UV-Visible Spectrophotometry

Authors Avatar
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)


UV-Visible Spectrophotometry


To determine phosphate composition in cola beverages by UV-Visible spectrophotometry and pH titrimetry.


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 ? are sometimes defined in terms of the natural logarithm instead of the base-10 logarithm.


Colorimetric Analysis:

At the beginning, 4cm3 decarbonated cola was transferred into a 100cm3 volumetric flask and diluted with distilled water. After, 25.0cm3 of the sample, 0mM (blank), 0.1mM, 0.2mM, 0.4mM and 0.6mM of phosphate standards (KH2PO4) were put into six 50ml volumetric flask separately. Then, 5ml of vanadate-molybdate reagent was added to each flask and all of the flasks were diluted with distilled water. Finally, the absorbance of each solution was measured at 400nm and recorded.
Join now!

Titrimetric Method:

25cm3 of undiluted decarbonated cola was transferred into a conical flask and a pH meter was used to measure the pH value. 0.02M of KOH was chosen to titrate with the sample. The pH values change were recorded and a graph of pH vs. volume of alkaline added was plotted.

Results and Graphs:

For the Colorimetric Analysis:

Concentration of phosphate standards

Absorbance at 400nm

0 mM


0.1 mM


0.2 mM


0.4 mM


0.6 mM



This is a preview of the whole essay

Here's what a star student thought of this essay


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.

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.

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.