Experiment 3 - Qualitative and Quantitative Analysis of Food Dyes >> Step 2

Experiment 3 - Qualitative and Quantitative Analysis of Food Dyes

Step 2 - Taking the spectrum of each dye for identification


Taking the spectrum of each dye for identification:
After collecting each dye into a separate vial (remembering to discard the intermediate mixture of the two dyes), water is added to fill each vial to about 2/3 of the height. This is to ensure that the light travels to the detector through the dye solution and not through the air when the vial is placed in the colourimeter. This is a qualitative measurement, so the exact amount of the dye is not important.

Once the spectrum of the unknown dye and its similarly colored standard dyes are taken, we can match them to determine the unknown’s identity.

Start the experiment with measuring the absorbance of the blank (water) and follow with the remaining vials.

​Blank
​Blue #1
​Green #3
​unknown
​Comment
​Repeat Exp.
Based on the shape of the unknown spectrum and the relative heights of the three highest peaks, what is the unknown dye?
Hint
Note that the highest bar appears at 620nm in the spectra of both Blue #1 and Green #3. Therefore, the identification is best achieved by comparing the relative heights of bars on both sides of that bar. This would be the bars at 605 and 644 nm in the spectra of the Blue #1 and Green #3.
Select
Green #3
Blue #1
Correct, the shape of the unknown dye most closely resembles the standard dye, blue #1
Your answer is incorrect, please check the spectra again, looking closely at the heights of the last three peaks, those at 605, 620 and 644 nm.
Hint:
In comparing the different sample spectra, please look closely at the heights of the last three peaks, those at 605, 620 and 644 nm.
 
get next hint
Hint:
In the unknown sample the height of the red line at 644 nm is slightly higher than the orange peak at 605 nm, which of the two known spectra mimics this situation?
 
get previous hint
get next hint
Hint:
The spectrum of Blue #1 has a red line at 644 that is higher than the orange line at 605, this is the identity of the unknown spectrum.
 
get previous hint

Since similar dyes have similar spectra, you have to carefully compare the spectrum of your unknown to the spectra of known dyes with similar colour. In the laboratory, the program called ColourMixer (similar to the simulation above) will help you with this process.

Now that we have identified our dyes, we can quantitatively determine their concentrations