09. The Fading of Bromophenol Blue

Goals

• • Determine the rate law for the fading of bromophenol blue in basic solution.

  • Better understand rate laws, rate constants, and orders of reaction.

Background

Bromophenol blue, H₂Bp, is an organic compound having two acidic protons (H⁺'s) which are readily abstracted to form a blue dianion, Bp^2-. The blue dianion has a molar extinction coefficient of about 65000 M^-1 cm^-1. In this lab, you will be supplied with an aqueous solution of Na₂Bp. The blue dianion, Bp^2-, in this aqueous solution reacts with excess hydroxide ion to form a colorless tri-negative ion Bp(OH)^3- as shown below in equation 1.

Equation 1: Reaction of Bp^2- with OH^-

The disappearance of the blue dianion in an excess of hydroxide proceeds at a rate that is easy to monitor as the half-life is on the order of minutes. The overall rate law that describes this reaction can be written as follows:

where k is the overall rate constant, [Bp^2-] and [OH^-] are the time-dependent concentrations of the bromophenol blue dianion and the hydroxide ion, respectively, m is the order of the reaction with respect to Bp^2-, and n the order with respect to OH^-. To determine the rate law, the unknowns k, m, and n must be found.

One way to determine the order of a chemical reaction is to monitor the concentration of the reactant(s) versus time. If a reaction is first order with respect to a particular species, X, then a plot of ln[X] vs. time is linear and the slope of the line is the negative of the rate constant. If the reaction is second order with respect to X, then a plot of 1/[X] vs. time is linear. Here, the slope is equal to the rate constant.

In a bimolecular reaction, such as the one above, determining the reaction order with respect to one species can be simplified by choosing reaction conditions in which the concentration of one reactant is so large that it is essentially constant during the course of the reaction. In determining the rate law for this reaction, the hydroxide ion should be in large excess so that only the concentration of Bp^2- decreases during any one experiment. Since Bp^2- is colored, its concentration can easily followed spectrophotometrically. By monitoring the concentration of Bp^2- in two or more experiments with different concentrations of excess hydroxide, one can determine the order with respect to the [OH^-] also. This permits evaluation of the overall rate law and the rate constant.

You will need to use the spectrum below in order to determine the wavelength at which to set your spectrophotometers. Notice that the maximum absorption is much higher than ideal for our spectrophotometers to give us an accurate measurement. Should we measure our solutions at 590nm? 610nm? 570nm? If we want our initial absorption readings to be less than 1.5, what wavelength should we use in lab?

Figure 1: Absorption spectrum of 0.03 g/L Bromophenol blue solution

Experiment

Materials and Equipment

• • 4 M NaCl

  • 4 M NaOH

  • 0.5 g/L bromophenol blue (Na₂Bp = Na₂C₁₉O₅Br₄SH₈ FW 669.96 g/mol)

• • 50.00 and 100.00 mL volumetric flasks

  • 25.00, and 50.00 mL volumetric pipettes

  • Graduated cylinders

  • Ocean Optics spectrophotometers

Experimental

With your lab partner, agree on a plan to determine the rate law for the reaction of Bromophenol Blue and sodium hydroxide. You should perform two reactions, Trial A & Trial B, each with a different (excess) NaOH concentration. Be sure to do each reaction one at a time, because the color will fade from the second experiment before you get to place it in the spectrophotometer. Use your prelab as a guide to determine which concentrations and amounts to use in each solution you make. The initial concentration of Bp^2- should be the same for each of your reactions, as described in your prelab. Be certain to keep the ionic strength, which is related to the total number of ions present in a solution, the same for both trials. This will be achieved in this experiment by keeping the sum of the [NaOH] and [NaCl] constant. For example, [NaOH] + [NaCl] = 2M.

We will be using computerized data collection for this lab. Spectrophotometer units by Ocean Optics will be attached to your laptops which have Logger Pro Software on them. Set up your laptop and spectrophotometer before making your solutions. Otherwise, the bromophenol blue will fade before you can make measurements. Using Logger Pro to measure absorption:

• 1. Open the Logger Pro program. The program will automatically detect the spectrophotometer and report real-time absorption values.

  2. Under the Experiment menu, open the "Data Collection" window. Set the Mode to Time Based, the experiment Length to 20 minutes, and the Sampling Rate to 2 samples/minute.

  3. Click on the spectrum icon (shown above) at the top of the screen; within the pop-up menu choose Configure Collection; then, within the next pop-up menu, you will be able to select the appropriate wavelength. Use the spectrum above (in the Background section) to determine the absorbance for bromophenol blue that starts below 1.5 absorbance units. Set the wavelength on the spectrophotometer accordingly.

  4. Blank the spectrophotometer:
     --Fill one cuvette (~ ¾ full) with water and place it in the spectrometer.
     --Under the Experiment menu, choose "Calibrate" and then "Spectrometer". Follow the directions in the window. Logger Pro should now indicate zero absorbance.

  5. Now prepare your Bromophenol Blue solution for Trial A or Trial B, use it to fill a cuvette, and place it in the spectrometer. Collect data (green button near top right of screen).

  6. When your data collection is complete, copy the data into Excel and send the file to yourself and your lab partner.

  7. Repeat for the other trial (Trial B or Trial A)