Experiment 4: Buffers and Indicators
Abstract
This experiment involved determining the pK
HIn
of the indicator bromocresol green by measuring
its percent transmittance, converted to absorbance, in various solutions of different pH values. The first part
of this experiment involved finding the wavelengths of maximum absorbance for both a solution of high
[HIn] and one of high [In

] which were determined to be 445 nm and 605 nm, respectively. All solutions
had absorbance measured at these two wavelength which would in turn create a trendline from which pK
HIn
can be determined. Measuring at 445 nm and 605 nm, it was determined that bromocresol green had pK
HIn
values of 4.75 ± 0.73 and 5.05 ± 0.84, respectively.
Introduction
The purpose of this experiment is to find the p
K
a
(p
K
In
), of the indicator (changes color at certain
pH, a weak acid or weak base) bromocresol green. This acidbase indicator is an organic acid which
dissociates with the following reactions:
HIn + H
2
O ↔H
3
O
+
+ In

Using the HendersonHasselbalch equation, the equilibrium expression for this reaction can be written as:

+
HIn
In
log
HIn
pH
pK
Which can be converted to:

In
log

HIn
HIn
pH
pK
This is equivalent to the line form of y = mx + b where y is log([In

]/[HIn]), x is pH, m (the slope of the
line) is equal to one, and b (how much the line is shifted vertically) is pK
HIn
. When this is log([In

]/[HIn]) is
plotted against pH, the following trend is obtained as shown in Figure 1.
1
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View Full DocumentFigure 1: Sample graph of log([In

]/[HIn]): plotting log([In

]/[HIn]) versus pH of the bromocresol
green indicator will give the following linear trend.
1
When y=0 or log([In

]/[HIn])=0, pH=p
K
HIn
which makes sense because when log([In

]/[HIn])=0, [In

]
/[HIn]=0, which means that the indicator is 50% disassociated.
This concept will be used in this experiment in order to find the p
K
HIn
. But instead of finding the
concentrations of the HIn and In

at certain pH’s, only the ratio needs to be established. This ratio does not
only represent the ratio of the concentrations between acid and base, but it also represents the ratio of the
absorbance between acid and base since absorbance is proportional to concentration using the equation
A=ξ
bc
(A= absorbance; ξ= molar absorbtivity (cm
1
M
2
); b= cell length (cm); and
c
= concentration of
component in solution):
:
base
acid
Absorbance
log

Absorbance
HIn
pH
pK
Because of this, one can measure the absorbance values of solutions of various concentrations of
bromocresol green with a spectrophotometer. This can then be plotted against their respective pH values
(measured with pH meter) to get a linear trend and the y intercept will equal the p
K
HIn
.
To be able to eliminate any error due to poor readings of too diluted solutions, the wavelength of
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 Fall '06
 Dai
 pH, Pk, percent transmittance, pKHIn

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