Chemical Sciences Molecular Absorption Spectroscopy Experiments

pH effects on absorption spectra: pKa determination by spectrophotometric method

Pre-Assignments

  1. Which states (ground vs. excited) of the chromophore for the transitions will have higher dipole moment and hence more solvated?
  2. Why are K a (acid dissociation constant) values expressed as pK a like the hydrogen concentration in pH? (Hint: power scale because K a values range over many orders of magnitude)
  3. Write the acid dissociation equation for a diprotic acid and express it in terms of pK a and pH. Under what conditions will pK a be equal to pH?
  4. Are all weak acids or bases indicators?
  5. Do all indicators change color at pH 7.0? Justify your answer.
  6. Explain how one obtains: A = A a × [InH + ]/C T + A b × [In]/C T as given in eq. (6).
  7. Show that: [In]/[InH + ] = (A a –A)/(A – A b )
  8. Find the difference between pK a and pH values, when 50% of a drug (which is a weak acid) is ionized.

Post-Assignments

  1. Can one use methyl orange indicator for any acid-base titrations? Justify your answer.
  2. What is the origin of color change in methyl orange during the pH change in solution?
  3. Identify the species InH + for methyl orange.
  4. What will be the color of methyl orange indicator in pH range 3.1 - 4.4 and why? (Hint: color related to indicator name.)
  5. What is the color of the methyl orange solution at pH=10?
  6. Why is it necessary to have two extreme pH values about two units higher or lower than the pK a of an acid (i.e., extreme pH = pK a ± 2)? Calculate the % of conjugate base of the acid under these conditions of extreme pH values.
  7. What is the slope of the plot of pH vs. log of the ratio [In]/[InH + ]?
  8. Which species of methyl orange - standard methyl orange or its conjugate acid ¡V has higher molar extinction coefficient? How did you infer it?
  9. How can you relate the wavelengths of maximum absorbance to the colors observed for standard methyl orange and its conjugate acid?
  10. At what pH of methyl orange solution would equal amounts of the acidic and basic forms be present?
  11. Are there any wavelengths at which the absorbance of methyl orange does not change with pH variation?
  12. How will the pH of the buffer solutions be affected due to the dilution caused by the addition of the indicator solution to the buffer solution?