# 4.2 Day 3 Procedure


Day 3 - Lambert-Beer Study

Prepare four to six solutions (each person should prepare two solutions) of known $$[Co(NH_3)_5Cl]Cl_2$$ concentration made up in 25 mL volumetric flasks. Weigh out your samples first on weighing paper to approximate the correct mass. Next, tare the flask (zero the balance with the flask in place), then carefully add the weighed quantity of $$[Co(NH_3)_5Cl]Cl_2$$ and finally weigh the flask with the cobalt complex to 0.1 mg. Rinse any compound adhering to the neck of the flask into the flask and fill to the 25 mL line with 0.5 M $$HNO_3$$ (Helpful hints: add part of the acid solution, swirl to mix getting most of the complex in solution, then use a Pasteur pipet to add the nitric acid solution dropwise until the meniscus rests on the calibrated line). Concentrations between 2.5 and 10 mM would be most useful. Why? Be sure to span a range of concentrations. Calculate the mass required for 2.5 and 10 mM solutions as part of the pre-lab.

On the UV-VIS spectrophotometerʼs computer screen select “General Scanning”. Check that the parameter file is set correctly, and then take a background reading using a glass cuvette filled with the 0.5 M nitric acid solution. (Glass cuvettes will be used due to the instability of the plastic cuvettes at the higher temperatures required by the kinetic runs). See Appendix 1 for step-by-step instructions for running the Cary100 spectrometer to obtain a UV-VIS Spectrum. Be sure the outer walls of the cuvette are dry and clean by wiping them with a Kimwipe®. Fill cuvettes with your solutions and measure their absorbance. Manually record the absorbance at 550 nm for each spectrum or transfer your data files to a USB drive for analysis.

Plot absorbance (A ) versus concentration (c). Don't forget to include the (0, 0) data point in your calculations. Include the least-squares line and correlation coefficient on the plot. If the Lambert-Beer equation is valid over this range of concentrations, a straight line should be obtained whose slope is equal to $$\epsilon l$$, or in fact to $$\epsilon$$ if l = 1 cm. What is the error in $$\epsilon$$ ? How should $$\epsilon$$ be reported?

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