Question

Explain why you can create an error in estimating the concentrations of benzoic acid and caffeine in the soft drink solution if you use merely two calibration curves constructed from measuring absorbances of benzoic acid at 230 nm and those of caffeine at 273 nm.

Answer 1

Spectrophotometric analysis is very useful in determining the concentration of an unknown compound in a given sample. Beer’s law in principle states that the concentration of a given compound is proportional to the absorbanceat a given wavelength. The fundamental operation of the spectrophotometer is to scan a given standard through a variety of wavelengths, then, choose the wavelength in which the absorbance for that compound is highest. The beam of light at a specific wavelength is shined through an unknown sample and the light fraction which penetrates the sample is picked up by the photodetector and reads out an absorbance. In this experiment, the method of external standards is utilized to create a standard curve from known concentrations of both caffeine and benzoic acid. The slope of the standard curves is equal to ε – molar extinction coefficient. When we prepare a calibration graph of absorbance versus concentration for each compound at each of the two wavelengths we should make 4 different graphs. Each graph should go through 0. The slope of the graph is the absorptivity at that wavelength. The UV spectra of caffeine and benzoic acid overlap at certain wavelengths, thus you will need to measure the absorbance of the unknown mixtures using two different wavelengths, and apply equations for four diffetent molar absorptivities to evaluate the concentrations of caffeine and benzoic acid. Therefore to verify Beer's Law by finding the peak absorbance for two different organic compounds and then running several solutions of different concentration through the spectrophotometer accurately we need to construct at least four calibration curves from maximum wavelength determined by experiment.