Equilibrium Constant, Keq
Objective : Calculate Keq of Bromothymol Blue using a spectrophotometer and pH meter.
Use the “LAB : HOW TO…” link from the class website if you need help with how to use balance,
Bunsen burner… and such.
INTRODUCTION
You may write this information in your lab notebook for your own reference. It can’t be cut and
pasted.
Indicators are substances whose solutions change color due to changes in pH. They are usually weak
acids or bases, but their conjugate base and acid forms have different colors due to differences in their
absorption spectra. Indicators are typically weak acids or bases with complicated structures. For
simplicity, we represent a general indicator by the formula HIn- ,and its ionization in a solution by the
equilibrium,
HIn- (aq) + H2O (l)  H3O+ (aq) + In2- (aq),
and define the equilibrium constant as Kai,
[H3O+][ In2-]
Keq = ---------[HIn-]
In this experiment we will determine the equilibrium constant (Keq) for the indicator Bromothymol blue
using a spectrophotometer and a pH meter. Keep in mind that Bromothymol blue is blue when in the
basic form (In2-) and yellow when in the acidic form (HIn-).
At some wavelengths bromothymol blue will absorb light intensely while at others it will be nearly
completely transparent. Our goal is to tune the instrument to the wavelength that will give us the best
signal. This will be accomplished by testing our calibration solutions and selecting the wavelength of
maximum absorbance for both the basic blue form and acidic yellow form. By using strongly acidic or
basic solutions, we can shift the equilibrium nearly completely toward the basic or acidic forms of the
indicator.
There are two common methods by which to measure the interaction of light with a sample; %
transmittance (amount of light to pass through the sample) or Absorbance (amount of light absorbed by
the sample). The below equation shows you the relationship between % transmittance and absorbance .
 %trans 
A   log 

 100 
Beer’s law states that absorbance (A) is directly proportional to concentration in molarity. Let’s simplify
Beer’s law to the below equation where n is a constant (a combination of the distance the light travels
through the solution and how intensely the chemical absorbs light) and c is molarity.
A  nc
The value of n varies from one chemical to another so we will actually have two different values of n in
this lab. The value of n for HIn- shall be referred to as ny and n for In2- shall be referred to as nb where the
subscripts refer to the colors of those species in solution. Measuring n will allows us to determine the
relationship between concentration and absorbance. Remember that each n will only be accurate at the
wavelength determined in Part 1.
A  nb [ In2-]
A  n y [HIn-]
___________________CUT HERE _______________________
Procedure :
Part 1 – Tuning the Instrument Wavelength
(1) Prepare 6 calibration solutions in medium test tubes using 1 and 5 ml graduated pipettes.
Calibration Solution
0.00025M Bromothymol Blue (mL)
0.010 M HCl (mL)
C1
1.00
4.00
C2
0.70
4.30
C3
0.50
4.50
0.00025M Bromothymol Blue (mL)
0.010 M NaOH (mL)
C4
1.00
4.00
C5
0.70
4.30
C6
0.50
4.50
(2) Prepare 5 standard solutions in medium test tubes using 1 and 5 ml graduated pipettes.
Solution
0.00025M
Bromothymol blue (mL)
0.10M
K2HPO4 (mL)
0.10M
KH2PO4 (mL)
-----------------------------------------------------------------------------------------------------------------------------------------------------------
S7
S8
S9
S10
S11


1.00
1.00
1.00
1.00
1.00
4.00
3.00
2.00
1.00
0.00
0.00
1.00
2.00
3.00
4.00
Using LeChatelier’s principle and HIn-+ H2O = H3O+ + In2-, show the ‘shift’ when you have the BTB in
acidic solutions like C1 to C3.
Using LeChatelier’s principle and HIn-+ H2O = H3O+ + In2-, show the ‘shift’ when you have the BTB in
basic solutions like C4 to C6.
(3) Obtain a spectrometer (SpectroVis) and computer. Turn on the computer and attach the
spectrometer via USB cable.
(4) In order to begin, click on the Logger Pro 3.7 icon on the desktop, once the software is up and
running it is necessary to calibrate the Spectrophotometer
(5) To calibrate the SpectroVis, choose Calibrate
Spectometer:1 from the Experiment menu.
