Emily Muller
Acid-Base Indicators: Spectrophotometric Methods Lab Report
Lab Partner: Tamara Afflerbach
7/22/12
Introduction:
Acids and bases can be transformed when their protons become dissociated in water.
Many times this change would go unnoticed if it weren’t for indicators. Indicators are used to
show a visible color change as the pH changes. The pH represents how acidic or how basic a
solution is. The indicator demonstrates the change in pH and therefore the change from more
acidic to more basic or vice versa.
The purpose of this experiment is to determine the equilibrium constant (K) for the
dissociation of the weak organic acid bromcresol green. In order to complete this experiment,
[H3O+] will be adjusted by changing the concentration of [HOAc] and [OAc]. Then the value for
the ration [B-]/[HB] can be found using the spectrophotometer. These two values can then be
combined to calculate Ka. This formula is Ka = [H3O+][B-] where [B-]
[HB]
and where [H3O+] = Ka [HOAc]
[HB]
= (Aobserved − AHB)
(AB- - Aobserved)
when Ka = 1.75 x 10−5.
[ OAc−]
To begin, the following chemicals will be needed: 1.000 M acetic acid (HOAc), 3.0 x 10-4 M
bromcresol green solution (BCG), 0.200 M sodium acetate solution (NaOAc), and 3 M
Hydrochloric acid (HCl) containing 1.5 x 10−5 M BCG. Add 25.00 mL HOAc solution to 100-ml
volumetric flask. Add 5.00 ml of BCG solution to this same flask. Dilute to the mark with
distilled water. This is Solution A. Mix 5.00 ml BCG solution and 5.00 ml NaOAc. Dilute to the
mark, this is Solution 1. Record the absorbance of Solution 1 versus a blank of distilled water.
Determine the wavelength of maximum absorbance and call it λB-. Pour sample back into the
flask for 100 ml of Solution 1. Pour Solution A into a clean, dry buret. Add 2.00 ml of Solution
A. This is Solution 2. Measure its absorbance at λB−. Put sample back to have 102.00 ml of
Solution 2. To Solution 2 add 2.00 ml of Solution A. This is Solution 3. Mix well and measure
the absorbance at all fourteen wavelengths used for Solution 1. Put sample back to have 104.00
ml of Solution 3. To Solution 3, add 2.00 ml of Solution A. This is Solution 4. Mix it well and
measure its absorbance at λB-. Repeat until 10 mL of Solution A has been added, taking
absorbance measurements after every 2 mL added. Then to 110.00 ml of Solution 6, add 1.00 ml
of 3 M HCl containing 1.5 x 10-5 M BCG. This is Solution 7. Measure the absorbance at all of
the wavelengths used for Solutions 1 and 3.
Data:
Solution 1
Solution 2
Solution 3
Wavelength Absorbance Wavelength Absorbance Wavelength Absorbance
400
0.118
400
0.134
400
0.155
425
0.07
425
0.15
425
0.214
450
0.023
450
0.139
450
0.252
475
0.03
475
0.118
475
0.187
500
0.062
500
0.09
500
0.109
525
0.121
525
0.081
525
0.044
550
0.208
550
0.102
550
0.013
575
0.325
575
0.158
575
0.001
600
0.487
600
0.223
600
-0.003
605
0.517
605
0.231
605
-0.002
610
0.541
610
0.242
610
-0.002
615
0.555
615
0.248
615
-0.003
620
0.549
620
0.234
620
-0.002
625
0.528
625
0.238
625
-0.003
Solution 1
0.6
Absorbance
0.5
0.4
0.3
Series1
0.2
0.1
0
0
200
400
Wavelength
600
800
Solution 3
0.3
Absorbance
0.25
0.2
0.15
Series1
0.1
0.05
0
0
100
200
300
400
500
600
700
Wavelength
Solution 7
0.3
0.25
Absorbance
0.2
0.15
Series1
0.1
0.05
0
-0.05
0
100
200
Average ka value: 2.65e-5
Percent Error: 26.8%
Standard deviation: 1.08e-5
300
400
Wavelength
500
600
700
Results and Discussion:
The purpose of this lab was to calculate a value for Ka. By following the procedure and
calculations such value was calculated. The average calculated value was 2.65e-5. The accepted
value found via the internet was 2.09e-5. This makes there a 26.8 % error in association with the
values for K. The standard deviation for the given set of data was 1.80e-5. This is a very small
number representing the numbers were very close. The relatively large percent error and the
small standard deviation demonstrates the procedure was held constant throughout the
experiment however there was a reason that caused the average calculated values of K to be off
the accepted value.
One possible source of error in lies in the fact that each solution is contingent upon the
previous reacting correctly. The reaction of protons can be different every time. This reaction
causes a different output absorbance. If this difference is multiplied by the 7 solutions created,
this can lead to a big difference in final values. Also, another possible source of error is the exact
concentration of the chemicals used from the beginning being different than they are labeled. For
example if [NaOAc] = 0.19 M instead of 0.20 M and [HoAc] = 1.01 M instead of 1.00 M then
the Ka value for Solution 2 would change from 1.73e-5 to 1.81e-5. This would affect the average
and the standard deviation value. However, assuming the numbers stay similar the new
calculated Ka value would still fall within the normal distribution of the standard deviation which
is from 6.25e-5 to 9.50e-6. Therefore, even slight differences in concentrations would not greatly
affect the final conclusion and representation of data. This leads to the conclusion the 26.8%
error may have come from the reaction of protons occurring in the chemicals. The reaction of
protons for this specific reaction may be very specific. This means many different factors could
affect the rate at which the protons move such as pressure, temperature, and orientation.
Using the values for [H3O+] the pH values were calculated. The range found was 3.764.46. Based upon the second listed internet source, bromcresol green should distinguish between
the pH of 3.8-5.4. The low end of the pH indication is very close. This is because the
concentration of [H3O+] in Solution 2 must be closer to the concentration in Solution 1 than the
concentration in Solution 6 is to Solution 7. The range 3.76-4.46 disregards the pH of Solution 1
and 7. This would represent the outliers of the pH range. The pH were B- and HB would be equal
is in between the values. This is where the solution is neither acidic nor basic. This value would
have to be included in the range because it is where the solution changes over. It cannot change
from acidic to basic or vice versa without passing this value.
http://www.chemicalforums.com/index.php?topic=21230.0
http://antoine.frostburg.edu/chem/senese/101/acidbase/indicators.shtml
Post-Lab:
1. What two properties (one chemical and one physical) combine to make bromcresol green
useful as an acidity indicator?
Bromcresol green is useful as an acidity indicator because it changes to yellow as the pH
becomes more acidic. Also, the bromcresol green protons are easily dissociated making
the color change visible and relatively quick.
2. What criteria are used to select a wavelength for measuring absorbances in these solutions?
Why?
The wavelength with the greatest absorbance is used because at the highest absorbance
the margin of error is the least. It is a constant which can be used throughout the whole
experiment for comparison usage and accurate graphs.