NOU Bromocresol Green Equilibrium Systems Lab Report
IneedhelpwithLabReport
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elow.
Question#1.Inyourownwords,summarizethegoalsofthisexperiment.
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A
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Question#2.Summarizetheprocedureusedtopreparethe
standard
solution
s;thatis,whatsolutions(identitiesand
concentration
s)andvolumeswere
combined
to
prepare
the
solutions?
Calculation
#1.Calculate
the
concentration
of
HIn
in
Standard
Solution
A.
Show
your
work
and
include
units.
Calculation
#2.Calculate
the
concentration
of
In–
in
Standard
Solution
B.
Show
work
and
label
units.
Question
#3.Complete
Report
Table
1
below.
Report
Table
1.
Chemical
Species
Present
and
Their
Concentrations
for
Standard
Solutions
Aand
B
standard
solution
species
in
(HIn
or
In−)
concentration
(M)
A
B
Question
#4.Record
your
visualobservations of the test tubes in Report Table 2 below. Note the colors of the solutions, and the different degrees of color intensity.
Report Table 2.Volumes of Reagents and Observed Colors for Sample Solutions 1-5 and an Unknown Solution of Bromocresol Green
| solution |
Sample 1
Sample 2
Sample 3
Sample 4
Sample 5
Unknown
Question #5.Use the colors of the solutions to predict which of the Sample Solutions 1-5 will have a pH closest to that of the
solution. Explain your answer.
Question #6.Complete Report Table 3 below.
|
442 |
|
617 |
| Unknown |
Question #8.How do the predicted (by color) and measured pH of the unknown solution compare?
DATA ANALYSIS AND RESULTS
Calculate Molar Absorptivities of HIn and In– at 442 nm and 617 nm
Calculation #3.Use the concentration and absorbance data for Standard Solution
to calculate the molar absorptivity of HIn at 442 nm and at 617 nm (i.e., ε(HIn)442 and ε(HIn)617). Enter your results in Report Table 4 below. Show a sample calculation here.
Calculation #4.Use the concentration and absorbance data for Standard Solution
to calculate the molar absorptivity of In‒ at 423 nm and at 617 nm (i.e., ε(In‒)442 and ε(In‒)617). Enter your results in Report Table 4 below. Show a sample calculation here.
Question #9.Complete Report Table 4 below. Refer to Report Tables 1 and 3,and Calculations #3 and #4 above.
Report Table 4. Concentrations, Absorbances, and Molar Absorptivities for Standard Solutions
and B
|
species in solution (HIn or In‒) |
concentration (M) | A442 | A617 |
Calculate Concentrations in Sample Solutions 1-5 and in Unknown Solution
Use the absorbance data for mixtures of HIn and In– to set up your
equations
to solve for the concentrations of HIn and In– in each sample solution and the unknown solution. (Review the EXPERIMENTAL DESIGN section for information about how to solve two equations and two unknowns.) Remember that the total absorbance at each wavelength is the sum of the absorbance of HIn and the absorbance of In–. Enter your equations in Question #10(Report Table 5) below.
Question #10.Complete Report Table 5 below. Note: substitute your numerical values for A and Ɛ; the path length of 1.00 cm can be omitted from the equations.
Report Table 5. Systems of Equations for Sample Solutions 1-5 and Unknown Solution
| Sample 4 | Sample 5 |
You must show all your work (including units) for [HIn] and [In−] in Sample Solution 1 in Calculation #5.
You may use Wolfram-Alpha or other calculator to do the remaining calculations and/or to check your work.
- https://www.wolframalpha.com/
- Set up your 2 equations and enter them using the following example format: 85x + 12y = 0.623, 6x + 106y = 0.485 (i.e., 2 equations with 2 variables separated by a comma).
- Click the “=” button.
- You may search “solve a system of linear equations” for more information.
Enter your results for [HIn] and [In−] in Sample Solutions 1-5 and the Unknown in Question #11/Report Table 6.
Calculation #5.Sample calculations of [HIn] and [In–] for Sample Solution #1.(Show all work and include units.)
Calculate Hydrogen Ion Concentration
Calculate the concentration of hydrogen ion, [H+] in those tubes where you measured the pH. Enter your calculation results in Question #11 (Report Table 6 below). Show work for a sample calculation in Calculation #6.
Note that the numerical result of a logarithmic function should have the same number of significant figures as the mantissa (i.e., the number to the right of the decimal point in a logarithm). For example, a pH of 1.58 has two significant figures (two digits to the right of the decimal point), and the H+ ion concentration would be expressed as 2.6 x 10‒2 M (two significant figures).
Calculation #6.Sample calculation of [H+] for Sample Solution #1.(Show all work and include units.)
Calculate Values of K
Calculate the values of the equilibrium constant, K, for Sample Solutions 1–5 and the Unknown Solution. Show a sample calculation of K for Sample Solution 1 in Calculation #7. Enter results for the remaining solutions in Question 11(Report Table 6).
Calculation #7.Sample calculation of the value of the equilibrium constant, K, in Sample Solution #1.(Show all work.)