(6) Fill a cuvette about ¾ full with distilled water and place it in the sample chamber of the SpectroVis.
(7) Follow the instructions in the dialog box to complete the calibration, and then click OK After
calibrating the spectrophotometer you will be ready to collect data.
(8) Fill your cuvette about ¾ full with your solution C1 and place in the sample chamber of the
SpectroVis. Click
to begin data collection.
(9) Click
to end data collection. Note the wavelength at which the absorbance is the highest.
This is reported to spot (1) in the data table.
(10) Fill your cuvette about ¾ full with your solution C4 and place in the sample chamber of the
SpectroVis.
(11) Click
.
(12) Click
to end data collection. . Note the wavelength at which the absorbance is the highest.
This is reported to spot (2) in the data table.
(13) Click on the Configure Spectrometer Data Collection button ( .)
(14) Selecting Abs vs. Concentration as the collection mode. Select BOTH wavelengths of maximum
absorbance HIn- and In2-determined in Part 1.
(15) Use the data table given here and manually write down TWO absorbances for each of C1-C6 and
S7-S11 solutions. You may not need all the values but it is better to have more data than missing
information.
DATA TABLE
Total [BTB]
max for
HIn- =
[BTB] in
acidic form =
[HIn-1]
max for
In2- =
(1)_____nm
(2)_____nm
Absorbance
at this 
Absorbance
at this 
[BTB] in
basic form
= [In-2]
C1
0.000050 M
_______
0.000050 M
optional
0
C2
0.000035 M
_______
0.000035 M
optional
0
C3
0.000025 M
_______
0.000025 M
optional
0
C4
0.000050 M
optional
0
_______
0.000050 M
C5
0.000035 M
optional
0
_______
0.000035 M
C6
0.000025 M
optional
0
_______
0.000025 M
S7
_______
_______
pH =_______
S8
_______
_______
pH =_______
S9
_______
_______
pH =_______
S10
_______
_______
pH =_______
S11
_______
_______
pH =_______
(16) Use the pH meter to record the pH of solutions S7 – S11. You may need to transfer the solution to a
large test tube in order to immerse the pH probe.
___________________CUT HERE _______________________
Prelab Questions :
(PL1) . 0.00025M Bromothymol Blue solution is diluted when it is mixed with HCl or NaOH. Calculate the
concentration of BTB after it is mixed with HCl or NaOH for C1-C6. Use M1V1=M2V2.
(PL2) For C1 –C3, the lab is assuming that 100% of BTB are in acidic form, leaving 0% of BTB in basic
form. What is the concentration of acidic form of BTB in C1 to C3?
(PL3) For C4 –C6, the lab is assuming that 100% of BTB are in basic form, leaving 0% of BTB in acidic
form. What is the concentration of basic form of BTB in C4 to C5?
(PL4) Calculate the concentration of bromothymol blue after K2HPO4 and KH2PO4 are added for
solutions, S7 – S11? Use M1V1=M2V2.
(PL5) For solution S7, if 30% of BTB are in acidic form, what is the concentration of acidic form of BTB in
S7? What is the concentration of BTB basic form?
+1
(PL6) If the solution reads pH 7.66, calculate the [H ] eq ?
Analysis :
A1) Use Excel to graph a calibration curve for A vs. [acidic form of bromothymol blue using C1 – C3.
Clearly label everything.
A2) Use Excel to graph a calibration curve for A vs. [basic form of bromothymol blue using C4 – C6.
Clearly label everything.
A3) Calculate (or measure) ny and nb. These are the slopes for each graph.
A4) Calculate the Keq using S7. Show your work clearly.
A5) Calculate the Keq using S8 to S11. You do not have to show your work if you are doing the same
thing as (A4).
A6) Calculate the average Keq.
Summary :
Present all data and calculated values neatly and creatively in data table(s).
Balanced chemical equation:
Keq expression:
Average Keq = _________________
Graphs – site the page numbers.
Abstract:
No more than 100 words stating why the lab was done, what was done, the major result(s), and
conclusion(s).
Reflection Statement:
One thing I might do differently next time is …… because…….