How to Determine pH Using Spectrophotometry

Prepare the Unknown and Known Solutions for Measurement

• 1

  Wear safety goggles and gloves. Pipet 10 mL of bromocresol green indicator into a 50 mL volumetric flask. Use a graduated cylinder to add 25 mL of the solution with the unknown pH and dilute it to the 50 mL mark with deionized water.

• 2

  Pipet 10 mL of bromocresol green indicator into a second 50 mL volumetric flask. Use a graduated cylinder to add 25 mL of 0.1 M HCl solution and dilute it to the 50 mL mark with deionized water.

• 3

  Pipet 10 mL of bromocresol green indicator into a third 50 mL volumetric flask. Use a graduated cylinder to add 25 mL of the 0.1 M NaOH solution and dilute it to the 50 mL mark with deionized water. You now have three separate 50 mL volumetric flasks containing three solutions to measure.

Establish Deionized Water Baseline

• 4

  Rinse the cuvette with tap water and rinse it again with deionized water.

• 5

  Fill the cuvette with deionized water and set it aside in a holder.

• 6

  Use the spectrophotometer to measure the spectra of the deionized water. This establishes a baseline spectra for comparison.

Measure the Spectra of the Unknown and Known Solutions

• 7

  Using the unknown sample solution in the first 50 mL volumetric flask, rinse the cuvette twice and then fill it with solution. Place the cuvette in the holder and measure and record the spectrum of the unknown sample.

• 8

  Using the HCl solution in the second 50 mL volumetric flask, rinse the cuvette twice and then fill it with solution. Place the cuvette in the holder and measure and record the spectrum of the HCl solution.

• 9

  Using the NaOH solution in the third 50 mL volumetric flask, rinse the cuvette twice and then fill it with solution. Place the cuvette in the holder and measure and record the spectrum of the NaOH solution.

Calculation of pH of Unknown Sample

• 10

  In Excel or by hand, enter the absorbances and pHs of the two known samples in a table as follows:

  pH of NaOH pH of HCl

  absorbance of NaOH absorbance of HCl

• 11

  Add a row to the table to calculate the log of the absorbance values for the NaOH and HCl solutions:

  pH of NaOH pH of HCl

  absorbance of NaOH absorbance of HCl

  log (absorbance of NaOH) log (absorbance of HCl)

• 12

  Graph the values of pH vs log (absorbance) for the known samples of NaOH solution and HCl solution. Place the pH data on the x-axis and the log (absorbance) data on the y axis. Draw a line through the two points to establish the linear relationship.

• 13

  Since the absorbance of the unknown sample was measured, calculate the log of the absorbance of the unknown sample.

• 14

  Plot the value of the log of the absorbance of the unknown sample as a point on the Y axis of the graph, and draw a horizontal line until it intercepts the linear graph. Then draw a vertical line down to the x-axis and read off the value of pH of the unknown sample.