Spectrophotometry Activity  

Purpose

Several AP Chemistry activities require the use of the spectrophotometer. In addition, AP students deal with color and its sources in several chapters. This activity introduces them to the dyes used in food stuffs and in crepe paper and is a lead-in activity to a chromatography project. This activity can meet several MSDE Core Learning Goals.

Prior Knowledge

Before this activity, students have worked with spectroscopes and ionized gases. They have discussed the source of color in these cases and how it is used as evidence of electron levels. They have no prior experience with the spectrophotometer unless they have taken AP Biology.

Time and Evaluation of Activity Students prepared the samples the day before. It takes a full 90-min period to get all the data and some students failed to finish during that time period due to difficulties with the equipment. However, the data gathered can be saved as a text file and data from the next day appended to it.

The editting of the text file and production of graphs using MatLab took students approximately 45 minutes.


MSDE CORE Learning Goals

  1. Goal 1: Skills and Processes
    1. Expectation 4: The student will demonstrate that data analysis is a vital aspect of the process of scientific inquiry and communication
      1. Indicator 6: The student will use spreadsheet, graphing, and database programs and probeware on computers and/or graphing calculators
    2. Expectation 5: The student will use appropriate methods for communicating in writing and orally the processses and results of scientific investigation
      1. Indicator 1: The student will demonstrate the ability to summarize scientific concepts, processes and data through written communications
      2. Indicator 5: The student will use computers and/or graphing calculators to produce tables, graphs and spreadsheet calculations
  2. Goal 4: Concepts of Chemistry
    1. Expectation 2: The student will explain that all matter has structure and the structure serves as a basis for the properties of and the changes in matter
    2. Expectation 5: The student will investigate the impact of Chemistry on society

Food Dye or Crepe Paper Spectroscopy

Using Vernier Spectrophotometer Program and MatLab

Purpose: to determine the absorption and transmittance spectra of various 
dyes and to relate these spectra to the observed color

Procedure:  Part A: Collecting the Data

1. Plug the ULI into the serial port of the computer and into the wall.  Attach
 the Spec20 to the ULI.  Turn the Spec20 on and turn the ULI on.

2. Prepare the dye samples as instructed by your teacher.

3. Start the Vernier Spectrophotometer Program.  From the initial screen choose 
the Spectrum/ Absorbance vs. Wavelength button. 

4.  Now follow the instructions given in the program.  Start at a wavelength of 
400 nm.  After you measure the first sample, ignore the program menu and place 
the second sample in the chamber.  Determine its measurement, tell the program 
to keep that value as well.  Repeat for the third, fourth, etc. samples.  Only 
go back to the program instructions given by the program after you have read 
all the samples at one wavelength. 

5.  Change the wavelength by 10, and repeat step 4.  BE SURE to do the samples 
in the precisely the same order everytime. Make a note of this order. If you 
make a mistake, keep track of the readings, mistakes can be edited out later.

6.  When you have finished taking all readings (400 -700 nm by 10 for all 
samples) click on STOP.  From the file menu, choose save.  Save the data as 
spec.txt.

Part B: Analyzing the Data

7.  To use the data in MatLab, the file must be edited.  Open MatLab and then 
choose Open M-file from the File menu.  Find the file spec.txt and open it.  
It should look something like this only much longer: 

Spectrophotometer Program
Absorbance vs Wavelength
Wavelength transmittance 	absorbance
nm 			%
500 	 	80.8 	 		0.092
500 	 	0.7 	 	 	2.120
500 	 	64.7 	 		0.188
500 	 	23.7 	 		0.624
520 	 	86.8 	 		0.061
520 	 	0.7 	 	 	2.122
520 	 	62.6 	 		0.202
520 	 	59.9 	 		0.222
540 	 	75.0 	 		0.124
540 	 	6.7 	 	 	1.172
540 	 	60.3 	 		0.219
540 	 	76.6 	 		0.115

You need to delete the text at top and to delete all the repeat wavelengths.  
The file should have all data for one wavelength on one line, spaces between 
the separate readings.  The file above would look like this when edited:

500 80.8 0.092 0.7 2.120 64.7 0.188 23.7 0.624
520 86.8 0.061 0.7 2.122 62.6 0.202 59.9 0.222
540 75.0 0.124 6.7 1.172 60.3 0.219 76.6 0.115

When you have the file edited, save it as spec.dat.  MatLab will ask a question 
about a .m extension, click on No. 

