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- acidic (HMR) at pH = 2
- basic (MR-) at pH = 10
* done to make sure that the methyl red exists in only one form
Spectral scanning was performed; the wavelength of maximum absorption of the acidic, HMR, and of the basic, MR-, form of methyl red was determined. RESULTS and DISCUSSION Calibrations curves were constructed to determine of HMR and MR at HMR and MR-. Using the absorbance obtained and the molar absorptivity constants, determined earlier, Beer’s law can be used in the analysis of multi-component system. Relationship between pH and ratio of concentrations is clearly shown in Henderson-Hasselbalch equation,
pH = log ([MR-])/([HMR]) + pKa Relating the equation of the best fit line (y = 1.1201x + 4.9972) with equation above, y-intercept = pKa
pKa = 4.9972 ; Ka = 1.01 x 10-5, which deviates by 0.65% from the theoretical value which is 1.00 x 10-5 SOURCES of ERROR the deviation of the obtained Ka from the theoretical Ka could be the result of the limitations of Beer-Lambert’s law. other variables that also influence the results are the nature of the solvent, pH of the solution, temperature, preparation of the solutions, scratches in the cuvettes, bubbles in the solution which causes the incident rays to scatter, improper handling of the cuvettes, and flaws in the spectrophotometer machine used (Skoog) WORKING EQUATIONS  Petrucci R.H. & Harwood W.S.General Chemistry: Principles and Modern Applications, 8th ed.; Prentice Hall Inc.: Upper Saddle River, New Jersey 1997; pp 658-686. Nurullah Sanli, Senem S., and Guleren A., Determination of Dissociation Constants of Folinic Acid (Leucovorin); 5-Fluoro uracil, and Irinotecan in Hydro-organic Media by a Spectrophotometric Method. Journal of Chemical & Engineering Dat Online 2010 55 (8), 2695-2699, http://pubs.acs.org/doi/pdfplus/10.1021/je100072na (accessed Mar 11, 2013)
 Esguerra, M. and Chem 28.1 Instructors. Analytical Chemistry Laboratory Manual. 7th ed.; Institute of Chemistry, UP Diliman, Quezon City. 2007.
 Skoog, D. A., West, D. M., Holler, F. J., Crouch, S. R. Fundamentals of Analytical Chemistry, 8th ed.; Brooks/Cole, Singapore. 2004.
 Sommer L. Analytical Absorption Spectrophotometry in the Visible and Ultraviolet: The Principles, Elsevier Science Publishers: Hungary 1989; pp 5-25.
 UV/Visible Spectrophotometer. Online, http://www.yteach.co.uk/page.php/resources/view_all?id=absorption_light_colorimetry_colour_metal_compound_block_periodic_table_absorption_spectra_UV_t_page_14&from=search (accessed March 14, 2013) *for video and sample.prob.1.
REFERENCES SAMPLE PROBLEMS Problem 2 Prepared and presented by: * ELLAINE DATU * DIANNE CORSINO CHEM 28.1 THWX1
* MA'AM SARAH G. YES! FINALS NA LAAAANG! Calibration standard solutions (solutions 1-6) were prepared; then, absorbance of the solutions were taken at (lambda)HMR and (lambda)MR-.
* matched cells were used to ensure that the absorbance subtracted by the reference cell (cuvette with distilled water) to the sample cell is consistent. Properties of the cuvettes should be the same (i.e. absorbance, surface reflection, and path length)
pH of the sample solutions 7-10 was determined using a pH meter
*pH values are necessary since it is the ordinate of the Henderson-Hasselbalch plot to be used for the determination of Ka of methyl red.