Week 4: The Spectra of Conjugated Dyes and Investigation of Beer's Law Overview Getting Started Techniques Procedure FAQ Full Lab Manual Introduction & Goals Chemistry & Background Key Questions Prelab Problems Safety Procedure In Your Write-up Experiments Index ChemLab Home		Procedure In this week's experiments you will work with a partner to collect the data. The goals and general procedures for both experiments are outlined below. You will need to decide on the detailed steps of your experiments in collaboration with your partner. Spectra of Conjugated Dyes Measuring the Absorption Spectrum with a Spec 20 Obtain a solution of one of the two conjugated dyes in a cuvet and note its color in your lab notebook. Estimate the wavelength of maximum absorbance assuming that the dye has one absorption peak. Use this estimate as a starting wavelength to measure and plot the absorption and transmission spectra, using a Spec 20. Your two spectra should include the entire visible range, but need not have the same wavelength resolution in all regions. You and your partner must decide at what wavelengths to measure the absorbance and transmission to map out the spectrum adequately in a reasonable period of time. The Spec 20 must be calibrated with a blank of deionized water at each wavelength. Measuring the Absorption Spectrum with a Scanning Spectrometer Obtain a solution of the other conjugated dye in a cuvet and measure its absorption spectrum with a scanning spectrometer. Your TA will show you how to operate the instrument, but you should make the measurements. Make a photocopy of the spectrum for one partner's lab notebook in Kresge library. Label the spectrum with the wavelength of maximum absorption. Determination of Food Dye Concentration in Drink Mix Your first goal is to determine how many and which dye(s) are present in the drink. You will answer these questions by measuring the absorption spectrum with the scanning spectrometer and by performing a simple paper chromatography experiment. These experiments can be done in any order. Obtain a packet of the drink mix supplied in your lab and record its flavor. Make up 25 mL of solution at the concentration given on the label using an analytical balance and volumetric flask. Record the mass of drink mix used and calculate the concentration of the solution in g/L. The solution will be used for spectral measurements and some of the remaining powder will be used for the chromatography experiment. Absorption Spectrum of Drink Solution Place some of your drink solution in a cuvet and measure its absorption spectrum with the scanning spectrometer. Your TA will show you how to operate the instrument, but you should make the measurements. Make a photocopy of the spectrum for one partner's lab notebook in Kresge library. Label the spectrum with a wavelength scale and label the wavelength(s) of maximum absorption. Paper Chromatography Experiment For the chromatography experiment, place a small amount of powdered drink mix on a watch glass and add a drop or two of water. Mix this together into a uniform, concentrated slurry. Use a rectangular strip of filter paper and apply a small spot of the concentrated drink solution with a wooden stick, about 1 inch from the end of the strip. Allow the spot to dry and apply another spot of solution to the same place until the color of the spot is clearly visible. The following figure shows the experimental set-up for running your chromatogram. Suspend the strip of paper, with the spot end down, in a beaker containing a thin layer of water, with the bottom of the paper just touching the water and the spot out of the water. Allow the water to diffuse up the paper, until you are satisfied with the results and have a clear answer to your questions. After you have completed the chromatogram and the spectrum, identify the dye(s) in the mix, as observed by each technique. Record your results in your lab notebook. Paper Chromatography Beer's Law Experiment for Food Dyes Your next goal is to determine the concentration of the dye(s) in the drink solution, using Beer's Law. To start, you will produce a "standard curve" for each dye found in your drink mix. Your data will be absorbance values for several solutions of precisely known concentration of the individual dye(s) under study. Use a Spec 20 to measure the absorbance at the wavelength of maximum absorbance for solutions with a range of concentrations. Dilutions of the initial stock solution should be done with a volumetric flask, volumetric pipets, and your best analytical technique. The Spec 20 should be calibrated with a water blank between readings. After you have completed the measurements of absorbance vs. concentration for the set of known dye solutions, graph a Beer's Law plot(s) by hand or on a graphing calculator, to check for linearity. If more than one dye is present in your drink mix, plan to share data with another group of students. You will need a set of Beer's Law data at the appropriate wavelength for each dye present in your drink mix. Absorbance Measurements of Koolaid Solution Your final goal is to determine the concentration of the dye(s) in the Koolaid solution. To do this, use a Spec 20 to measure the absorbance of your drink solution at all relevant wavelengths. Don't forget to calibrate with a water blank at each wavelength. For your uncertainty analysis, you will need to record the uncertainty with which you can measure the absorbance of your solution.
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