Number 356
Spec 20 part 3: Ka of bromocresol green
Michael Jansen <mjansen@crescentschool.org>
Crescent School, Willowdale ON M2L 1A2
[The third lab activity in Michael Jansen’s series using the Spec 20.]
Many acid-base indicators, bromocresol green included, are themselves weak acids. This lab determines
the Ka of bromocresol green with the data collection taking less than an hour. (continued on page 7)
Spec 20 with lab materials
1
pH 0.0
pH 4.0
pH 5.0
pH 14.0
Absorbance of Bromocresol Green at Different pH Values
0.9
0.8
0.7
Absorbance.
April 2008
pH 14.0
0.6
0.5
0.4
pH 5.0 Buffer Solution
0.3
pH 4.0 Buffer Solution
0.2
0.1
pH 0.0
0
380
400
420
440
460
480
500
520
540
560
580
600
620
640
660
680
700
720
Wavelength (nm)
Sample of graph based on student data
INSIDE THIS ISSUE:
• Spec 20 part 3: Ka of bromocresol green, pages 1 and 7-10
• Quinic acid in fruit juices, pages 4-6
• For new teachers: Composition of a
mechanical mixture, pages 11-12
• A DNA crostic, pages 14-15
• What is maple syrup? page 18
(continued from page 1)
Since the stock solution of bromocresol green is known only to
one significant figure, 0.04% (m/v), we carry out the experiment
using droplet quantities of bromocresol green and a graduated
cylinder; no volumetric flasks or pipets are needed. This makes
for a fast lab. To one significant figure, the determined Ka is
remarkably accurate. Don’t be surprised if you need to spend
time going over how to do the calculations. Students enjoy the
challenge of this experiment.
The photocopy-ready lab starts on the next page and is
complete with pre-and post-lab questions. I begin with a halfhour class discussion, explaining what the experiment is about
and how the data will be collected. Part of this discussion
focuses on the post-lab questions. The pre-lab questions are
intentionally imbedded in the background and procedure, to
encourage students to read this section. Students do the prelab questions for homework; we take them up before beginning
the experiment. This ensures that students know what they are
doing and why they are doing it. Students need to have this
handout with them while they perform the experiment; they will
also need it to carry out the calculations.
Page 10 is a sample of student collected data (Sidney Chow,
class of 2007). Teachers may use these data if a “dry lab” is
desired. If this activity is run as a dry lab, students need to know
that 12 drops of the bromocresol green stock solution were
made up to 10 mL of solution for each of the four pH values
tested. Approximately 20 drops of the 0.04% (m/v) stock
solution of bromocresol green equals 1 mL.
Many high schools have one or more Spectronic 20s (Spec 20),
visible spectrophotometers developed by Bausch & Lomb in
1954. These simple instruments require manual setting of the
wavelength. Readings are displayed with either a moving
needle (analog) or, for newer models, digitally. The unit is
rugged, accurate, and easy to use. Our September 2007 issue
has a “get acquainted” lab for the Spec 20. Teachers may also
consult www.chemistry.nmsu.edu/Instrumentation/Spectronic_20.html for a complete set of operating instructions.
A-
[BG] =
0.4 g L-1
698 g mol
-1
×
(12 / 20) mL
= 3.4 × 10 −5 mol L-1
10 mL
5.
λmax = 440 for pH = 0.0; λmax = 620 nm for pH 14.0
6.
At 620 nm, only A- absorbs. At 440 nm, both species
absorb, so we choose 620 nm.
7.
A- has an Absorbance of 0.762 in the pH 14.0 spectrum at
620 nm, and [A-] = 3.4 x 10-5 mol/L
ε = 1.8 x 104 L mol-1 cm-1 for A- at 620 nm.
8.
With the pH 5.0 buffer, Absorbance = 0.495 at 620 nm; then,
[A − ] =
0.495
4
1.8 × 10 L mol -1 cm -1 × 1.25 cm
= 2.2 × 10 −5 mol L-1
[HA] = 3.4 x 10-5 - 2.2 x 10-5 = 1.2 x 10-5 mol L-1, and
[H3O+] = 1.0 x 10-5 mol L-1
[H3 O + ][ A − ]
1.0 × 10 −5 × 2.2 × 10 −5
=
= 1.8 × 10 −5
[HA]
(3.4 × 10 −5 − 2.2 × 10 −5 )
pKa = 4.7
Ka =
9.