Question #11.Enter your results from your calculations of [HIn], [In–], and [H+] in Report Table 6 below.
Calculation #8.Calculate your average equilibrium constant (Kave) and the standard deviation of your K values (by using the function =stdev.p(range of K values)in Excel). “The standard deviation is a measure of how widely values are dispersed from the average value (the mean).” Values of K can be considered constant if each K value differs from the average by less than 2 times the standard deviation.
DISCUSSION AND ANALYSIS
Question #12.Explain whether your experimental data support the hypothesis that the value of the equilibrium constant is a constant at a given temperature. If not, identify possible sources of error in the procedure that might account for the variation. Justify your claims using evidence (i.e., your data).
Question #13.Explain why it was necessary to measure the absorbance of each solution at two different wavelengths.
Question #14.Suppose the measurements for Sample Solutions 1-5 had each been done at a different temperature. Do you have enough information to be able to predict how the value (magnitude) of the equilibrium constant would have been affected, if at all, in each case? If not, describe what additional information you would need to make such predictions.
Non-tech_Lab 5: Bromocresol Green Equilibrium Systems
(CHM 11600 – Spring 2021)
Lab 4 (25 pts.) consists of the following:
1. A pre-lab quiz (10 pts)
2. A Lab Report (15 pts)
Record the names of your group members.
PRELAB PRACTICE QUESTIONS
As part of your individual preparation for lab, read the experiment and answer the following
questions. (Your answers will not be collected or graded.)
You will take a quiz on Brightspace related to these concepts.
Review “Appendix D: Spectroscopy: An Introduction” at Brightspace > CHM 11600-Merge
> Labs > Reference Materials for information about absorption spectra, calibration plots,
and use of the spectrophotometer.
1. Write the chemical equation and the equilibrium constant expression for the reaction you
will study in this experiment. Define any abbreviations you use. Describe what method(s)
you will use to determine the concentrations of each of the species in the equilibrium
constant expression.
2. Do you expect the value of the equilibrium constant, K, to be constant for the equilibrium
mixtures you will prepare in this experiment? Why or why not?
3. Define, in words, the following terms that appear in equations (5) and (6) below. For
example, “ε(HIn)423 is the molar absorptivity of bromocresol green at a wavelength of 423
nm.”)
• A423
• ε(HIn)423
• ε(In‒)423
• [HIn]
• [In‒]
• A617
• ε(HIn)617
• ε(In‒)617
1
4. When you prepare the HIn
by combining 25.00 mL of 0.10 M HCl and 5.00
mL of the NaIn (bromocresol green sodium salt) stock solution, the only species in solution
is essentially HIn. Explain why you can assume that this solution contains only HIn and no
In‒.
5. Calculate the concentration of In‒ in Standard
if the concentration of the NaIn stock
solution is 3.6 x 10‒4 M.
6. Write the expression for the Beer-Lambert Law and define the variables it contains.
7. Calculate the molar absorptivity of iron thiocyanate ([Fe(SCN)6]3‒) in a solution that is 1.56 x
10‒5 M [Fe(SCN)6]3‒ given that the solution has an absorbance of 0.142 at a wavelength of
480 nm.
8. Calculate the value of the equilibrium constant, K, for the equilibrium system H++ In‒ ⇌ HIn
given the data below.
[HIn] = 1.68 x 10‒5 M
[In‒] = 1.42 x 10‒5 M
pH = 4.580
9. Solve the following system of algebraic equations, which contain two variables. Show all of
your work.
4x + 2y = 8
5x + 3y = 9
10. A Chemistry 11600 student was charged with the task of determining the concentrations of
FD&C Blue #1 dye and FD&C Red #40 dye in a sample of artificially-colored grape drink. She
used a spectrophotometric procedure like the one described in this lab and obtained the
following data.
dye
molar absorptivity at 500 nm
molar absorptivity at 580 nm
FD&C Red #40
22,000 M‒1 cm‒1
250 M‒1 cm‒1
FD&C Blue #1
150 M‒1 cm‒1
41,300 M‒1 cm‒1
sample
absorbance at 500 nm
absorbance 580 nm
grape drink
0.600
0.400
2
Calculate the concentration (in M) of each dye in the sample of grape drink. Use the method of
two equations and two unknowns, and the equations below.
A500 = εred (500)· [red] + εblue(500) ·[blue]
A580 = εred(580)· [red] + εblue(580)· [blue]
11. Read the procedure below and write an experimental procedure that you would use in lab.
The procedure should be an abbreviated form of the instructions, such as a list of steps or
an outline. (Note: the prelab quiz will include questions about the procedure.)
INTRODUCTION
The purpose of this experiment is to determine whether an equilibrium constant is truly
constant. We will examine mixtures of the indicator bromocresol green, the bromocresol
green anion, and hydrogen ion (H+) at equilibrium with different concentrations to see if each
gives the same value for the equilibrium constant.
To calculate the equilibrium constant, we need to determine the concentrations of each of
these species. Bromocresol green and the bromocresol green anion are colored, so we will
measure their concentrations with a spectrophotometer. We will measure the concentration
of hydrogen ion (H+) with a pH electrode.
