Tetrabromophenolsulfonphthalein, bromophenol blue, is an acid- base indicatorwhose useful range as an indicator lies between pH 3.0 and 4.6. It changesfrom yellow Species I at pH 3.0 to purple Species II at pH 4.6, and thisreaction is reversible.
Bromophenol blue undergoes another reaction at higher pH which is slow andpH dependent. The reaction of Species II with hydroxide ion is first orderwith respect to each reactant and second order overall. The rate law forthis reaction is:
(In the write-up the structures of I, II, and III (kinetics product) aregiven)
Rate = k2 [II] [OH1-] where [II] is the molar concentration ofSpecies II and k2 is the second order rate constant.
We run this reaction with 1.0 and 0.50 M NaOH. Both concentrations are verymuch larger than the indicator and result in pseudo first order rates forthe 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 IIat 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 inthe 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 lograther than ln to avoid student confusion because the conversion from %Tto 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 ofbromophenol blue stock solution. Since the initial concentration is not critical,we don't worry about preparing an initial molar concentration of bromophenolblue. 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 0.50M in KNO3 (to maintain ionic strength) to which is added 9 drops of bromophenolblue stock solution.
The stock bromophenol blue solution is 0.05 percent aqueous bromophenol blueand form II absorbs at 590 nanometers.
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 BromophenolBlue: A combined synthesis and spectrophotometric kinetics experiment", RandallWinans and Charles Allan Brown (Baker Laboratory, Cornell University) inJournal of Chemical Education, Vol. 52, pages 526 and 527, 1975.
Frank M. Lanzafame
Department of Chemistry
Monroe Community College
1000 East Henrietta Rd
Rochester, NY 14623