Abstract
Determination of caffeine content of tea and instant coffee brands found in the Kenyan market
H. N. Wanyika*, E. G. Gatebe, L. M. Gitu, E. K. Ngumba and C. W. Maritim
Department of Chemistry, Jomo Kenyatta University of Agriculture and Technology, P. O. Box 62 000 – 00200, Nairobi, Kenya. Accepted 12 April, 2010
Caffeine (1, 3, 5-trimethylxanthine), a mild addicting drug though used for medicinal purposes is the active ingredient that makes tea and coffee valuable to humans. In this study, the levels of caffeine in certain coffee (nescafe, africafe, dormans) and tea (chai mara moja, kericho gold, sasini, finlays premium) brands found in the Kenyan market were determined using high performance liquid chromatography (hplc) and UV/ Vis spectrophotometric methods. The levels of caffeine in all the tea and coffee brands were found to be within the documented range. The order of caffeine concentration in tea samples was found as follows: chai mara moja > finlays premium > kericho gold > sasini. In coffee it was found that the caffeine content of africafe > nescafe > dormans. Generally, higher concentration of caffeine in all the samples were realized with the UV/ Vis spectrophotometric method compared to hplc method indicating that acidified water was a better caffeine extractor than pure water.
Based on the Absorbance Spectrum in Figure 1, what is the maximum absorbance wavelength for caffeine?
Construct a Standard Reference Graph in Excel for the Caffeine using the following data:
Determine the concentration of caffeine from the Standard Reference Graph for the following beverages given their absorbance values when tested with a spectrophotometer.
Absorbance
Chai Mara Moja Tea .26
Nescafe instant coffee .13
Finlay Tea .18
Africafe coffee .12
Helpful website to construct and understand the standard reference graph:
https://www.nku.edu/~whitsonma/Bio150LSite/Lab%202%20Water/SpectrophotometryII.html
Helpful video on how to Excel to construct the standard reference graph.
Questions:
1. Look at the standard reference graph. What is the independent variable and dependant variable?
2. Explain how the relationship between absorbance and concentration based on Beer’s Law.
3. If the wavelength is changed, what will happen to the absorbance value?
4. What would be another real world application for spectrophotometry?
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