In addition, we will see if we can determine the pH of an unknown solution by adding
bromocresol green and then measuring the absorbance of the sample.
LAB LEARNING OBJECTIVES
Overall Objectives
•
•
Use spectrophotometry and pH measurements to determine the equilibrium constant, K,
for a system.
Determine if the equilibrium constant, K, is truly constant at a certain temperature.
Skills/Techniques/Knowledge
•
•
•
•
•
Calculate the concentration of a standard solution prepared by simple dilution of a stock
solution of known concentration.
Determine the molar absorptivity, ε, for a compound at a specific wavelength by using
spectrophotometry.
Calculate the molar concentration of H+ from pH measurements.
Use a system of two equations with two unknowns to calculate the concentrations of
reactant and product in an equilibrium system.
Identify potential sources of error in the procedure.
3
The Bromocresol Green Equilibrium System
Bromocresol green is a common acid-base indicator. In aqueous solution, an equilibrium
between a mixture of bromocresol green (which we will abbreviate HIn (i.e., protonated
Indicator)), its anion (In–; deprotonated Indicator), and hydrogen ion (H+) is established as
rapidly as the mixture can be stirred, which is represented by the chemical equation below.
The expression for the equilibrium constant is shown below using the abbreviations HIn and In –
in place of the complex chemical formulas.
𝐻 + + 𝐼𝑛− ⇌ 𝐻𝐼𝑛
𝐾=
[𝐻𝐼𝑛]
[𝐻 + ][𝐼𝑛− ]
We can follow or observe this reaction visually. For instance, when a colorless solution of H+
(acid) is added to a blue solution of In–, the color of the solution changes to yellow. The color
change is directly related to a change in the H+ concentration (or pH of the solution).
As we vary the concentration of H+, and thus the pH, a shift in the equilibrium will occur and we
will observe a change in the color that is related to the relative concentrations of HIn and In – in
the solution. We can determine the concentrations of each of these species by using a
spectrophotometer. Using the (absorbance) data obtained from the spectrophotometer, we
can calculate the concentrations of both HIn and In– in the equilibrium mixture.
In addition, we can use pH measurements made in the lab and the expressions below to
calculate the concentration of H+.
𝑝𝐻 = − log[𝐻 + ]
𝑜𝑟
[𝐻 + ] = 10−𝑝𝐻
Once we have calculated the concentrations of H+, HIn, and In– in various equilibrium mixtures,
we can calculate the value of the equilibrium constant, K, and answer the question, “Is the
4
value of the equilibrium constant, K, truly constant for systems where the equilibrium
concentrations are different?”
EXPERIMENTAL DESIGN
Determination of Molar Absorptivities for HIn and In– From Standard Solutions
We will not use a calibration plot (standard curve) for this experiment; instead, we will
determine molar absorptivities from absorbance measurements of standard solutions of HIn
and In–.
Recall that the concentration of a species is related to its absorbance at a particular wavelength
by the following equation (Beer-Lambert Law):
𝑎𝑏𝑠𝑜𝑟𝑏𝑎𝑛𝑐𝑒 = (𝑚𝑜𝑙𝑎𝑟 𝑎𝑏𝑠𝑜𝑟𝑝𝑡𝑖𝑣𝑖𝑡𝑦) 𝑥 (𝑝𝑎𝑡ℎ 𝑙𝑒𝑛𝑔𝑡ℎ) 𝑥 (𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛)
𝐴=𝜖𝑥𝑙𝑥𝑐
𝑚𝑜𝑙𝑎𝑟 𝑎𝑏𝑠𝑜𝑟𝑝𝑡𝑖𝑣𝑖𝑡𝑦 =
𝑎𝑏𝑠𝑜𝑟𝑏𝑎𝑛𝑐𝑒
𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 𝑥 𝑝𝑎𝑡ℎ 𝑙𝑒𝑛𝑔𝑡ℎ
𝑝𝑎𝑡ℎ 𝑙𝑒𝑛𝑔𝑡ℎ = 1 𝑐𝑚
𝜖=
𝐴
𝑐
You will prepare two standard solutions (A and B) from a stock solution of bromocresol green
indicator. The pH of the standard solutions will be adjusted using either acid (to increase the
concentration of H+) or base (to decrease the concentration of H+) to shift the equilibrium either
to bromocresol green (HIn) or to the anion of bromocresol green (In‒).
