Spectrophotometric Determination of the Acid Dissociation Constant of Methyl Red

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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 analyze participating specie which cannot be isolated in a solution.The analysis of two different components in a sample can be done by choosing an optimum wavelength for each of the component wherein one of the specie has a higher absorbance compared to the other.[2] The sample analyzed in this experiment is Methyl red which primarily exists in three forms:H2M+,a cation which appears in an intense red color,HMR,a neutral molecule known as a zwitterion (consists of a positive and a negative charge) which also has a red color and MR-,an anion with a yellow color(which results in the addition of base causing methyl red to loss proton).[5]These colored forms of methyl red absorbs light on the visible region of the spectrum and have minimum interference from other substances. Methyl red is a weak acid widely used as an acid-base indicator.Its ability as an indicator is based on the equilibrium that exists between HMR and MR-.The chemical equation for the equilibrium is :

HMR⇌MR-+H+
From the chemical equation the acid dissociation constant Ka can be represented as: Ka=[MR-][H+] (2)
[HMR]
In order to experimentally determine the acid dissociation constant,the concentration of HMR and MR- were obtained through the use of a spectrophotometer and application of Beer’s Law which directly relates the concentration of the colored analyte and the amount of light it absorbs. The equation below shows the relationship between equation 1 and Beer’s Law. A=log (Io/I) = εbc (3) Where:

A is the absorbance
ε is the molar absorptivity coefficient expressed in cm. per M b is the path length in cm(1cm in this experiment)
c is the concentration of the absorbing substance
Even though there exists a relationship between transmittance and absorbance,Beer’s Law is still measured by Absorbance since it is unitless and is proportional to the concentration of the solution.On the other hand, Transmittance is...
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