Photosynthesis Lab Report

The majority of the experimentation in this project relied on the idea that there is a relationship between concentration and absorbance of a solution. This is easy to understand if you can visualize the particles in the solution; and this is especially true if the particles in solution portray a distinct color. The more particles there are in the solution, the more they will obstruct the path of the light. This occurs the most at the analytical wavelength, where the most light is absorbed by the particles. When light of this wavelength hits the particles, it is absorbed and there is no chance that it will reach the sensor in the spectrophotometer. However, if a solution is analyzed with a wavelength corresponding to the color of the particles,
concentration has much less of an effect on absorbance. This is because even though the particles obstruct the path of the light, they reflect the light instead of absorbing it. When the light is reflected, is can bounce around the inside of the “chamber” and still reach the sensor. This is why it is so important to analyze solutions using the analytical wavelength and not just any wavelength. In this particular procedure, the phosphate ions and AVM formed a complex together that had a distinct yellow color in solution.
This solution had an analytical wavelength of 380 nm, because that is the wavelength at which the complex absorbs the most light; allowing the lowest amount to reach the sensor. The source of phosphate for making our standard curve was potassium hydrogen phosphate, and this wavelength allowed us to target and analyze only the phosphate-AVM complex while yielding the least impact from the potassium ions. As for the colas, they are made with so many ingredients that it was very important to have a good analytical wavelength. In addition to phosphate, there is caffeine, sugar, corn syrup, vanilla, and many other ingredients in colas (Field); all of which could have an effect on a percent transmittance reading from the Spec 20 at a poorly selected wavelength. Using the analytical wavelength minimizes the effect that other factors in the solution have in the reading, which in turn minimizes error in the
data.
The main goal of project 13 was to determine the relationship between concentration of a substance and absorbance measured by a spectrophotometer; and to use this knowledge to determine the phosphate concentration in colas. We were able to accomplish this over three weeks of experimentation. The first week was, for the most part, a practice week to get us used to the procedures and techniques that we would be using in weeks two and three. In week one we performed the exercise with the food coloring and the potassium permanganate, and also formed a standard curve for potassium permanganate.
The results of the food coloring exercise are seen in Figure 1 of the group report. The red food coloring displayed the highest percent transmittance in the long wavelengths closest to red light, which was expected. With the same logic, it was expected that the blue food coloring would have the highest percent transmittance in the higher energy wavelengths associated with blue light. In fact, the trend lines for the red and blue food coloring look almost like opposites. As for the potassium permanganate, we were interested in the wavelength that resulted in the lowest percent transmittance. This wavelength, which was 550 nm, was our analytical wavelength. This wavelength yields the most accurate results when analyzing potassium permanganate and also provides for the least error from other substances in the solution. Choosing the proper analytical wavelength for phosphate was very important for testing the colas since there are so many different ingredients that go into
them. Using the analytical wavelength of 550 nm, we determined the relationship between concentration and absorbance for potassium permanganate. Absorbance is inversely related to transmittance, so the analytical wavelength provides the lowest percent transmittance while having the highest absorbance number. The data showed that as the concentration of a substance increases, the absorbance of the solution also increases. Lower concentrations of potassium permanganate resulted in the lowest absorbance while the higher concentrations resulted in higher absorbance; indicating a positive relationship between concentration and absorbance. Based off of the data in Figure 2 of the group report, Microsoft Excel provided an equation for the relationship. The graph in Figure 2, plotting concentration vs. absorbance, is known as a standard curve. In week two we more or less performed the same procedure as week one, except with phosphate. After determining the analytical wavelength for phosphate + AVM to be 380 nm, we were able to use various dilutions of potassium hydrogen phosphate and AVM to form a standard curve for this combination. In week three, we used the standard curve from week one to determine the concentration of phosphate ions in three different colas. From our week two data, Microsoft Excel related the absorbance and concentration in the equation y=1193.1x, where x is the phosphate concentration and y is the absorbance number. When we got absorbance numbers for our diluted cola samples, we simply had to plug that number in for y and solve for x in that equation. The resulting concentration number then had to be multiplied by 50 to account for the dilution that was made for testing. This gave the phosphate concentration of each cola based on our previous data.