𝐻 + (𝑎𝑞) + 𝐼𝑛− (𝑎𝑞) ⇌ 𝐻𝐼𝑛 (𝑎𝑞)
•
If you add excess acid (HCl) to the system above, initially at equilibrium, the concentration of
In‒ will decrease and the concentration of HIn will increase (i.e., the equilibrium “shifts to the
right”). If the amount of HCl added is sufficiently large, essentially all of the In‒ will be
converted to HIn. (Standard Solution A)
•
If you add excess base (NH3) to the system above, initially at equilibrium, the concentration
of In‒ will increase and the concentration of HIn will decrease (i.e., the equilibrium “shifts to
the left”). If the amount of NH3 added is sufficiently large, essentially all of the HIn will be
converted to In‒. (Standard Solution B)
5
Therefore,
A will contain only bromocresol green indicator in its protonated
form (HIn), and Standard Solution B will contain only the deprotonated form of bromocresol
green (In‒), as summarized in the table below.
standard solution
chemical species in solution
A
acidic
HIn
B
basic
In‒
You will measure the absorbances of Standard Solutions A and B at two different wavelengths,
and then you will use the concentration of each Standard Solution to calculate four values of
molar absorptivity, ε, as listed in the table below. You will use these values to calculate the
concentrations of HIn and In‒ in equilibrium mixtures that you will prepare.
standard solution
λ of absorbance measurement
use absorbance at λ to calculate…
A
442 nm
ε(HIn)442
(molar absorptivity of HIn at 442 nm)
A
617 nm
ε(HIn)617
(molar absorptivity of HIn at 617 nm)
B
442 nm
ε(In‒)442
(molar absorptivity of In‒ at 442 nm)
B
617 nm
ε(In‒)617
(molar absorptivity of In‒ at 617 nm)
Review “Appendix D: Spectroscopy: An Introduction” at Brightspace > CHM 11600-Merge
> Labs > Reference Materials for information about absorption spectra, calibration plots,
and use of the spectrophotometer.
6
Determination of the Concentrations of HIn and In‒ in Samples
In this experiment, you will prepare sample mixtures of HIn and In‒ by adjusting the pH of a
solution of bromocresol green indicator. Solutions of HIn and In– have strong absorbances at
different wavelengths, as shown in Figure 1 below.
Figure 1 Absorption spectra for HIn and In‒.
You will utilize the absorption spectrum differences to determine the concentrations of HIn and
In– in samples at various pH. You will measure the absorbance of each sample solution at two
different wavelengths (442 nm and 617 nm) and use the molar absorptivity values to calculate
the concentration of each species (HIn and In–) in the sample. These calculations involve
solving two equations with two unknowns; the unknowns are the concentrations of HIn and In–.
The total absorbance at 442 nm, A442, is due to the combined absorbances of HIn and In – at
this wavelength.
A442 = A(HIn)442 + A(In‒)442
(1)
The absorbance at 442 nm due to HIn, A(HIn)442, is equal to the product of the molar
absorptivity of HIn at 442 nm, ε(HIn)442, the path length of the cuvette, 1.00 cm, and the
concentration of HIn, [HIn].
A(HIn)442 = ε(HIn)442 x 1.00 cm x [HIn]
The absorbance at 442 nm due to In–, A(In–)442, is equal to the product of the molar absorptivity
of In– at 442, ε(In–)442, the path length of the cuvette, 1.00 cm, and the concentration of In –,
[In–].
7
A(In–)442 = ε(In–)442 x 1.00 cm x [In–]
Thus, we can write:
A423 = ε(HIn)442 x 1.00 cm x [HIn] + ε(In‒)442 x 1.00 cm x [In‒]
(2)
Similarly, the total absorbance at 617 nm, is due to the combined absorbances of HIn and In – at
this wavelength.
A617 = A(HIn)617 + A(In-)617
(3)
Where
A(HIn)617 = ε(HIn)617 x 1.00 cm x [HIn]
And
A(In-)617 = ε(In-)617 x 1.00 cm x [In-]
Thus, we can write:
A617 = ε(HIn)617 x 1.00 cm x [HIn] + ε(In-)617 x 1.00 cm x [In-]
(4)
Now we have a system of two equations with two unknowns ([HIn] and [In–]) that we can solve
for the concentrations. (Note: the 1.00 cm path length has been omitted for clarity.)
A442 = ε(HIn)442 x [HIn] + ε(In‒)442 x [In‒]
A617 = ε(HIn)617 x [HIn] + ε(In‒)617 x [In‒]
(5)
(6)
8
Determination of the Concentration of H+ in Samples
A pH electrode will be used to measure the hydrogen ion concentration. Remember that pH is
related to hydrogen ion (e.g., symbolized as either [H+] or [H3O+]) concentration as follows: pH
= –log[H+]
OVERVIEW
A summary of the action steps and measurements you will perform in this experiment is shown
in the table below.
Prepare/use
Measure
Standard solutions
A and B
Samples 1- 5
Absorbances at 442
nm and 617 nm
Absorbances at 442
nm and 617 nm
pH
Samples 1 – 5 and
Unknown
Use measured data to
determine…
ε(HIn)442, ε(HIn)617,
ε(In-)442, ε(In-)617
… using
[HIn] and [In-] in samples
1 – 5 and unknown
[H+]
System of two equations
and two unknowns
pH = -log [H+]
ε = A/c
Calculating the Value of the Equilibrium Constant
After you have calculated the values of [HIn], [In‒], and [H+] for each of the samples and the
unknown, you can calculate the value of the equilibrium constant for each of the mixtures.
𝐾=
[𝐻𝐼𝑛]
[𝐻 + ][𝐼𝑛− ]
SAFETY
•
Wear your goggles at all times in the laboratory.
