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The primary objectives of this comprehensive experiment were twofold: first, to elucidate the relationship between the color of a compound and the wavelength of light it absorbs, and second, to establish a robust methodology for correlating the amount of light absorbed with the concentration of species in solution. We harnessed the capabilities of the Spectronic 20 (also known as the Spec 20) to measure transmittance and convert it into absorbance values for visible light through solution samples.
The Spectronic 20 is a versatile device capable of precisely quantifying the transmittance of ultraviolet to visible light rays in various solutions.
This ability makes it an indispensable tool for conducting quantitative measurements in the laboratory, allowing us to explore the interplay between light and chemical properties.
Beer's Law serves as a fundamental equation underpinning our experimental approach. It articulates the relationship between the concentration of a chemical species in solution and its ability to absorb light. Mathematically, Beer's Law is expressed as A = εbc, where "A" represents absorbance, "ε" symbolizes molar absorptivity (a constant for a given substance), "b" is the path length of the sample (in centimeters), and "c" represents the concentration of the solution in moles per liter (mol/L).
The experiment spanned three weeks, each focused on specific aspects of our goals.
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In the first week, our primary aim was to become proficient in operating the Spectronic 20. We selected three different food coloring solutions as well as Potassium Permanganate as test samples. We measured the percent transmittance of these samples across a range of wavelengths, from 350 to 650 nm.
To establish a baseline, we employed water as the blank solution for calibration.
The results were as follows:
| Solution | Wavelength (nm) | Percent Transmittance (%) |
|---|---|---|
| Red Food Coloring | 400 | 0.11 - 0.01 |
| Blue Food Coloring | 450 | 0.04 - 0.49 |
| Red and Blue Mix | 500 | 0.12 - 0.27 |
| Potassium Permanganate | 550 | 0.62 - 0.07 |
After measuring percent transmittance, we calculated absorbance using the formula A = 2 - log(%T). This calculation allowed us to identify the optimal wavelengths for determining both the percentage transmittance and absorbance of diluted potassium permanganate.
In the second week, we performed absorbance measurements over a wider range of analytical wavelengths, spanning from 370 nm to 610 nm. The obtained absorbance values ranged from 0.487 A to -0.010 A. These values were critical in calculating the percent transmittance for our second dilution, which needed to fall between 32.6% and 102.2%. We employed the equation 101xV1 = 0.00101 x 100 to calculate the concentration for each volume, repeating this process for each dilution throughout the experiment.
Week three introduced ammonium (AVM) as an additive. We realized that the phosphate solution was transparent, making it necessary to add AVM to induce color formation, thereby facilitating spectrophotometer measurements. We created graphs to visualize the percent transmittance calculations.
While our experiment proceeded with relative smoothness, we encountered a few challenges. Some calculations had to be redone due to inaccuracies in transmittance-to-absorbance conversions. Additionally, we had to repeat part of week one's experiment because of incorrect mixing of ammonium with phosphate instead of introducing it into the blank solution. This erroneous step resulted in negative calculations, which were not mathematically plausible given the proper utilization of AVM. Week two and week three suffered from significant calculation errors, leading to inaccurate graph representations.
The selection of the optimal wavelength was based on the measurement of the highest absorbance value. When different wavelengths of light pass through a sample, various colors become observable. This phenomenon can be visualized by splitting white light using a prism, which reveals a spectrum composed of distinct constituent wavelengths, each corresponding to a unique color.
By converting percent transmittance to absorption levels and plotting these values on a graph, we were able to determine the final concentrations of our solutions, allowing us to draw meaningful conclusions about the relationships between color, concentration, and absorbance.
The results of the Analysis of Colas Project 13 affirm the validity of our research hypothesis. As transmittance percentages increased, absorption levels decreased. The calibration curve, generated from absorption measurements, further emphasized the inverse relationship between percent transmittance and absorbance. This relationship indicates that higher absorbance corresponds to a more concentrated solution, whether in water or another medium. This insight is in line with the established principle: "There is a direct relationship between absorbance and concentration; the higher the absorbance of a substance, the more concentrated its solution will be in water or another medium" (Reference 1).
Color-Absorbance Relationship Study with Spectrophotometry. (2024, Jan 17). Retrieved from https://studymoose.com/document/color-absorbance-relationship-study-with-spectrophotometry
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