A spectrophotometer measures the amount of light absorbed by a solution at different wavelengths of light emitted. Beer’s Law says that absorbance is equal to molar absorptivity times the thickness of the sample times the concentration of the sample. Beer’s law also states that conformity of a solution is able to be determined by plotting its absorbances versus its concentrations, and if a straight line results crossing through the origin, the solution has conformity. Using this information, it is possible to determine an unknown concentration of a solution by finding its absorbance, or if given its concentration, its absorbance can be found without the use of a spectrophotometer.
First, a spectrophotometer was turned on, allowed to warm up for about 15 minutes, and was set at a wavelength 400 nm. A cuvette filled with deionized water was used for blanking the spectrophotometer. A second cuvette was filled with a solution of potassium permanganate which was provided. Each cuvette was wiped with a Kimwipe before being placed in the spectrophotometer in order to eliminate smudges which could affect the light passing through. The spectrophotometer was blanked at 400 nm and the cuvette with the potassium permanganate solution was placed in, and its absorbance was read and recorded. It was taken out, and the spectrophotometer was then blanked at 410 nm. The cuvette with the potassium permanganate solution was once against placed in the spectrophotometer. Its absorbance was read and recorded again. This process was repeated, increasing the wavelength of the spectrophotometer by 10 nm until it reached 640 nm when recording ceased. The wavelength with the highest absorbance was used for the rest of the experiment.
Four volumetric flasks were then used to make solutions of KMnO4. Flask 1 was a 100 mL volumetric flask that contained 10 mL of 3.170 x 10-4 M KMnO4, which was dispensed into the flask using a buret. Flasks 2 through 4 were all 50 mL volumetric flasks that contained 20 mL, 30 mL, and 40 mL respectively of 3.170 x 10-4 M KMnO4. All four volumetric flasks were filled to the line on the neck with deionized water. All the flasks were agitated, and cuvettes were filled with each sample. Each cuvette was placed in the spectrophotometer and their absorbances were all recorded.
Next the unknown was placed into a 250 mL beaker and 10 mL of concentrated nitric acid was added to it. Then 0.5 g of potassium periodate was dissolved in 40 mL of deionized water. This solution was heated with a hot plate in order to aid the dissolving process. The contents of the 250 mL beaker were emptied into this solution and were heated for about 10 minutes, but the solution was never brought to a boil. After heating, the solution was put on ice and brought back to room temperature. A cuvette was then filled with this solution and its absorbance was determined and recorded using the spectrophotometer.
Absorption vs. Wavelength for Maximum Absorbance Determination:
|Standard solution||Initial buret reading||Final buret reading||Volume added (mL)|
Unknown Number: 14
|Standard solution||Concentration of KMnO4 (M)||Absorbance|
|Standard #1||3.170 x 10-5 (10%)||0.038|
|Standard #2||1.268 x 10-4 (40%)||0.313|
|Standard #3||1.902 x 10-4 (60%)||0.453|
|Standard #4||2.536 x 10-4 (80%)||0.605|
|Standard #5||3.170 x 10-4 (100%)||0.834|
|Unknown Solution||1.370 x 10-4||0.322|
To find the concentration of the standards, I figured out how much the KMnO4 was diluted in each volumetric flask. I did this by taking the amount of KMnO4 added, then divided by the total volume on the volumetric flask. I then multiplied this percentage by the original concentration of KMnO4, which was 3.170 x 10-4. To find the concentration of the unknown solution, I first got the equation of the standard curve line, which was y = 2701.2x – 0.048. I then substituted the absorbance I found for the unknown, which was 0.322, for y. I could then find the value of x, which was the concentration.
Potassium permanganate does indeed seem to follow Beer’s Law. When I plotted the absorbances found against the concentrations, I was left with nearly a straight line that goes almost directly through the origin. It is only 0.048 absorbances away from going through the origin, and the best fit line is very close to hitting every point plotted. This is one way to prove conformity and Beer’s Law.
Sources of error in this experiment could occur many different ways. If the cuvettes are not wiped off before being placed in the spectrophotometer, there could be smudges or fingerprints that would cause error. The wavelength on the spectrophotometer had to be set by eye, so there is some room for error there, too. If the dilutions are made inaccurately, that would also cause error in absorption readings. Overall, if anything measured in this experiment was measured inaccurately, that would cause error. Also, if the solution with the unknown in it was boiled, that may cause it to form something different than we wanted to measure and that would cause error, too.