9
PROCEDURE
To complete the experiment in the allotted time, group members can divide the following
tasks and complete them simultaneously:
•
•
•
•
•
•
Prepare Standard Solutions A and B
Prepare Solution U (unknown pH)
Prepare Sample Solutions 1-5
Set up the SpectroVis Plus spectrophotometer
Set up and calibrate the pH electrode
Measure room temperature using a temperature probe attached to the LabQuest 2 unit
Equipment Used
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
pH probe (1); front benchtop (in plastic pitcher)
50-mL buret (2); lower cabinet
plastic cuvette (1); lower cabinet
25-mL volumetric pipet (2); lower cabinet
5-mL volumetric pipet (1); lower cabinet
Parafilm 1″ square (8); side benchtop
100-mL beaker (4); student drawer
250-mL beaker (3); student drawer
ring stand (2); table
buret clamp (1); table drawer
pipet bulb (1); table drawer
utility clamp (1); table drawer
wood support block (4); table drawer
8-inch test tube (8); upper cabinet
LabQuest 2 (1); Vernier equipment box (lower cabinet)
power adapter (1); Vernier equipment box (lower cabinet)
SpectroVis spectrophotometer (1); Vernier equipment box (lower cabinet)
temperature probe (1); Vernier equipment box (lower cabinet)
Reagents Used – see instructions below for dispensing
•
•
•
•
•
•
50 mL 0.10 M HCl
50 mL 0.10 M NH3
50 mL stock solution of bromocresol green (provided as the sodium salt, NaIn)
100 mL 0.10 M CH3COOH
100 mL 0.10 M CH3COONa
50 mL solution of unknown pH
10
Preparing Solutions
Each group will prepare eight solutions: 2 standard solutions, 5 sample solutions and 1
unknown solution. Each individual in the group should prepare at least one of the solutions.
When you finish preparing solutions you will have test tubes of sample solutions 1 – 5 with
varying pH values as well as two standard solutions (A and B) and a solution of unknown
concentration (U).
Obtain eight clean and dry, large test tubes or ignition tubes; label them A, B, U, and
numerically for the five sample solutions that your group will prepare. Place them in test tube
racks.
Dispense the indicated amounts of the “Reagents Used” (see above) into beakers, and take
them back to your lab bench. Use 250-mL beakers for the 0.10 M CH3COOH and 0.10 M
CH3COONa solutions, and 100-mL beakers for the other solutions.
Review “Volumetric Measurement Techniques” at Brightspace > CHM 11600-Merge >
Labs > Reference Materials.
Use the NaIn stock solution you obtained to prepare standard solutions A and B, samples 1 – 5
and the unknown solution, as described below. Never stick a pipet into a reagent bottle or
pour unused reagent back into the bottle, as this can result in contamination of the reagent.
Standard Solution A (HIn)
Pipet 25.00 mL of 0.10 M HCl and 5.00 mL of the stock solution of bromocresol green (NaIn)
into Test Tube A (acidic). Record the concentration of the stock solution of bromocresol green
(NaIn).
Note: the concentration of HCl in this test tube is sufficiently large that essentially all of the
In– is converted to HIn.
Standard Solution B (In–)
Pipet 25.00 mL of 0.10 M NH3 and 5.00 mL of the stock solution of bromocresol green (NaIn)
into Test Tube B (basic).
Note: the concentration of the base, NH3, in this test tube is sufficiently large (and the H+
concentration is sufficiently small) that essentially none of the In– is converted to HIn.
11
Solution U (unknown pH)
Pipet 25.00 mL of a solution of unknown pH and 5.00 mL of the stock solution of bromocresol
green (NaIn) into Test Tube U. Record the ID number of the unknown, if one is given on the
bottle.
Sample Solutions 1 – 5 (Containing Mixtures of HIn and In–)
Prepare Sample Solutions 1 – 5 using the information given in Table 1 below by following the
steps listed below the table. Follow the detailed instructions give below the table.
Table 1. Volumes of Reagents Used to Prepare Sample Solutions 1 – 5.
sample
solution
volume of 0.10 M
CH3COOH (mL)
volume of 0.10 M
CH3COONa (mL)
volume of NaIn stock
solution (mL)
1
21.00
4.00
5.00
2
17.00
8.00
5.00
3
13.00
12.00
5.00
4
9.00
16.00
5.00
5
5.00
20.00
5.00
•
•
•
•
•
•
Review “Volumetric Measurement Techniques at Brightspace > CHM 11600-Merge >
Labs > Reference Materials for detailed information on how to rinse, fill and use
burets.
Clean two burets and put them in buret holders on ring stands.
Fill one buret with the acetic acid solution, and one with the sodium acetate solution.
Using the burets, add the appropriate volumes of acetic acid solution and sodium acetate
solution (Table 1) to each labeled test tube.
Add 5.00 mL of the NaIn stock solution to each test tube using a volumetric pipet.
When all solutions have been prepared, cover the tubes with Parafilm and invert to mix
the contents.