8.  Click on the Command Window (the window where the >> is).  Type 

	load spec.dat;
	wave = spec(:,1);   the wavelengths are in the variable wave

The odd columns in your data file are absorbance columns, the even are 
transmittance.  We want to load each column and we want to know the color.  
For instance, if you collected the samples in the order red, yellow green and 
blue, you might type the following:

	redt = spec(:,2);
	redA = spec(:,3);
	yelt = spec(:,4);
	yelA = spec(:,5);
	grnt = spec(:,6);
	grnA = spec(:,7);
	blut = spec(:,8);
	bluA = spec(:,9);

where t stands for transmittance and A for absorbance.  You MIGHT have to adjust
the above commands for your data.

9.  The transmittance data will be plotted on one graph and the absorbance on 
another.  The command to plot absorbance looks like this:

 plot(wave,redA,'r',wave, yelA,'y',wave ,grnA,'g',wave,bluA,'b');

where wave is the x-variable, the A variables are on the y and the plot colors
 are changed by the letters in single quotes ('r' = red, 'y' = yellow, etc.)

10.  The absorbance graph should have some peaks and valleys.  Type:

	ginput(4)   (4 because there were four colors in the example 
		above. This number should equal the number of curves)

 a cursor will appear on the graph and you can point and click at the major peak
 for each color.  The values for the peaks will appear in the command window.

11. Plot the transmittance graph and determine the peaks for it.


Sample data

400 26.1 0.582  7.9 1.099 51.6 0.287 0.0  2.7
410 24.1 0.617  8.3 1.080 54.5 0.263 0.0  2.7
420 24.1 0.617  8.6 1.061 63.2 0.198 0.0  2.7	  	 
430 23.0 0.637 10.3 0.986 72.6 0.138 0.0  2.7
440 16.9 0.771 11.6 0.934 77.2 0.111 0.0  2.7
450 14.3 0.843 17.5 0.756 76.8 0.114 0.2  2.592
460  9.4 1.025 23.8 0.622 75.1 0.124 0.0  2.7
470  5.7 1.243 35.1 0.454 72.6 0.138 0.0  2.7
480  1.7 1.751 51.2 0.290 66.5 0.176 1.3  1.856
490  1.6 1.784 64.6 0.189 64.2 0.192 6.0  1.216
500  1.2 1.893 81.4 0.088 64.3 0.191 23.2 0.633
510  1.1 1.900 83.0 0.080 61.5 0.210 43.9 0.356
520  1.0 1.996 80.0 0.090 60.5 0.218 60.0 0.221
530  2.5 1.596 79.1 0.101 60.1 0.220 70.3 0.152
540  6.9 1.159 74.0 0.130 58.9 0.229 77.3 0.111
550 15.0 0.821 65.7 0.182 52.3 0.280 83.5 0.078
560 33.9 0.468 52.7 0.277 44.2 0.354 87.1 0.059
570 52.9 0.276 42.7 0.368 33.8 0.470 90.1 0.045
580 68.8 0.161 29.9 0.523 24.4 0.611 91.7 0.037
590 76.3 0.117 25.0 0.601 17.9 0.745 94.4 0.024
600 82.4 0.083 18.6 0.729 12.6 0.896 94.6 0.023
610 83.5 0.078 11.9 0.921  8.0 1.094 96.1 0.016
620 81.2 0.090  7.8 1.104  3.9 1.405 95.3 0.020
630 82.5 0.083  5.6 1.247  3.0 1.510 97.3 0.011
640 82.1 0.085  8.5 1.066  5.0 1.296 95.5 0.019
650 84.9 0.070 18.0 0.744 11.7 0.929 95.2 0.021
660 85.5 0.067 35.1 0.454 31.4 0.502 92.5 0.033
670 86.3 0.063 55.8 0.253 53.0 0.275 86.9 0.060
680 86.2 0.063 70.6 0.150 69.5 0.157 83.7 0.077
690 82.3 0.084 76.2 0.117 79.0 0.101 75.4 0.122

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