Microscale Spectrophotometric Titration of a Mixture of m-Nitrophenol and p-Nitrophenol

Microscale Spectrophotometric Titration of a Mixture of m-Nitrophenol and p-Nitrophenol

The concentration of two nitrophenols will be determined by spectrophotometric titration. The titration will be executed in microscale inside the cuvette.

Materials:

 

  • Mixture of p-nitrophenol and m-nitrophenol
  • ~0.1M NaOH (see bottle for actual concentration)
  • ~0.5M HCl
  • Cuvettes
  • Pipets
  • Eppendorf micropipet and tips

Pre-Lab Preparation:

Assuming that the nitrophenols are both present at 0.02 M, calculate the volume of 0.1 M NaOH required to titrate 2.0 mL of the mixture. Multiply this volume by 1.3. This will be the total volume of NaOH you will add during the course of your titration. Divide this value by 20 top determine the approximate size of the aliquots that will be added during the titration.

Procedure:

  1. Collect a background spectrum in the UV-Vis using water as a blank.
  2. Pipet 1.0 mL of the nitrophenol mixture into a cuvette, add ~1-1.5 mL of 0.5M HCl and collect a UV-Vis spectrum of the acid-form of the nitrophenols.
  3. Rinse the cuvette and pipet another 1.0 mL of the nitrophenol mixture into the cuvette. Add ~1-1.5 mL of 0.1M NaOH and collect a UV-Vis spectrum of the basic-form of the nitrophenols.
  4. After consultation with the instructor, determine the appropriate wavelength to use for your titration.
  5. Pipet 2.0 mL of the nitrophenol mixture into a cuvette and determine the absorbance of the sample at the wavelength selected above.
  6. Using an Eppendorf pipet, add an aliquot of 0.1M NaOH corresponding to the volume calculated in the Prelab Preparation, cap the cuvette and mix gently for ~15-30 seconds to ensure complete mixing and reaction. Return the cuvette to the instrument and determine the absorbance.
  7. Repeat step 6 until the total volume determined in the prelab calculation has been added.

 

Calculations:

 

  1. Create two plots, one of absorbance as a function of the volume of NaOH added, and a second of corrected absorbance as a function of volume NaOH added.
  2. Extrapolate the linear portions of the plots to determine the two equivalence points for each plot.
  3. Based on these equivalence points, determine the concentration of each nitrophenol in the mixture.

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