Title: Spectrophotometric analysis of caffeine and benzoic acid in soft drink.
Name: Tan Herh Lim
Name of partner: Sia Ting Wai, Chong Zheng Yee
Lecturer: Dr. Neo Kian Eang
Practical class: P1
Objective: To obtain the absorbance of the caffeine and benzoic acid in soft drinks. Introduction
Soft drinks that commonly found in our daily life contain caffeine and sodium benzoate. The caffeine act as a stimulant and the sodium benzoate acts as a preservative of soft drink by inhibits grow of bacteria. In this experiment, hydrochloric acid is added to all solution to protonate the sodium benzoate to produce benzoic acid. Since the caffeine has no appreciable basicity, so it is neutral at pH 2. In this experiment, a spectrophotometric analysis of caffeine and the benzoic acid in soft drink is conducted to determine concentration of the caffeine and the benzoic acid mountain dew. The UV/Vis spectrometer is used and the ultraviolet absorbance is chosen to conduct the spectrophotometric analysis in this experiment.
The UV/Vis spectrophotometer work by passing the light by strong lamp through a monochromator, which split the light into its component colours using a gratings. Then, the desired wavelength of light beam is direct through the sample compartment which the cuvette is placed by using the mirror and slit. At other side of the compartment, there is a detector which can determine the amount of light beam pass through the solution in the cuvette, thus the absorbance of the solution in the cuvette can be determined. In addition, the absorbance of light is determined by three factors which is the nature of the solution, concentration of the solution, and the distance of light travel through the solution (path length) which stated in the Beer-Lambert Law.
For each wavelength of light that passing through the sample, the absorbance of a solution or sample is determined by calculate from the negative logarithm of ratio of the intensity of light passing through the sample cell (I) to the intensity of light that passing through the reference cell. (Io) The equation is shown as below:
A= -log I/Io The absorbance of the sample also can be measured using the following equation according to the Beer Lambert law :
ɛ= molar absorptivity/ molar absorption constant
l= length of the solution that light pass through(path length) c= concentration of the solution
Beer lambert’s law describes the linear relationships between the absorbance and the concentration of a sample. The molar absorptivity is a measure of amount of light absorbed per unit concentration. This can be proved by rearrange the Beer’s Law equation ɛ= The molar absorptivity also can be defined as how strong a chemical species attenuates given wavelength of light.
In this experiment, the type of soft drink that chosen in the experiment is very important. This is because the type of the solution used may affect the results obtained in the experiment. A light colour soft drinks such as mountain dew and sprite that used in this experiment is suitable in the spectophotometic analysis and try to prevent using dark colour soft drink due to the colorants in the soft drinks have an ultraviolet absorbance which may contibute to systemic error in the experiment. Besides that, the diet soft drinks also should be avoid since the sugar subsitute aspartame in the diet soft drink have some ultraviolet absorbance which may interfere the results obtained.
After the experiement, a calibartion curve will be plotted. Two calibration curve will be plotted which is graph aborbance of benzoic aicds agaisnt concentration of benzoic acid and graph of absorbance of caffiene agaisnt concentration of caffiene. By using the curve, the concentation of the benzoic acids and the caffiene in...