Chemistry Lab III: “Absorption & Emission Spectroscopy”Experiment 1: Absorption SpectroscopyDo this lab in pairs. Read the following carefully, at leasIn this lab we will record absorption spectra of several fooBlock Diagram of a Diode Array SpectrometerDo this lab in pairs.Experiment 3: Application of Emission Spectroscopy - Making Background information about sparklersWeighing the IngredientsInorganic ingredientsPreparing the starch mixturePreparing the sparklerIgniting your sparklersIntroduction to Natural Science (2006/07) Winter 2007 Quarter Chemistry Lab III: “Absorption & Emission Spectroscopy” Prepared by: Dr. Dharshi Bopegedera1Experiment 1: Absorption Spectroscopy Do this lab in pairs. Read the following carefully, at least 4 times, before coming to lab. In this lab we will record absorption spectra of several food dye solutions and “unknown solutions” using a diode array spectrometer. Light, when passing through a solution, can be absorbed, transmitted, reflected and scattered. These processes are shown in the following diagram. Transmitted light beam (I) Incident light beam (I0) Reflection losses at interfaces Reflection losses at interfaces scattering losses in the solution The diode array spectrometer is based on Beer-Lambert’s law. Beer-Lambert’s law relates the absorbance of light passing through a medium (a solution, in this case) to the path length and concentration as follows. L C II log - A 0λε=⎥⎦⎤⎢⎣⎡= where A = absorbance ελ = molar absorptivity coefficient (or molar extinction coefficient) which is wavelength dependant C = concentration of the sample (a solution, in this case) L = path length through which light travels I = intensity of the transmitted radiation I0 = intensity of the incident radiation Transmittance is defined as T = I/I0 Notice from the equation: A = ελ C L that absorbance is: • directly proportional to concentration (C) • directly proportional to path length (L) Therefore we can increase the absorbance by either increasing the concentration of the sample or by increasing the path length (since molar absorptivity coefficient is an inherent property of matter and we have no way of changing it).2At high concentrations of the sample however deviations from the direct proportionality between absorbance and concentration occur frequently. Therefore when recording absorption spectra we use samples of low concentration so that absorbance stays below 1. Pre-Lab Assignment: 1. What is Beer-Lambert’s Law (mathematical expression, with terms defined)? 2. Derive the relationship between absorbance (A) and transmittance (T). 3. The absorbance of a solution of 0.354 M concentration of a red dye is 0.250 at a given wavelength λ. If the concentration of the dye is increased to 0.935 M without changing the path length, what is the absorbance of the concentrated dye solution at the same wavelength? Block Diagram of a Diode Array Spectrometer L Light Source detector Sample Slit Grating • The diode array spectrometer has a UV/visible light source. • The liquid sample is contained in a plastic cuvette, of 1 cm width. • The detector is a diode array. • The grating performs the same function as a prism (i.e. it separates light into different wavelengths). • The resolution of the instrument is 2 nm. Although we are using a UV/visible diode array spectrometer, we will only use it in the visible region. We can do that by selecting the wavelength we want to work in (350-800 nm work well). In this lab we will record absorption spectra, which are graphs of absorbance versus wavelength. The spectrum tells us what wavelengths of light are absorbed by a given sample. Note that ⎥⎦⎤⎢⎣⎡=0II log - A The spectrometer records I and I0. Then it computes the value of absorbance (A) using the above mathematical relationship.3For the food dye sample: Record the intensity of the incident beam by using DI water in the cuvette. This is called a blank spectrum (I0). Intensity of the transmitted beam is recorded by putting the food dye sample in the cuvette. This is called the sample spectrum (I). The instrument uses the sample spectrum and the blank spectrum to generate the absorption spectrum of the sample. The absorption spectrum is then displayed on the screen. 0.4 absorbance 0.5 wavelength (nm) 800300 0.3 0.2 0.1 Once you have obtained a spectrum you need to save it to your INS network folder. Instead of saving the spectrum (or the graph) we will save the data points as a table, so we can later export it into Microsoft Excel and print the spectrum. Wavelength (nm) Absorbance 300 302 304 306 308 310 312 314 etc. Record the spectrum of one dye sample first. Then save the data to your network folder with an appropriate name. Then move on to the next dye solution. Continue in this manner until you are done with all of the dye solutions. Always remember to record a blank spectrum (I0) before starting on a dye solution (I). Also remember to record the colors (naked eye observations) of the dye solutions. Each student must have their own data saved to their network folders before leaving lab. Write down the concentrations of the “known dye solutions” before leaving the lab.4 Post Lab: 1. Using Microsoft Excel, plot the absorption spectra of the “known dye solutions”, one spectrum for each dye, on separate sheets (not on the worksheet but as a chart). Add your name and the name of the solution to the title of the spectrum. 2. Create the following table for each “known” dye solution. Food dye color: _______________(as seen by naked eye) Peak wavelength Peak absorbance Relative intensity of peak (Relative intensity means “strong”, “medium” or “weak” for a given peak). 3. Based on your observations, decide which wavelengths are transmitted and which wavelengths are absorbed for each of the “known dye solutions”. 4. Correlate the wavelengths that are transmitted with the color (naked eye observation) of each of the “known dye solutions”. 5. For the primary colors only (red, blue and yellow dyes), determine the molar extinction coefficients for each of the “known dye solutions” at each of the wavelengths were light is absorbed. What are the units of the molar extinction coefficient? Show all