Gravimetric Determination of Moisture and Phosphorous in Fertilizer Samples

SPECTROPHOTOMETRIC DETERMINATION OF THE EQUILIBRIUM CONSTANT OF A REACTION

M. L. SANGA
CHEMICAL ENGINEERING, COLLEGE OF ENGINEERING
UNIVERSITY OF THE PHILIPPINES, DILIMAN, QUEZON CITY, PHILIPPINES
DATE PERFORMED: JANUARY 15, 2014
INSTRUCTOR’S NAME: MOISES JEROME GARRERO

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ABSTRACT The experiment aimed to determine the equilibrium constant Keq of the formation of Fe(SCN)2+ from Fe3+ and SCN- in the equation Fe3+(aq) + SCN-(aq) ⇌ Fe(SCN)2+(aq), through spectrophotometric analysis. Acidified solutions (using 0.1 M HCl) of FeCl3 and KSCN were prepared for the calibration of the UV-Vis spectrophotometer (using standard solutions) and the determination of Keq of the Fe(SCN)2+ formation (solutions with unknown concentrations). Absorbance readings of the standard and unknown solutions were obtained through the spectrophotometer. The equation of the trend line of the calibration curve (obtained through the plot of the absorbance readings vs. thiocyanatoiron (III) ion concentration of the standard solutions) is given by y = 4041x – 0.049. Using the absorbance readings and the application of the Beer-Lambert Law on the trend line equation, the average experimental Keq was computed to be 190.85 with a 30.72 % deviation from the theoretical value. Based on the results of the experiment, it can be assessed that the experiment was successful.
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INTRODUCTION

Spectrophotometry, a branch of spectrometry, is a group of instrumental techniques that measures the atomic and molecular energy levels of a substance [1]. In spectrophotometry, the emission and absorption spectra’s wavelengths identify and measure the concentration of a substance. The wavelength at which the substance’s absorbing ions or molecules has a

References: [1] Silberberg, M. S. Chemistry: The Molecular Nature of Matter and Change 5th ed. The McGraw-Hill Companies, Inc., New York. 2009. [2] Zumdahl S. S., Zumdahl S. A. Chemistry 9th ed. Brooks Cole, Canada, USA. 2010. [3] Alyea H. N. The Halogens: Chlorides. http://pubs.acs.org/doi/pdf/10.1021/ed046pA355.1 (accessed January 24, 2014). [4] Lodge, Jr., J. P. Methods of Air Sampling and Analysis. CRC Press, Florida. 1988. [5] Study Notes: Analytical Wavelength. (n. d.). Retrieved (January 28, 2014) from http://toolboxes.flexiblelearning.net.au/demosites/series5/508/Laboratory/StudyNotes/snAnalyWavelength.htm [6] Reusch, W. Visible and Ultraviolet Spectroscopy. Retrieved (January 24, 2014) from http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/UV-Vis/spectrum.htm [7] Los Angeles Harbor College. Chemical Equilibrium. Retrieved (January 27, 2014) from http://www.lahc.edu/classes/chemistry/arias/Lab4Equilibriasp11.pdf [8] Cobb, C. L., Love, G. A., J. Chem Educ. 1998, 75, 92.