A coffee shop in Seattle wishes to determine the amount of caffeine in each 8oz

Question

A coffee shop in Seattle wishes to determine the amount of caffeine in each 8oz cup of coffee they sell. In general cup of coffee is thought to contain 100 mg-150mg of caffeine. Using a caffeine standard (a solid sample with high purity) and the same the materials provided for the Bradford Assay protein lab please construct a method to determine the amount of caffeine in each cup. They require a 5 point calibration curve and a literature search has shown that the spectrophotometric detection of caffeine (with a UV-V is) is linear in the range of 3-18 mg/L. For full credit please be specific with volumes and amounts of each reagent needed to make the calibration curve and how the coffee will need to be diluted or if it can be measured at its brewed strength.

Explanation / Answer

UV Vis absorption spectoscopy can be used to determine the unknown amount of caffeine. Caffeine has an absorption around 273 nm and the absorbance is noted here. (international journal of analytical chemistry 2015 article id 170239)

Standard solution for the calibration curve is made by finding the absorbance of known concentrations of caffeine at this wavelength. For that purpose 5 different standard flasks of 100 mL were taken and standard solutions were made.

Preferably 5 concentrations are needed for this purpose. Let's fix the concentrations as 3,6,9,12,15,18 mg/liter. Since our standard flasks are of 100mL, 0.03, 0.06,0.09,0.12,0.15,0.18 mg of caffeine was weighed accurately and made them to 100 mL.

Absorbance at 273 nm is noted at each case and plotted aginst concentration . This will be a straight line.

Since a cup of coffee contains 100-150 mg/L concentration of caffeine, approximately in 100 mL, per mL it contains approximately 1 mg/mL or 1000 mg/ L. Thus it has to be diluted .

Since our calibration cutves concentration ranges from 3-18 mg/L, this has to be diluted 100 times. That is take 1 mL from coffee and make up to 100 mL. Take absorption spectra and from calibration curve one can easily find the concentration of caffeine .

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