pKaMethylRed - pKa of Methyl Red1-3 Purpose: The pKa' of...

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pK a of Methyl Red 1-3 Purpose: The pK a ' of methyl red will be determined by measuring absorbance spectra as a function of pH. Introduction Methyl red (4-dimethylaminobenzene-2’-carboxylic acid) is a commonly used indicator for acid-base titrations. We will measure the visible absorption spectra of the acidic and basic forms of this compound. Next we will prepare a series of buffered solutions of methyl red at known pH. By following the change in absorbance as a function of pH we will determine the acid dissociation constant, or pK a . This technique is not restricted to indicators, and can be used with any substance whose absorption spectrum changes with pH. The acid form of the indicator, which we will designate as HMR, is zwitter ionic, Figure 1. The basic form is designated as MR - . N N H + N C H 3 C H 3 O O N N H N + C H 3 C H 3 O O N N N C H 3 C H 3 O O H + OH acid form (HMR) red basic form (MR - ) yellow Figure 1. Acid and base forms of methyl red. The equilibrium of interest is HMR + H 2 O MR - + H 3 O + 1 The equilibrium constant is the acid dissociation constant:
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pKa Methyl Red -2- K a ' = [H 3 O + ][ MR - ] [HMR] 2 The prime indicates that we have used concentrations rather than activities. Activities are necessary in true thermodynamic equilibrium constants. Using concentrations, instead, gives the effective or conditional equilibrium constant. By definition pH = – log [H + ] and pK a = – log K a . Taking the (– log) of both sides of Eq. 2 gives: pK a ' = pH – log [MR - ] [HMR] 3 In this experiment we will determine this equilibrium constant, pK a ' , by varying the pH and measuring the ratio [MR - ]/[HMR]. We will use acetic acid-acetate buffers to control the pH, since the K a value for acetic acid is in the same range as the K a ' value for methyl red. The pH of these buffers force methyl red to distribute itself somewhat evenly between the two colored forms. The absorption of light is governed by the Beer-Lambert Law: A = ε l [X] 4 where A is the absorbance, ε is the molar absorption coefficient, l is the path length of the cell in centimeters, and [X] is the concentration of the absorbing species in moles per liter. The absorbance of mixtures is the sum of the separate absorbencies. In mixtures of the acid and base forms of methyl red the total absorbance is A = A MR - + A HMR 5 The absorption spectra of HMR and MR - are given schematically in Figure 2. For two components in solution, the absorbance must be measured at two different wavelengths. The best wavelengths to choose for the analysis are where one form absorbs strongly and the absorbance of the other form is negligible. Examination of Figure 2 reveals that there are no wavelengths where one form, acid or base, absorbs exclusively. For this case, we need to set up two equations in two unknowns, one equation for each wavelength. Call the two wavelengths λ 1 and λ 2 . The absorbance at λ 1 is A 1 and at λ 2 is A 2 .The two measurements then provide two simultaneous
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This note was uploaded on 10/30/2011 for the course CHEM 432 taught by Professor Greise during the Spring '11 term at Wellesley College.

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pKaMethylRed - pKa of Methyl Red1-3 Purpose: The pKa' of...

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