In this laboratory, we will use UV/Visible spectroscopy to quantitatively determine the amount of copper in an unknown sample. Using the Spec-20 instruments, there are several steps in this analysis. Initially, using a standard Cu(II) solution with a concentration of 0.2mg/mL the absorbance of the Cu(II) solution is measured as a function of wavelength. The Absorbance of a sample is a function of the energy of light that is incident on it. For the best analysis, we want to have the maximum absorbance (e.g. we want to conduct the analysis at the wavelength that gives the maximum interaction with the sample).
This wavelength will then be used for the rest of the analysis. The absorbance of the sample is also a function of the concentration of the analyte in the sample. This functionality is described by Beer's Law. In Beer's law, the absorbance of the sample is a linear function of concentration. By measuring the absorbance of a series of Cu(II) standard solutions (e.g. solutions of known concentration), then the linear relationship between the absorbance and concentration should be apparent. A plot of the measured absorbance vs. the known concentrations of the standards is a calibration curve and shows the linear relationship between absorbance and conc. This linear relationship allows one to determine the functional dependence of the Absorbance on the sample concentration, and provides the key for determining the concentration of the unknown solution from its measured absorbance
NOTE that in preparing the unknown sample for measurement you diluted the unknown until its appearance fell into the range of the standards. The concentration of the actual unknown that you are to report, therefore, must account for this dilution.
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