Phosphate Cola Analysis

Phosphate Cola Analysis

Erin Pickens

Chemistry 101 Laboratory, Section 025 Instructor: Craig Goodman November 8th 2012

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Discussion and Scientific Explanation: On our lab groups first day of experimentation, our first task was to become acquainted with the Spec twenty machine through a practice activity using different colored solutions. The Spec 20 Machine measures the absorbance and transmittance of light in solutions. On day one our lab group was asked to make three different solutions of one hundred milliliters of water with one drop of food coloring. Each solution had a different color. We then were asked to make a third solution of potassium permanganate. We then found the absorption and percent transmission of each at different wavelengths ranging from 350 nm to 650 nm. Later we plotted these values to give us an idea of which solution had the highest absorbance and highest transmittance. These values are recorded in Tables 1 and 2 and are graphed in Graphs 1, 2, and 3. We graphed the data because it is a lot easier to compare in graph form. We compared our data for the colored solutions with values given on page 87 in our lab manual. Our data was comparable. At certain wavelengths certain colors would display a higher transmittance or higher absorbance than other colors. This corresponds to the visible light spectrum. . In addition to becoming familiar with Spec 20 and seeing how color and wavelength are related, this first part of the lab allowed us to derive the analytical wavelength (the wavelength where there is the most absorption by a substance) for potassium permanganate. We found this analytical wavelength to be 550 nm. Then using the analytical wavelength, we prepared five solutions of potassium permanganate with different dilutions ranging from .00007 M to .0025M. We used the Spec 20 machine to find the percent transmittance and absorbance of each of our dilutions. The whole purpose of this part of the lab was to help us later determine how the concentration of a solution affects its absorption. As seen in our results in Table 3 and Graph 3, in general as concentration increases, absorbance increases. This makes sense because a more

concentrated solution, one with more atoms of solute per volume, will obviously have less open space for light to freely pass through. This concept is known as Beers law. It also works inversely so that it could be said that as concentration increases, transmittance of light decreases. Our data reflects this law fairly accurately with the exception of the transmittance recorded for potassium permanganate concentration .001M. As our second largest dilution this should have been slightly more translucent that our largest dilution .0025M and slightly less transparent than our third largest dilution, .0005M. However, it was found to be a lot more transparent than the .0005M solution and did not correlate with the other data we received. This could be possible due to many issues in the experimental procedure. Maybe we calculated proportions for the dilution incorrectly or calculated correctly and then executed the actual mixing incorrectly. Regardless it was almost certainly an error in process and this point should probably be left out. In addition, it should be noted that our different concentrations of potassium permanganate used were very awkward intervals. We kind of randomly created different dilutions using the formula (Initial Molarity*Initial Volume=Final Molarity*Final Mass). If this experiment were redone I would probably intentionally choose more evenly distributed concentrations because it would be a lot easier to compare the data. On day two of experimentation, our lab used the knowledge we had gained from the activities of day one to run experiments on a different compound, mono-potassium phosphate. Our data is displayed in Tables 4, 5, and 6, and Graphs 4, 5, 6, 7, 8, and 9. As seen in Table 5, we first tested a .0002M concentration of mono-potassium phosphate at wavelengths from 350nm to 650nm in 50nm increments just as we had done before with potassium permanganate. We did this to find the analytical wavelength to use for further testing. Absorbance was highest at 350nm so this was what we determined to be the analytical wavelength. We used this

analytical wavelength to, like we did on potassium permanganate, test different concentrations and determine the relationship between concentration and wavelength. This experiment was different, however, because we used our data from the concentration testing to create a calibration curve for phosphate content which would later be used when examining the phosphate content of the colas. In Table 4 and Graph 6, our data for the percent transmittance of concentrations from .0002M to .002M of mono-potassium phosphate can be seen. In this part of the experiment we created dilutions just like we had previously done with potassium permanganate. However, this time we had to add AVM to our solutions so that the solutions could be read in the Spec 20 machine. Without AVM the Spec 20 machine would have showed 100 percent transmittance because mono-potassium phosphate is colorless. Similarly to the potassium permanganate testing, in the testing of mono-potassium phosphate we did a poor job of creating evenly dispersed dilutions. The concentrations of .0003M and .0004M, and .002M and .001M had very similar results to one another and therefore did not provide for very descriptive data and therefore created some issues later on when creating graphs. However, we continued with the experiment. We used this percent transmittance data to calculate absorbance for the concentrations using a formula that we found using Excel. This absorbance graph (Graph 7) was then used to calculate a calibration curve for phosphate content. The calibration curve is basically a line of best fit for the data. We encountered two big issues when creating this curve. Our first issue was one that we have already somewhat discussed, the issue regarding our uneven concentration distribution. Our concentrations did not provide us with a wide range of data so therefore our calibration curve or line of best fit it less accurate for not close to the numbers we used. Our second issue was data we received for the mono-potassium phosphate

concentration .0004M. For this concentration it can be seen that we received an absorbance value of .6577. Because we had two concentrations so close together (.0003M and .0004M), when we created the line of best fit, this point disrupted the line. It fell too far from the line of best fit and therefore we decided it would be best to leave it out of the graph. The graph without this value is Graph 9. This Graph we used to find the phosphate content of the Colas. Finally, after all this experimenting and testing we could finally determine the phosphate content of the colas. We received three different colas and created dilutions to be tested at the analytical wavelength we found for mono-potassium phosphate, 350nm. The data we found from the three colas is displayed in Table 7. As we had done with mono-potassium phosphate, we took the percent transmittance numbers of the colas and calculated absorbance. Before using the Spec 20 machine to find percent transmittance however, we had to mix the cola dilutions with AVM just as we had done to the mono-potassium phosphate so that we could compare the results to one another. This being said, you did not need AVM to get a reading from the colas because the colas are colored. We then plugged the absorbance numbers into the equation of the calibration curve found in Graph 9. This would give us the molar concentration of each colas displayed in Table 7. This gave us the information we had been seeking from the beginning of the experiment and allowed us to determine which colas had the most phosphate. The formula y=.3106x-.1214 was used where the absorbance of each cola at 350nm was plugged in as variable x. Ultimately, to conclude, we found that soda 2 had the most phosphate, followed my soda 3 and soda 1 as a close third.

References 1. Cooper M. M., Cooperative Chemistry Laboratories, McGraw-Hill New York, NY, 2008