In the pH 4.0 buffer, Absorbance = 0.144, so
[A-] = 0.8 x 10-5 mol L-1, [HA] = 2.6 x 10-5 mol L-1, and
[H3O+] = 1.0 x 10-4 mol L-1, and
Ka = 3.1 x 10-5, and pKa = 4.5.
10. The average pKa is 4.6, to one significant figure, in good
agreement with the value of 4.7 in the Merck Index.
Editor’s Note:
Lew Brubacher points out that one can analyze data at any
wavelength, even if both species absorb. Without presenting
the proof, he notes that it’s possible to evaluate the ratio of
[A-]/[HA], as follows. In this example the data is from 440 nm,
using the pH 4.0 buffer.
[A − ]
(absorbance at pH 0.0) - (absorbance at pH 4.0)
=
[HA ] (absorbance at pH 4.0 ) - (absorbance at pH 14.0)
0.395 - 0.361
=
= 0.1197
0.361 - 0.077
K a = [H 3 O + ] ×
[A − ]
= 1.0 × 10 − 4 × 0.1197
[HA ]
It should also be noted that if both HA and absorb at a given
wavelength, the calculations to determine concentrations will be
complicated. Students are led to use a maximum wavelength
where only one species (HA or A-) absorbs.
pKa = 4.92
Selected answers (based on student data table on page 10.)
Alternatively, using data at 560 nm, for example, we get
Pre-lab
Q2. Assumption: Temperature remains constant
Molar mass of bromocresol green: 698 g/mol
Q3. pKa = -log Ka = 4.7;
Q4. 5.7 x 10-4 mol/L
+
4
Q6. [H3O ] = 1.0 x 10 mol/L in a buffer solution of pH 4.0
Q9. HA predominates; [HA] = 3.4 x 10-5 mol/L in HCl solution
Q10. A- predominates; [A-] = 3.4 x 10-5 mol/L in NaOH solution
Post-lab assignment
4. In the cuvette the bromocresol green (BG) concentration is
= 1.197 × 10 −5
[ A − ] 0.018 - 0.093
=
= 0.2192 ,
[HA] 0.093 - 0.435
and pKa = 4.66
These are both close to the accepted value of 4.7.
This procedure does not require one to know the concentration
of bromocresol green! So the limitation to just one significant
figure does not apply in this analysis.
Student activity on next page
⇒
April 2008/Chem 13 News 7
Determination of the Ka of bromocresol green
Answer pre- and post-lab questions and record data on a
separate page.
Background and procedure
Background information, pre-lab questions and the experimental
procedure are combined in this section.
Many acid-base indicators, bromocresol green included, are themselves weak acids. Since bromocresol green is a complex
molecule, we will represent it simply as HA, where A– represents
the anion. (Think about hydrochloric acid, HCl(aq). The anion is
Cl–.)
Weak acids ionize in aqueous solution:
HA(aq) + H2O(l) ⇄ H3O+(aq) + A-(aq)
eq 1
Notice that this is an equilibrium.
Q1.
determined with eq 4. Remember that regardless of pH, the
individual concentrations of each component, when substituted
into the Ka expression, will yield the same value.
Q2.
In this experiment we will determine, semi-quantitatively, the Ka
value for bromocresol green, using a Spec 20. We have a stock
solution of bromocresol green whose concentration is given as
0.04% (m/v).
NB You can look up the molar mass of bromocresol green in the
Merck Index under bromcresol green — note the spelling
difference. Answer Q3 while you’ve got the Merck Index open.
Q3.
= Kc = Ka =
There are a few equations that you will need to use with the Spec
20. The absorbance can be determined using the equation
Absorbance = –log(%T/100).
eq 3
Some Spec 20s have Absorbance as a reading along with %T
(transmittance).
Recall the Beer-Lambert Law
Absorbance = l•c•ε
Q4.
Calculate [bromocresol green], in mol/L, in the stock
solution (0.04% (m/v)).
Q5.
Why can we determine the Ka of bromocresol green only
semi-quantitatively?
We will prepare 4 different solutions of bromocresol green: in
1.0 mol/L HCl; 1.0 mol/L NaOH; pH 4.0 buffer; pH 5.0 buffer.
Each solution will be prepared by adding, with a glass medicine
dropper, 12 drops of the indicator to a 10.0 mL graduated
cylinder and filling to the 10.0 mL line with the appropriate
solution.
