44 10.3 5.41 18.4 6.94 32 9.47 10.6 5.51 18.7 6.98 32.5 9.49 10.9 5.66 19 7.02 33 9.52 11.2 5.74 19.5 7.06 33.5 9.54 11.5 5.83 20 7.18 34 9.57 Questions and Calculations: 1) The calibration curve: Absorbance at 400nm vs. [H3PO4] in mM was plotted. 2) From the calibration curve, the sample has 0.440 absorbance at 400nm. The concentration of the sample was found to be 0.164 mM (0.164 � 10-3 M) from the graph. However, the sample was diluted 50 times as the finally. Thus the concentration of phosphoric acid in the original sample should be: 0.164 � 10-3 M � 50 = 8.2 � 10-3 M 3) The pH titration curve (the titration of 25cm3 of cola with 0.0200M KOH) was plotted. 4) From the titration curve, the equivalent point was found at pH 6.98 5) ...read more.

Conclusion

However, the colorimeter analysis does not have such reading problem. 8) The device can only detect the absorbance of the solution between 0 to 2 A. Since the absorbance is proportional to the concentration of the sample. If the concentration of the solution is so high, then the absorbance of the sample will be out of the range between 0 to 2 A. Thus, the sample should be diluted in the colorimetric analysis so the concentration of the diluted sample can be found in the range. Discussion: The linearity of the Beer-Lambert law is limited by chemical and instrumental factors. Causes of nonlinearity include deviations in absorptivity coefficients at high concentrations due to electrostatic interactions between molecules in close proximity, scattering of light due to particulates in the sample, fluoresecence or phosphorescence of the sample, changes in refractive index at high analyte concentration, shifts in chemical equilibria as a function of concentration, non-monochromatic radiation, deviations can be minimized by using a relatively flat part of the absorption spectrum such as the maximum of an absorption band and stray light. Conclusion: By colorimetric method, the concentration of phosphoric acid is 8.2 � 10-3 M. By titration method, the concentration of phosphoric acid is 4.99 � 10-3 M. ...read more.

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Response to the question

The student has very clearly and logically answered the set question. They have supported their answer well and the calculations are clearly explained. I would suggest explaining why the calculations are being done (as in what each step means) but ...

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Response to the question

The student has very clearly and logically answered the set question. They have supported their answer well and the calculations are clearly explained. I would suggest explaining why the calculations are being done (as in what each step means) but this is not compulsory. The student shows a very solid and strong understanding of the topic and of the experiment. Their answer is coherently displayed and justified.

Level of analysis

The student shows high analytical skills towards understanding the experiment and this is all supported with relevant evidence. Their judgements were all sensible from the data that they collected. However, one flaw of this that I picked up on was the inconsistency of decimal places or significant figures in the table. This is key as inconsistency can ruin an experiment. Yet, I do not think this would have affected the mark greatly. Their conclusion was logical and followed from their previous paragraphs. This is good as the conclusions should not be a surprise to the examiner.

Quality of writing

The spelling, grammar and punctuation are fine. The technical terms could have been put into a glossary which would further show the understanding of the student. This would also be a place for explaining the steps for calculating e.g. moles. Such further explanations will make a piece of work stand out as other students are less likely to do it. Glossaries show the understanding of the technical terms used (unless they have been integrated into the piece of work itself). The student follows the typical layout of the piece of work and this is recommended as it is the easiest and best way to present such work in a logic and coherent manner.


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