Observations
Obtain the temperature of the laboratory room by using the temperature probe attached to
the Lab Quest 2.
12
Line up the test tubes containing Sample Solutions 1 – 5 that your group has prepared in
numerical order. Note the colors of the solutions and the different degrees of color intensity.
Examine the test tubes visually and record your observations. Then, examine the test tube
containing the unknown solution and record its color too.
DATA COLLECTION
Overview of Absorbance Measurements
1. Set the spectrophotometer to zero absorbance with a blank solution (DI water).
2. Set the spectrophotometer to make the absorbance measurements at the specified
wavelengths (i.e., 442 nm and 617 nm).
3. Record absorbance values for Standard Solutions A and B, Sample Solutions 1 – 5, and the
Unknown Solution at the specified wavelengths
1. Connect the SpectroVis Plus spectrophotometer to the computer.
•
•
•
Restart the computer.
Start the Logger Pro application by double-clicking on the Logger Pro icon on the
desktop.
Connect the SpectroVis Plus spectrophotometer to the computer with the USB cable.
LabQuest 2 Troubleshooting Tips
Issue:
Battery will not charge or hold power.
Solution:
Batteries are not supplied for the LabQuest 2. You must use the power
supply and connect the LabQuest 2 to an electrical power outlet. Note:
The LabQuest 2 does not receive power through the USB connection.
Issue:
Screen is not responsive to taps or acts as if you tapped the wrong
location.
Solution:
Press and hold the Home button (not the icon) for 5 seconds or until the
calibration screen appears.
13
SpectroVis Plus / Logger Pro Troubleshooting Tips
Issue:
Logger Pro is not recognizing the LabQuest 2 or sensor or is just
generally not responding.
Solution:
Close all programs, disconnect the LabQuest 2 and restart computer.
Force the log off of all previous users. Then, log back in, connect the
LabQuest 2 and start Logger Pro again.
If this does not work, try connecting to a different USB port on the
computer.
Issue:
Logger Pro does not recognize the SpectroVis Plus spectrophotometer.
Solution:
Close Logger Pro. Disconnect the SpectroVis Plus spectrophotometer.
It is important that you perform the following steps in order.
1. Start Logger Pro.
2. Connect the SpectroVis Plus to the computer with the USB cable.
Issue
A wooden block is stuck in the SpectroVis Plus sample holder.
Solution:
Find the two friction clips inside the sample holder that must be pushed
in to release the wooden block. Place a cuvette in the
spectrophotometer and push the cuvette against the clips (they are on
the lamp side of the opening). While pushing the clips in, turn the
spectrophotometer over and push the block out of the holder with a
pen or pencil.
Issue:
Logger Pro does not respond, or unexpectedly quits.
Solution:
Do not open multiple windows or instances of Logger Pro.
14
2. Set the SpectroVis Plus spectrophotometer to zero.
Before the absorption measurements can be made, it is necessary to obtain an absorption
spectrum for a blank solution (a solution with no bromocresol green; in this case DI water).
A blank corrects for absorbance by the cuvette, solvent, impurities, the optics of the
instrument, etc. The absorption spectrum of the blank is stored internally, and when the
absorbance of a sample (such as bromocresol green) is subsequently measured, the
absorption of the blank solution will be automatically subtracted from the absorbance of
the sample solution for each wavelength. This approach is sometimes referred to as a
“background correction”.
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This experiment uses plastic, disposable cuvettes.
The blank solution for this experiment is DI water.
Rinse the cuvette with DI water. Discard the rinse.
Fill the cuvette about 3/4 full with DI water.
Wipe the outside of the cuvette with a Kimwipe.
From the Experiment menu, choose Calibrate > Spectrometer: 1.
The calibration dialog box will display the message: “Waiting 90 seconds for lamp to
warm up.”
When warmup is complete, place the cuvette with the blank in the sample chamber of
the SpectroVis Plus.
Click Finish Calibration.
Click OK.
3. Set the spectrophotometer to make absorbance measurements at the specified
wavelengths (442 nm and 617 nm).
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•
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Click on Configure Spectrometer.
Select Absorbance vs. Concentration as the collection mode.
Use the drop-down menu to change Single 10nm Band to Individual Wavelengths.
Click the Clear Selection button to deselect any wavelengths.
Click on the boxes for 442 nm and 617 nm (or as close as the spectrophotometer will
allow).
Click OK.
4. Record absorbance values for Standard Solutions A and B, Sample Solutions 1 – 5, and the
Unknown Solution at the specified wavelengths (442 nm & 617 nm).
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For all the solutions (A, B, 1 – 5, and U), do the following:
Rinse the cuvette with the solution.
Fill the cuvette about 3/4 full with the solution.
Wipe the cuvette with a Kimwipe.
Insert the cuvette into the spectrophotometer.
15
•
Wait 10 seconds and record the absorbance values displayed in the boxes in the
lower left hand corner for both wavelengths.
pH Measurements
You must first calibrate the pH electrode using buffer solutions of known pH. The buffers
remain at a stable pH so they can be used to calibrate a pH electrode.