Q6.
a) What is a buffer solution? See your text.
b) What is the [H3O+] in a buffer solution of pH 4.0?
Q7.
How do you plan to determine the number of drops/mL
delivered by the glass medicine dropper?
eq 4
where l, the pathlength, is the distance that the light travels
through the solution, i.e., the diameter of the curvette. For the
Spec 20, l=1.25 cm, and is constant. The molar absorptivity
value for a species is represented by ε. It is a constant value for
each solute at a given wavelength.
In order to determine the concentration from the Beer-Lambert
Law, the molar absorptivity value ε will first need to be
determined for both HA and A-. This is why you will first
determine the absorbance spectrum of bromocresol green in a
1.0 mol/L HCl solution and then in 1.0 mol/L NaOH solution.
Keep in mind that when an acid-base indicator is added to an
aqueous solution, the [H3O+], or pH, of the solution will,
according to LeChâtelier’s Principle, determine which form of the
indicator, HA or A- , will predominate.
Once the ε has been calculated, the concentrations of the HA
and A- concentration in the buffered solutions can be
8 CHEM 13 NEWS/April 2008
b) What is the literature value of the pKa for bromocresol
green (Merck Index)?
eq 2
The molecular, or acidic, form of the indicator, HA, is a different
colour than its anionic form, A-. This colour difference will allow
you to determine the concentration of both species with the
Spectronic 20 (Spec 20). With the concentrations of HA and Aand eq 2, the Ka of bromocresol green can be determined at a
given pH.
The pKa of bromocresol green is given in the Merck
Index.
a) What relationship does the pKa bear to the Ka?
Write the Ka, or Kc, expression corresponding to eq 1.
Keq
What assumption must be made in order for this to be
strictly true?
Each solution will be transferred to a separate, labelled cuvette.
With a Spec 20, the %T profile of each of the four solutions,
from 400 nm to 700 nm — in 20 nm increments — will be taken.
Measure and record the %T of all four solutions at each
wavelength. Don’t forget to zero the machine at each new
wavelength.
Q8.
On a separate sheet of paper, prepare a data table with
spaces for λ and %T for each of the four solutions.
Alternatively, you can enter data directly into a
spreadsheet. Have at least one of these ready prior to
the experiment. You will also need a column for
Absorbance.
Q9.
In 1.0 mol/L HCl, what form of bromocresol green, HA or
A-, predominates? Explain briefly. What are the
concentrations of HA or A- in this solution? Remember to
take into account the dilution of the stock solution.
Q11. What equation can be used to determine the ε value of a
compound given its Absorbance at λmax?
Q10. In 1.0 mol/L NaOH, what form of bromocresol green, HA
or A-, predominates? Explain briefly. What are the
concentrations of HA or A- in this solution. Remember to
take into consideration the dilution.
From the Absorbance versus λ graphs of the indicator in
1.0 mol/L HCl and in 1.0 mol/L NaOH, you can determine the
value for the molar absorptivity, ε, of both the HA and A- forms
of the indicator at their respective λmax.
After you have collected the data
You will only use one λmax for your calculations to find the
concentrations of HA and A-. You will use the λmax that has
only one of the species (HA or A-) absorb. This will avoid
complicated calculations. With the concentrations and the pH,
you calculate the Ka for bromocresol green. Use the post-lab
questions to guide you through the calculations for Ka.
Plot a single graph of Absorbance (reading or calculate from
% T) vs wavelength for all four solutions. You may want to
colour-code each curve for easy reading.
Post-Lab Assignment: Spectrophotometric determination of the Ka
(Answer on a separate sheet)
*If this is done as a dry lab, use the data provided by the teacher. Remember that each of the four solutions was prepared by adding
12 drops of the stock bromocresol green solution to a graduated cylinder and making up the solution to a total volume of 10 mL with the
appropriate solution. (Note that 20 drops = 1 mL)
1.
After you have answered the questions below, write a suitable abstract for this experiment.
2.
Append your graphs (all on one set of axes). Include: descriptive title, labeled axes (with units where applicable), 4 labeled
“curves”, axes properly sized.
3.
See prelab assignment. [bromocresol green] in stock solution = ____________________ mol/L
4.
Calculate [bromocresol green] in the cuvette. Remember to take into account dilution of the stock solution and to convert drops to
mL, and ultimately to L.
[bromocresol green] in cuvette = ____________________ mol/L
5.
