Transcript Lab 7 A series of buffered bromothymol blue (BTB) solutions with increasing pH is prepared. The ratio [In-] / [HIn] is determined with.

Lab 7
A
series of buffered bromothymol blue (BTB)
solutions with increasing pH is prepared.
The ratio [In-] / [HIn] is determined with the aid
of spectrophotometry by measuring the
absorbance for each solution at a wavelength of
635 nm.
The pKa of BTB is then determined by plotting
log ([In] / [HIn]) versus pH.
In- = basic form of the indicator
HIn = acidic form of the indicator
Consider a generic indicator (HIn).
At a low pH the indicator is completely in the HIn form and the
absorbance is at a maximum at wavelength 1 (Figure 1A).
At a high pH the indicator is completely in the In- form and the
absorbance is at a maximum at wavelength 2 (Figure 1B).
At an intermediate pH, the solution contains appreciable
concentrations of both HIn and In-, and exhibits an absorbance
spectrum containing contributions from both forms.
Figure 1. Example absorbance spectra of an acid base indicator in (A)
acidic solution, (B) basic solution , and (C) solution of intermediate pH.
BTB indicator in pH acidic, neutral, and basic solutions (left to right).
 pH
indicators are useful to estimate the pH of a
solution and to visually detect the equivalence
point of an acid / base titration.
 Bromothymol
blue is mostly used in measuring the
pH of substances that would have relatively low
acidic or basic levels (near a neutral pH). It is
often used in pools, fish tanks, or measuring the
presence of carbonic acid in a liquid.
 BTB
is also used for observing photosynthetic
activities or respiratory indicators (turns yellow
as CO2 is added).
A
common demonstration of BTB's pH indicator
properties involves exhaling through a tube
into a neutral solution of BTB. As carbon
dioxide is absorbed from breath into the
solution, forming carbonic acid, the solution
changes color from green to yellow.
 Bromothymol
of 7.1.
 We
blue is a pH indicator with a pKa
want to determine this experimentally.
 The
pH at which an indicator changes color
depends on its pKa.
 An
indicator is chosen such that the pKa of
the indicator is that of the equivalence point
pH.
 The
color change takes place over a pH range
of the pKa ± 1.
HIn
yellow
low pH
In-
+
blue
high pH
H+
In  H 
Ka = HIn
 
pH = pKa + log
In 
HIn
pH = pKa when
In 
HIn
=1
We graph:
y = mx + b;
log
In 
HIn
Where
= m(pH) + b
In 
HIn
= 1, log
b
pH = pKa = m
In 
HIn
=0
 Prepare
8 solutions using the indicated
concentration of BTB and buffer solutions A
through H.
 Measure
the absorbance of each solution.
 Measure
the pH of each solution.
 Make
up a spreadsheet and graph.
 Determine
the pKa of BTB.

Reagents:



Ingestion:


May act as a sensitizer. Inflammation, pain, burns,
scarring, and redness
Inhalation:


Vomiting, lethargy, diarrhea, cardiac effects, CNS
effects, respiratory effects, tingling in extremities,
irritation of the GI tract, and burns
Skin Contact:


Potassium Phosphate / Sodium Phosphate
Bromothymol blue
Respiratory irritation, coughing, shortness of breath,
bronchitis, pneumonitis, and pulmonary edema
Eye Contact:

Irritation, redness, pain, blindness, and corneal
damage
Dispose of waste in the appropriate waste
receptacles.


Acid and basic solutions / waste need to be
disposed in the acid/base waste container in the
fume hood.
Solutions with a pH between 6 and 8 can be
disposed down the drain.
 Lab
8 is next.