Care and Handling of pH Electrodes
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•
•
Keep the electrode wet at all times. When you are not using the electrode, it should be in
the tube containing storage solution.
The electrode has a thin membrane at the bottom that is easily broken. When you move
the electrode from one solution to another, be very careful of the tip of the electrode.
Never dry an electrode by rubbing it with a paper towel. Just “dab” water droplets with a
lint-free paper such as a Kimwipe.
You will find the pH electrode in a tube of storage solution next to the computer. The pH probe
should be connected to the port labeled CH1 on the LabQuest 2 interface. Turn on the
LabQuest 2 by pressing the power button.
At the top of the display, click Sensors > Calibrate > CH1: pH. This will open a new window
called Sensor Settings. Click Calibrate Now.
Remove the pH electrode from the storage solution. Rinse the electrode with DI water (use a
small beaker to collect the rinses), blot it dry with a Kimwipe and gently put the electrode into
the pH 4 buffer solution.
Swirl the jar gently for about 15 seconds. In the box for Known Value 1: enter “4” and click
Keep to store the calibration point.
Remove the electrode from the pH 4 buffer, rinse it with DI water and blot dry. Place the
electrode in the pH 7 buffer solution. Swirl the jar gently for about 15 seconds. Enter “7” in the
box for Known Value 2: and click Keep. Click OK.
Remove the pH probe from the pH 7 buffer, rinse, and place the pH probe in DI water or
storage solution. Your pH electrode is now calibrated and ready to measure the pH of
solutions.
Measure the pH value for Sample Solutions 1 – 5 and the Unknown Solution and record the
results. Rinse the probe after each measurement.
16
WASTE DISPOSAL AND CLEANUP
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All solutions (except for the pH buffers) can be poured down the drain with (tap) water.
Used plastic cuvettes should be rinsed with water and disposed in the regular trash.
Disconnect the pH electrode, rinse it with DI water, and place the electrode in the tube of
storage solution.
Return the LabQuest 2 instrument box to your Graduate Instructor, after you have checked
that all of the parts are present.
Rinse all glassware at least three times with DI water and return to your Graduate Instructor
or appropriate location.
DATA ANALYSIS, RESULTS, DISCUSSION AND ANALYSIS
Answer the questions and complete the calculations found on the Lab 5 Report
Form. The assignment is in the Labs module on Brightspace. Make sure you use
the version of the report form for on campus students.
When you have completed your report, upload it to Brightspace in the Lab 5
portal. It is due Mon. Mar. 1 by 11:59 PM ET.
17
Lab 5: Bromocresol Green Equilibrium Systems
Report Form – for on-campus students
Due Mon. Mar. 1 at 11:59 PM. Upload MS Word or .pdf file to Brightspace.
Group Member Names:
Question #1.In your own words, summarize the goals of this experiment.
DATA
Question #2. Summarize the procedure used to prepare the standard solutions; that is, what
solutions (identities and concentrations) and volumes were combined to prepare the solutions?
Calculation #1. Calculate the concentration of HIn in Standard Solution A.
Show your work and include units.
Page 1 of 9
Calculation #2. Calculate the concentration of In– in Standard Solution B.
Show work and label units.
Question #3. Complete Report Table 1 below.
Report Table 1. Chemical Species Present and Their Concentrations for Standard Solutions A
and B
standard
solution
species in solution
(HIn or In−)
concentration
(M)
A
B
Question #4. Record your visual observations of the test tubes in Report Table 2 below. Note
the colors of the solutions, and the different degrees of color intensity.
Report Table 2. Volumes of Reagents and Observed Colors for Sample Solutions 1-5 and an
Unknown Solution of Bromocresol Green
solution
CH3COOH CH3COONa
(mL)
(mL)
bromocresol
green stock
solution (NaIn)
(mL)
Sample 1
Sample 2
Sample 3
Sample 4
Sample 5
Unknown
Page 2 of 9
observed color
of solution
Question #5. Use the colors of the solutions to predict which of the Sample Solutions 1-5 will
have a pH closest to that of the Unknown solution. Explain your answer.
Question #6. Complete Report Table 3 below.
Report Table 3. Absorbance and pH data
solution
absorbance at 442 nm
(A442)
absorbance at 617 nm
(A617)
pH
Standard A
—
Standard B
—
Sample 1
Sample 2
Sample 3
Sample 4
Sample 5
Unknown
Question #7. Which Sample Solution (1 – 5) had a pH closest to the measured pH of the
Unknown Solution?
Question #8. How do the predicted (by color) and measured pH of the unknown solution
compare?
Page 3 of 9
DATA ANALYSIS AND RESULTS
Calculate Molar Absorptivities of HIn and In– at 442 nm and 617 nm
Calculation #3. Use the concentration and absorbance data for Standard Solution A to
calculate the molar absorptivity of HIn at 442 nm and at 617 nm (i.e., ε(HIn)442 and ε(HIn)617).
Enter your results in Report Table 4 below. Show a sample calculation here.
Calculation #4. Use the concentration and absorbance data for Standard Solution B to
calculate the molar absorptivity of In‒ at 423 nm and at 617 nm (i.e., ε(In‒)442 and ε(In‒)617).