In order to calculate Ka, you need to find the concentrations of HA, A- and H3O+ in equilibrium. We can do this using Absorbance
data at a λmax setting. What is the λmax for each of the pH 0.0 and pH 14.0 spectra? Label these on your graph.
6.
If both the HA and the A- species absorb at a given λmax setting, it makes the calculations complicated. You want to choose a λmax
at which only one species absorbs, in order to simplify the calculations.
At the λmax for the pH 14.0 spectrum, does HA absorb? (Look at the pH 0.0 curve to tell.)
At the λmax for the pH 0.0 spectrum, does A- absorb? (Look at the pH 14.0 spectrum to tell.)
Select the λmax at which only one species absorbs. Which one is it?
7.
Which species absorbs at the λmax you have selected? What is the absorbance of this species at this wavelength? Use this value
to calculate the value of ε for this species, using your answer to pre-lab question 11 for this determination. The concentration you
need was determined in pre-lab question 9 or 10.
8.
For the pH 5.0 buffer, what is the absorbance of the absorbing species at your selected λmax? Using this value and the value of ε
you have just calculated, calculate the concentration of the absorbing species. Then using the fact that the total bromocresol green
concentration equals the sum, [HA] + [A-], calculate the concentration of the non-absorbing species at this pH. What is [H3O+] in
this pH 5.0 buffer? Use these concentrations to calculate Ka and pKa. Include a set of neat, well-organized calculations.
9.
Repeat step 8 with the pH 4.0 buffer using the absorbance at the same λmax.
10. Average the two pKa values. How does this average compare with the value found in the Merck Index? Comment.
April 2008/Chem 13 News 9
Determination of Ka bromocresol green: Data page
Sample student data
HCl
(yellow)
NaOH
(dark blue)
pH 4.0 buffer
(lime green)
pH 5.0 buffer
(light blue)
λ
%T
A
λ
%T
A
λ
%T
A
λ
%T
A
400
420
440
460
480
500
520
540
560
580
600
620
640
660
680
700
52.2
45
40.3
41
50.5
65
80.2
91
96
98.1
100
100
100
100
100
100
0.282
0.347
0.395
0.387
0.297
0.187
0.096
0.041
0.018
0.008
0
0
0
0
0
0
400
420
440
460
480
500
520
540
560
580
600
620
640
660
680
700
63.1
70.8
83.7
89.4
85.6
75.8
63.9
49.5
36.7
25
18.6
17.3
29.5
58
82.9
94.3
0.200
0.150
0.077
0.049
0.068
0.120
0.194
0.305
0.435
0.602
0.730
0.762
0.530
0.237
0.081
0.025
400
420
440
460
480
500
520
540
560
580
600
620
640
660
680
700
44.9
46.9
43.6
45.5
53.7
65.9
76.3
81.2
80.7
76.6
73.8
71.8
79
90.2
97.6
99
0.348
0.329
0.361
0.342
0.270
0.181
0.117
0.090
0.093
0.116
0.132
0.144
0.102
0.045
0.011
0.004
400
420
440
460
480
500
520
540
560
580
600
620
640
660
680
700
59.3
59.8
63.8
67
70.1
72
69.9
62.7
52.8
41.4
34.8
32
45.1
70.8
88.9
97.1
0.227
0.223
0.195
0.174
0.154
0.143
0.156
0.203
0.277
0.383
0.458
0.495
0.346
0.150
0.051
0.013
Double displacement
su-chem-du
Ba
Na
Hidden is this su-chem-du are the products for the
following chemical reaction:
Na2SO4 (aq) + BaCl2 (aq) →
(s)
Cl
To solve the puzzle, remember each of the nine
characters is to appear once in each row, column and
each 3x3 square. Once solved, circle the hidden
solution to the above equation. At the bottom of the
puzzle are all the numbers, states of matter and an
addition sign that you will need to use.
Cl2
SO4
2
+
Cl
(s)
Ba
+
We’ll draw the winner from the correct entries on May
10, 2008. Send your entry to Chem 13 News, Double
displacement su-chem-du, Department of Chemistry,
University of Waterloo, Waterloo ON N2L 3G1. Fax:
519-888-9168. E-mail: kjackson@uwaterloo.ca. ∎
Cl2
2
Na
10 CHEM 13 NEWS/April 2008
Cl
(aq)
Cl2
Ba
Ba
Na
Cl2
Na
SO4
SO4
(aq)
(s)
2
+