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Electromagnetic spectrum, Spectroscopy or colorimetry, Solution concentration, Beer's law.
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If you have ever added a powdered drink mix to water, you will realize that the more concentrated the drink, the deeper the color of the solution. Analytical chemists routinely use a quantitative approach called spectroscopy that uses a photometer to measure light intensity to determine the concentration of solute in a solution. A colorimeter is an instrument that measures light at specific wavelengths in the color spectrum. In this lab, you will investigate and design an experimental procedure that utilizes spectroscopy and quantitative analysis to determine the concentration of FD&C food dyes in sports drinks.
Color surrounds us every moment of every day. It affects our emotions, behaviors, even the choices we make of food or drink, both consciously and unconsciously. The color of matter is a physical property that can be used for both qualitative and quantitative analysis. In a solution, the color of matter becomes an important tool for scientists to analyze the identity and quantities of components contained in that solution. Many of the most popular, commercially available sports drinks contain FD&C food dyes that make them more attractive to the consumer. Spectroscopy and colorimetry are commonly used methods for quantitating how much food dye is contained in a sports drink. Both methods measure % transmittance and absorbance of light through a sample of a colored solution but differ in the range of wavelengths over which measurements can be made. A spectrometer measures over a range of 380 nm to 950 nm at a wavelength interval of about 2 nm. A colorimeter measures % transmittance or absorbance at specific wavelengths of the visible spectrum between 450 nm – 650 nm. This experiment can be performed using either instrument.
Spectroscopy is the study of the interaction of electromagnetic radiation and matter. In spectroscopy and spectrophotometry, two terms are inescapable: transmittance and absorbance. Transmittance T is defined as the ratio of the intensity of light after it passes through a medium being studied ( I ) to the intensity of light before it encounters the medium ( I o).
0
Chemists more commonly refer to the percent transmittance % T , which is simply 0
Because the percent transmittance is exponentially related to concentration of solute, the use of absorbance, which gives a linear relationship, is often preferred.
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Note that A = –2 log (% T ).
If one knows the percent transmittance, one can calculate absorbance and vice versa. Most modern spectrophotometers have both a % T and an absorbance scale. With a digital instrument, it is simply a matter of changing modes to display either value.
Beer’s law is one of the most fundamental and widely applied spectroscopic laws. It relates the absorbance of light to the concentration (c ) of the solute, the optical path length ( b) and the molar absorptivity ( a ) of a solution.
An operation statement of Beer’s Law can be represented as
A = abc
The molar absorptivity is a constant that depends on the nature of the absorbing solution system and the wavelength of the light passing through it. A plot that shows the dependence of A on wavelength is called a spectrum.
Pre-Lab Questions
The visible absorbance spectrum of a 0.1 M Co(NO3)2 solution is shown in the figure below.
What wavelength would be optimal for measuring the absorbance versus concentration of a series of solutions of Co(NO3)2? Explain your answer.
A series of solutions of Co(NO3)2 were prepared to construct a calibration curve. Using the dilution information below from a 0.1 M Co(NO3)2 stock solution, determine the concentration of each of the dilutions. Hint: M1V1 = M2V
Dye Stock Solution (A)
Concentration 0.1 M Water (mL) 0 2 4 6 7 8 9 10 Stock Solution (mL) 10 8 6 4 3 2 1 0
Initial Investigation
Determination of Optimal Wavelength for Measuring Absorbance of FD&C Blue # 1
Calibration Curve of FD&C Blue # 1 Food Dye
Prepare a series of dilutions of FD&C Blue 1 food dye from a stock solution of known concentration prepared for you by your instructor, and distilled water. Prepare 10 mL of each dilution, calculate and record the concentration of each sample in the table below. Using a colorimeter, you will measure absorbance of each solution in the Initial Activity.
Table 1: Calibration Curve of Stock FD&C Blue # 1 (Absorbance at 600 nm) Blue #1 Sample Stock Solution (mL)
Distilled Water (mL)
Concentration (M)
Absorbance (-log T)
%Transmittance
1 10 0
2 8 2 3 6 4
4 4 6 5 3 7
6 2 8 7 1 9
8 0 10
Synthesis Questions
AP® Chemistry Review Questions
A. The distance the light must travel through the solution (path length) B. The amount of solute in each volume (concentration) C. The wavelength of the light that is interacting with the solution D. All of the above
A. 4. B. 2. C. 1. D. Not enough information