Enter your results in Report Table 4 below. Show a sample calculation here.
Question #9. Complete Report Table 4 below. Refer to Report Tables 1 and 3, and
Calculations #3 and #4 above.
Report Table 4. Concentrations, Absorbances, and Molar Absorptivities for Standard Solutions
A and B
Standard
Solution
species in
solution
(HIn or In‒)
concentration
(M)
A442
A
B
Page 4 of 9
A617
𝜺442
𝜺617
Calculate Concentrations in Sample Solutions 1-5 and in Unknown Solution
Use the absorbance data for mixtures of HIn and In– to set up your equations to solve for the
concentrations of HIn and In– in each sample solution and the unknown solution. (Review the
EXPERIMENTAL DESIGN section for information about how to solve two equations and two
unknowns.) Remember that the total absorbance at each wavelength is the sum of the
absorbance of HIn and the absorbance of In–. Enter your equations in Question #10 (Report
Table 5) below.
Question #10. Complete Report Table 5 below. Note: substitute your numerical values for A
and Ɛ; the path length of 1.00 cm can be omitted from the equations.
Report Table 5. Systems of Equations for Sample Solutions 1-5 and Unknown Solution
solution
equations
general form
A442 = ε(HIn)442 x [HIn] + ε(In‒)442 x [In‒]
A617 = ε(HIn)617 x [HIn] + ε(In‒)617 x [In‒]
Sample 1
Sample 2
Sample 3
Sample 4
Sample 5
Unknown
Page 5 of 9
You must show all your work (including units) for [HIn] and [In−] in Sample Solution 1 in
Calculation #5.
You may use Wolfram-Alpha or other calculator to do the remaining calculations and/or to
check your work.
•
https://www.wolframalpha.com/
•
Set up your 2 equations and enter them using the following example format: 85x + 12y =
0.623, 6x + 106y = 0.485 (i.e., 2 equations with 2 variables separated by a comma).
•
Click the “=” button.
•
You may search “solve a system of linear equations” for more information.
Enter your results for [HIn] and [In−] in Sample Solutions 1-5 and the Unknown in Question
#11/Report Table 6.
Calculation #5. Sample calculations of [HIn] and [In–] for Sample Solution #1. (Show all work
and include units.)
Page 6 of 9
Calculate Hydrogen Ion Concentration
Calculate the concentration of hydrogen ion, [H+] in those tubes where you measured the pH.
Enter your calculation results in Question #11 (Report Table 6 below). Show work for a
sample calculation in Calculation #6.
Note that the numerical result of a logarithmic function should have the same number of
significant figures as the mantissa (i.e., the number to the right of the decimal point in a
logarithm). For example, a pH of 1.58 has two significant figures (two digits to the right of the
decimal point), and the H+ ion concentration would be expressed as 2.6 x 10‒2 M (two
significant figures).
Calculation #6. Sample calculation of [H+] for Sample Solution #1. (Show all work and
include units.)
Calculate Values of K
Calculate the values of the equilibrium constant, K, for Sample Solutions 1–5 and the Unknown
Solution. Show a sample calculation of K for Sample Solution 1 in Calculation #7. Enter
results for the remaining solutions in Question 11 (Report Table 6).
Calculation #7. Sample calculation of the value of the equilibrium constant, K, in Sample
Solution #1. (Show all work.)
Page 7 of 9
Question #11. Enter your results from your calculations of [HIn], [In–], and [H+] in Report
Table 6 below.
Report Table 6. Concentrations and Values for the Equilibrium Constant for Sample
Solutions 1-5 and Unknown Solution
solution
[HIn]
[In–]
[H+]
K (at
°C)
Sample 1
Sample 2
Sample 3
Sample 4
Sample 5
Unknown
Calculation #8. Calculate your average equilibrium constant (Kave) and the standard deviation
of your K values (by using the function =stdev.p(range of K values) in Excel). “The standard
deviation is a measure of how widely values are dispersed from the average value (the mean).”
Values of K can be considered constant if each K value differs from the average by less than 2
times the standard deviation.
Average K = __________ at __________ °C
Std. Dev. = _______________
Page 8 of 9
DISCUSSION AND ANALYSIS
Question #12. Explain whether your experimental data support the hypothesis that the value
of the equilibrium constant is a constant at a given temperature. If not, identify possible
sources of error in the procedure that might account for the variation. Justify your claims using
evidence (i.e., your data).
Question #13. Explain why it was necessary to measure the absorbance of each solution at
two different wavelengths.
Question #14. Suppose the measurements for Sample Solutions 1-5 had each been done at a
different temperature. Do you have enough information to be able to predict how the value
(magnitude) of the equilibrium constant would have been affected, if at all, in each case? If not,
describe what additional information you would need to make such predictions.
Upload your completed report in MS Word or .pdf format to the Lab 5
assignment portal in Brightspace. It is due Mon. Mar. 1 by 11:59 PM.
Page 9 of 9
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Report Table 3. Absorbance and pH data
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16
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