Spectrophotometric Determination of Manganese Introduction The purpose of the conducted experiment was to utilize new equipment, such as a spectrophotometer (an instrument that measures light absorbance in a sample), to obtain data necessary to establish the relationship between the absorption of radiant energy (electromagnetic radiation; with properties of wavelength and frequency) and concentration of a solution of KMnO4 (potassium permanganate). Beer’s Law (log(P / Po) = -abc) was then used to determine the concentration of Manganese in an unknown solution, as the law bridges the relationship between concentration and absorption of a certain substance. To test the conformity of the substance to Beer’s Law, a plot of absorbance vs. concentration of various concentrations of KMnO4 was constructed, and the concentration of potassium permanganate in an unknown solution after being oxidized (2Mn2+ + 5IO4- + 3H2O → 2MnO4- + SIO3- +6H+) was determined.
Experimental Initially, a digital spectrophotometer was turned on and allowed 15 minutes to warm up. Next, a clean 100 mL volumetric flask was filled with approximately 10 mL of 3.170 x 10-4 M KMnO4 solution using a buret suspended from an iron clamp and stand. It was labeled Standard 1. Next, three clean 50 mL volumetric flasks were filled with 20 mL, 30 mL, and 40 mL of 3.170 x 10-4 M KMnO4 solution using the same method as Standard 1. They were labeled Standard 2, Standard 3, and Standard 4, respectively. Each of the volumetric flasks was then diluted with distilled water until they were filled to their respective volume marks. In order for the spectrophotometer readings to be accurate, a blank (a solution containing everything except what is being analyzed) was created by filling a clean test tube with distilled water. The spectrophotometer was then set to a wavelength of 540 (given wavelength of maximum absorbance) and one by one, each of the four volumetric flasks had their absorbance measured by transferring some solution to a clean test tube. The spectrophotometer was blanked by placing the blank test tube in the instrument and waiting for an absorbance of 0. A test tube containing solution from Standard 1 was then placed in the spectrophotometer and its absorbance recorded. The same procedure was conducted for each of the Standard solutions, using the blank to zero the spectrophotometer in between readings. A test tube filled with undiluted 3.170 x 10-4 M KMnO4 solution (Standard 5) was then placed in the spectrophotometer and its absorbance was recorded. To determine the concentration of potassium permanganate in an unknown solution, a glass container (unknown 113) containing an unknown concentration was emptied and rinsed with distilled water into a clean 250 mL beaker. 10 mL of nitric acid was then transferred to the beaker using a 10 mL graduated cylinder. Approximately 40 mL of distilled water was added to the solution, as well as 0.50 g potassium periodate (measured with an electronic balance). The beaker was then placed on a hot plate and constantly stirred until the solution had a pink tint. It was then stirred and heated (below boiling) for an additional 10 minutes until the solution was a dark purple color. After cooling, the contents of the beaker was transferred to a 250 mL volumetric flask, and distilled water was added until it was filled to the volume mark. The solution was then mixed before being poured into a clean test tube. After blanking the spectrophotometer, the absorbance of the unknown solution was obtained.
Results Table I: Standard Solutions 1-4 Standard Solutions Initial buret reading, (mL) Final buret reading, (mL) Volume added, (mL) Standard 1 1.24 11.29 10.05 Standard 2 11.29 31.32 20.03 Standard 3 2.28 32.38 30.10 Standard 4 4.99 45.08 40.09
Table II: Spectrophotometer Data Standard Solutions Conc. of KMnO4 (M) Absorbance Standard 1 3.19 x 10-5 .065 Standard 2 1.27 x 10-4 .318 Standard 3 1.91 x 10-4 .421 Standard 4 2.54 x 10-4 .670 Standard 5 (undiluted) 3.170 x 10-4 .799 Unknown Solution (113) 1.50 x 10-4 .366
Figure 1: Concentration vs. Absorbance
Calculations To determine the desired properties of the experiment, certain calculations were made using the compiled data to obtain further information. The first calculation made was to determine the exact volume of potassium permanganate released from the buret into the volumetric flasks. It was calculated: Final buret reading – Initial buret reading = Volume added Example 1: The precise volume of potassium permanganate in the four standard solutions was calculated: Standard 1 Final buret reading – Initial buret reading = Volume added 11.29 mL – 1.24 mL = Volume added Volume added = 10.05 mL Standard 2 Final buret reading – Initial buret reading = Volume added 31.32 mL – 11.29 mL = Volume added Volume added = 20.03 mL Standard 3 Final buret reading – Initial buret reading = Volume added 32.38 mL – 2.28 mL = Volume added Volume added = 30.10 mL Standard 4 Final buret reading – Initial buret reading = Volume added 45.08 mL – 4.99 mL = Volume added Volume added = 40.09 mL Before absorbance readings were obtained, the exact concentration of potassium permanganate in each of the 4 standard solutions was calculated: Concentration = M KMnO4 x Vol. Added / Size of container Example 2: The exact concentration of potassium permanganate in each of the 4 standard solutions was calculated: Standard 1 Concentration = M KMnO4 x Vol. Added / Size of container Concentration = (3.170 x 10-4 M)(10.05 mL) / (100 mL) Concentration = 3.19 x 10-5 M Standard 2 Concentration = M KMnO4 x Vol. Added / Size of container Concentration = (3.170 x 10-4 M)(20.03 mL) / (50 mL) Concentration = 1.27 x 10-4 M Standard 3 Concentration = M KMnO4 x Vol. Added / Size of container Concentration = (3.170 x 10-4 M)(30.10 mL) / (50 mL) Concentration = 1.91 x 10-4 M Standard 4 Concentration = M KMnO4 x Vol. Added / Size of container Concentration = (3.170 x 10-4 M)(40.09 mL) / (50 mL) Concentration =2.54 x 10-4 M The exact concentration of potassium permanganate in the unknown solution (113) was then calculated using the equation of the line generated by the data obtained of absorbance (y) and concentration (x), y = 2604.8x – 0.0252. Because the absorbance of the unknown solution was obtained with the spectrophotometer, it can be plugged in as the variable y and the equation can be solved: y = 2604.8x – 0.0252, where y = .366 .366 = 2604.8x – 0.0252 .3912 = 2604.8x x (concentration) = 1.50 x 10-4 M
Discussion/Conclusions Data obtained in the experiment proved to be both precise and accurate, as all of the calculations made yielded reasonable data. The standard solutions used yielded sufficient data to create a graphical representation of the absorbance vs. concentration, which in turn was used to determine whether or not the unknown solution conformed to Beer’s Law. After calculating the concentration, the set of x and y coordinates for the unknown concentration were traced on the graph, positioned almost exactly on the generated line (Beer’s Law was successful at showing the relationship between absorbance and concentration). New equipment and procedures, such as the proper utilization of spectrophotometers and blanks to obtain an accurate absorbance of a solution, was learned. Also, properties such as radiation, energy, wavelength, and frequency were introduced and a foundation in such terms and relations was developed. The ability to calculate concentrations was strengthened, as well as proof of the relationship between such different properties as concentration and absorbance was justified (Beer’s Law). Although the experiment yielded accurate data, there is always the possibility of error in the procedure, in the form of human and experimental error. Incorrect usage of equipment such as electronic balances, spectrophotometers, and measuring equipment could increase the amount of error in the data. Also, human errors involved with measurements, preparation of necessary solutions, and boiling of certain solutions could render some of the data erroneous.
-------------------- "For everything to be consummated, for me to feel less alone, I had only to wish that there be a large crowd of spectators the day of my execution and that they greet me with cries of hate."
|