Using Absorbance to Determine the Concentration of CuSO4

This experiment was carried out to explore the relationship between the absorbance and concentration of colored solutions. After determining the λmax was 635 nm for CuSO4, the absorbance of six solutions of CuSO4, ranging from 0.00 to 0.50 M, was found using a colorimeter. Graphical analysis revealed that the relationship between absorbance and concentration was directly proportional with a molar absorptivity of 2.81 Mcm. The data obtained reinforced the connection between the color of a solution and the wavelengths of light which the solution absorbs and reflects. In addition, knowledge of the linear relationship between absorbance and concentration allowed for the determination that unknown solution #285 was 0.726 M CuSO4. Introduction The world is full of a multitude of colors that are used for both practical and aesthetic purposes. The colors that are seen when looking at different objects are due to the ability of the compounds in those objects to absorb specific wavelengths of light. In other words, the colors that are seen are the ones that are not absorbed. The wavelengths of light that are absorbed are determined by the electrons in a compound. As electrons move around, they can absorb energy and become excited. The energy, and thus the wavelength of light, the electrons absorb is determined by the type of atoms found in the compound and how those atoms are bound together. Different environments for electrons will also determine how much of a particular wavelength of light can be absorbed, a parameter which is reflected in the molar absorptivity of the compound. Because the color of a species is due to its ability to absorb light, the color should become darker or more intense as the concentration increases. The increase in concentration leads to more electrons in the sample which can then absorb more light at a particular wavelength. Thus, there should be a relationship between the concentration of the compound being studied and its absorbance. This relationship is best determined using a wavelength of light in a region of the visible spectrum where the maximum absorbance is observed. This wavelength is known as λmax and is most sensitive to the changes in concentration. The purpose of this experiment was to study the absorbance of CuSO4. Because copper compounds tend to be blue in color, it was hypothesized that the CuSO4 would have a λmax with a longer wavelength corresponding to a color more in the range of orange or red. Once the best wavelength for studying CuSO4 was determined, it could then be used to examine the relationship between the absorbance and concentration of the compound and use that relationship to calculate desired information for unknown solutions. Experimental Procedure Six samples were prepared for analysis by diluting a 0.50 M CuSO4 stock solution. 4 The amounts of the CuSO4 stock solution and water used in each sample is indicated in Table 1. A Vernier colorimeter was attached to the LabPro interface and the LoggerPro software was adjusted to display absorbances. The absorbance of the 0.50 M CuSO4 was determined using a 1 cm cuvette at each of the four wavelengths on the colorimeter (Appendix 1), and 635 nm was chosen as the λmax. The absorbance of each of the standard solutions was then determined at 635 nm. Unknown solution #285 was obtained and its absorbance was measured. Because the absorbance was higher than that of the 0.50 M standard, the unknown was diluted in a 1:1 ratio with DI water, and the absorbance was measured again. Results The absorbances of the CuSO4 standard solutions were directly proportional to their concentrations (Table 1, Figure 1). The slope obtained for the graph is equivalent to the molar absorptivity since the cuvette had a pathlength of 1 cm. Thus, the molar absorptivity of CuSO4 at 635 nm is 2.81 M cm. This molar absorptivity could be used to determine the concentration of unknown #285. The absorbance of the undiluted solution was much higher (1.683) than the 0.50 M standard solution. After diluting 2.00 mL of the unknown with 2.00 mL of DI water, the absorbance obtained was 1.021. This corresponded to a concentration of 0.363 M CuSO4 for the diluted solution. The undiluted unknown #285 would therefore be twice that of the diluted, or 0.726 M CuSO4. Table 1. Preparation of standard CuSO4 solutions and their absorbances at 635 nm. Concentration of Standard CuSO4 Solution (M) Volume of 0.50 M CuSO4 Stock Solution (mL) Volume of DI Water (mL) Absorbance at 635 nm 0.00 0.00 5.00 0.000 0.10 1.00 4.00 0.406 0.20 2.00 3.00 0.638 0.30 3.00 2.00 0.854 0.40 4.00 1.00 1.202 0.50 5.00 0.00 1.276 Figure 1. Determination of molar absorptivity of CuSO4. The blue diamonds are from the standard solutions. The red square is the diluted sample of unknown #285. Abs = 2.81 Conc R2 = 0.96 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 0 0.1 0.2 0.3 0.4 0.5 0.6 A b so rb an ce