Tetrabromophenolsulfonphthalein, bromophenol blue, is an acid- base indicator whose useful range as an indicator lies between pH 3.0 and 4.6. It changes from yellow Species I at pH 3.0 to purple Species II at pH 4.6, and this reaction is reversible.
Bromophenol blue undergoes another reaction at higher pH which is slow and pH dependent. The reaction of Species II with hydroxide ion is first order with respect to each reactant and second order overall. The rate law for this reaction is:
(In the write-up the structures of I, II, and III (kinetics product) are given)
Rate = k2 [II] [OH1-] where [II] is the molar concentration of Species II and k2 is the second order rate constant.
We run this reaction with 1.0 and 0.50 M NaOH. Both concentrations are very much larger than the indicator and result in pseudo first order rates for the indicator.
k1 = k2 [OH1-]
Rate = k1 [II]
k1 is the pseudo first order rate constant. The integrated rate equation is:
log [II] = -k1 t / 2.303 + log [II]o
where t is the time, [II]o is the concentration of Species II at t=0.
From Beer's Law, A = c [II]; where A is Absorbance and c is the constant. Substitute (A/c) for [II], and eliminate the log(c) term. This results in the following:
log A = -k1 t / 2.303 + log Ao
From a plot of log A verus t, k1 is determined, and then k2. (We use log rather than ln to avoid student confusion because the conversion from %T to Absorbance involves log.)
We use two reaction mixtures:
Mixture 1 consists of 25.0 ml of 1.0 M NaOH to which is added 15 drops of bromophenol blue stock solution. Since the initial concentration is not critical, we don't worry about preparing an initial molar concentration of bromophenol blue. We just need to get to a convenient absorption range.
Mixture 2 consists of 25.0 ml of a solution which is 0.50 M in NaOH and 0.50 M in KNO3 (to maintain ionic strength) to which is added 9 drops of bromophenol blue stock solution.
The stock bromophenol blue solution is 0.05 percent aqueous bromophenol blue and form II absorbs at 590 nanometers.
Sample Data:
| t (min.) | 0.0 | 2.0 | 4.0 | 6.0 | 8.0| | 10.0 | 12.0 | 14.0 | 16.0 | 18.0 | 20.0 |
| #1 %T | 6.6 | 9.6 | 13.2 | 17.7 | 21.9 | 27.4 | 32.2 | 38.5 | 43.0 | 48.5| | 52.6 |
| #2 %T | 25.6 | 28.0 | 32.1 | 33.7 | 37.9 | 39.6 | 42.0| | 45.3 | 48.0 | 51.4 | 52.4 |
SAMPLE RESULTS MIXTURE #1: [OH1-] = 1.00 M
slope = -0.0316 +/- 0.0002
intercept = 0.069 +/- 0.003
k1 = 0.0728 +/- 0.0005
k2 = 0.0728 +/- 0.0005
SAMPLE RESULTS MIXTURE #2: [OH1-] = 0.500 M
slope = -0.0165 +/- 0.0004
intercept = -0.231 +/- 0.004
k1 = 0.0380 +/- 0.0009
k2 = 0.0760 +/- 0.0020
The above experiment was adapted from the article "Fading of Bromophenol
Blue: A combined synthesis and spectrophotometric kinetics experiment", Randall
Winans and Charles Allan Brown (Baker Laboratory, Cornell University) in
Journal of Chemical Education, Vol. 52, pages 526 and 527, 1975.
Questions? Comments??
Frank M. Lanzafame
Department of Chemistry
Monroe Community College
1000 East Henrietta Rd
Rochester, NY 14623
(716) 292-2396