Spectroscopy Laboratory

Analysis of Food Dye in Beverages

Abstract:

A solution's color can provide important qualitative and quantitative information about its composition. By measuring the transmittance of different concentrations of FD&C Blue 1 diluted in water by using a spectrophotometer, we were able to determine the relationship between transmittance and absorbance. This relationship allowed us to implement graphical analysis in order to learn the concentration of dye in blue Gatorade. Results showed that the concentration of FD&C Blue 1 dye in our sample was approximately 1.915 x 10-6M. In other words, Gatorade contains approximately1.52 milligrams/liter of Blue 1 dye.

Introduction:

Spectrophotometry is an important assay that has long provided scientists with the means to determine properties of various substances. A spectrophotometer, an instrument that analyzes the transmission of radiation by a sample, gives readings of both transmittance and absorbance. Transmittance is the fraction of incident light that passes through the sample and absorbance is the amount of light absorbed by the sample. By first measuring different dilutions of FD&C Blue 1 dye, we established the concentration of the stock dye and the correct relationship between transmittance and absorbance for a Beer's law calibration curve. We then used spectrophotometry to investigate the concentration of FD&C food dye in a sports drink.

Experimental:

Materials:

See Lab "Analysis of Food Dyes in Beverages"

Procedure:

The beaker was filled with 50mL of stock solution, and using the serological pipet and pipet bulb, the following dilutions were accurately created.

Dilutions of Stock Solution in Water to be Tested

Solution Stock (A) B C D E F G Blank (H)

Water (mL) 0 2 4 6 7 8 9 10

Stock Soln (mL) 10 8 6 4 3 2 1 0

The "blank" test tube was used to calibrate the spectrophotometer by setting it to 100% transmittance when inserted. The spectrophotometer was set at 630 μm because that is the optimum wavelength for measuring the absorbance of FD&C Blue 1 dye. This is because 630 is the wavelength of orange light, which is complementary to blue Solutions A-G were then tested using the spectrophotometer and percent transmittance values were recorded. Using our raw data and Beer's law (Absorbance = molar absorptivity x path length of sample cell x concentration), the precise concentrations of the dilutions were calculated and entered into the program Logger Pro. Using Logger Pro, five separate graphs were created, all of them being some variation of transmittance versus concentration. Lastly, the concentration of blue Gatorade was determined by measuring the transmittance, using the calibration curve created, and Beer's law.

Results and Discussion:

Table of manipulated transmittance values

(T = Transmittance, DC = Dye Concentration)

Linear graph of -log T graphed against dye concentration

When comparing the graphs of %T vs. DC, T vs. DC, 1/T vs. DC, logT vs. DC, and -logT vs. DC, we found that -logT vs. DC show the most linear relationship. Of course the graph of logT vs. DC shows the exact same relationship, we are concerned with only positive values. This observation makes sense because we know that absorbance is equal to the -log of transmittance. Although our graph shows a linear relationship, it is very unusual that our value for the dye concentration of the stock solution was only approximately 3.18μM when it should have been somewhere around 7.0μM. In comparison to the results of other groups, our spectrophotometer readings are consistently different. This consistent error shows that there was systematic error, perhaps with the spectrophotometer, which resulted in precise, but inaccurate data. This seems to make sense because we found the correct relationship, but with incorrect values. It is also possible that there was some human error, such as making the dilutions slightly incorrect, but this should not have had such a major impact on our data, and thus cannot be accounted for the extreme inaccuracy. The graph of -logT vs. Dye Concentration would provide the most accurate means to determine the concentration of an unknown solution whose transmittance has been measured spectroscopically. Because -logT is equal to absorbance, the slope of the graph above demonstrates