Spectrophotometric Determination of the Acid Dissociation Constant of Methyl Red
Date Performed: September 9 ,2011
SPECTROPHOTOMETRIC DETERMINATION OF THE ACID DISSOCIATION CONSTANT OF METHYL RED
M.C. Caligagan ,M.N.Q. Tolentino and M.Q. Clores
Institute of Chemistry, College of Science
University of the Philippines, Diliman, Quezon City, Philippines
Received :September 21,2011
ABSTRACT
This experiment aims to determine the acid dissociation constant value of Methyl Red by means of spectrophotometry. Ten samples were spectrophotometrically analyzed under a UV-Vis spectrophotometer and from the absorbance values acquired, calibration curves were constructed to determine the concentration of the species of interest particularly HMR(acidic methyl red) and MR-(basic methyl red). The principle of Beer’s Law was used since it establishes the direct proportionality between the absorbance and concentration.A pH vs. log(MR-/HMR) curve was also constructed to discern the pKa which is the y-intercept of the equation of the line. A pKa value of 4.9414 was obtained .By comparing the experimental value to the literature value of pKa which is 5.00, a 1.172% deviation was calculated.Based on the aforementioned results, it was confirmed that spectrophotometry is a feasible technique in the determination of the acid dissociation constant of a methyl red .
INTRODUCTION
Spectrophotometry is a technique used to obtain measurements on how much an analyte absorbs radiant energy at a certain wavelength by passing a beam of light through the analyte.The analyte absorbs a fraction of the light and transmits the rest. [3]. The absorbed light or Absorbance (A) can be equated to the logarithmic ratio between Io and I, the intensity of the monochromatic light entering the sample(Io) and the intensity of this light emerging from the sample (I).It is represented by the equation:
A = log (Io/I) (1)
By Spectrophotometry one or more specie present in the sample can be quantitatively determined which gives it the advantage of having the ability to
SPECTROPHOTOMETRIC DETERMINATION OF THE ACID DISSOCIATION CONSTANT OF METHYL RED
M.C. Caligagan ,M.N.Q. Tolentino and M.Q. Clores
Institute of Chemistry, College of Science
University of the Philippines, Diliman, Quezon City, Philippines
Received :September 21,2011
ABSTRACT
This experiment aims to determine the acid dissociation constant value of Methyl Red by means of spectrophotometry. Ten samples were spectrophotometrically analyzed under a UV-Vis spectrophotometer and from the absorbance values acquired, calibration curves were constructed to determine the concentration of the species of interest particularly HMR(acidic methyl red) and MR-(basic methyl red). The principle of Beer’s Law was used since it establishes the direct proportionality between the absorbance and concentration.A pH vs. log(MR-/HMR) curve was also constructed to discern the pKa which is the y-intercept of the equation of the line. A pKa value of 4.9414 was obtained .By comparing the experimental value to the literature value of pKa which is 5.00, a 1.172% deviation was calculated.Based on the aforementioned results, it was confirmed that spectrophotometry is a feasible technique in the determination of the acid dissociation constant of a methyl red .
INTRODUCTION
Spectrophotometry is a technique used to obtain measurements on how much an analyte absorbs radiant energy at a certain wavelength by passing a beam of light through the analyte.The analyte absorbs a fraction of the light and transmits the rest. [3]. The absorbed light or Absorbance (A) can be equated to the logarithmic ratio between Io and I, the intensity of the monochromatic light entering the sample(Io) and the intensity of this light emerging from the sample (I).It is represented by the equation:
A = log (Io/I) (1)
By Spectrophotometry one or more specie present in the sample can be quantitatively determined which gives it the advantage of having the ability to
References: [3] Parkampus, H.H. UV-VIS Spectroscopy and Its Apllications. Springer-Verlag Berlin Heidelberg, Germany. 1992. =6.015523509 Log([MR-]/[HMR])=Log(6.015523509)