SECTION 5: DIFFUSION EXPERIMENT

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DIFFUSION EXPERIMENT
Ruth Logan, 2003
The purposes of this experiment are to:
1. Establish a measurement system and apply it in a consistent manner.
2. Study the effect of three variables (time, temperature, and molecular weight) on
the rate of diffusion.
3. Transform data in order to study several dimensions of the same information.
4. Practice writing the following parts of a lab report:
a. Titles and captions for figures.
b. Discussion sections that contain both error and graph analysis.
c. A conclusion that incorporates information from three different graphs
into a single concept.
Experiment 1: Diffusion of KmnO4 into water.
Each student will construct a diffusion system consisting of a beaker of water containing a
permeable “cell” of KMn04. Different students should use different temperatures of water.
Consult with students near you so that at least three different temperatures of water are
used.
1. Use a china marker to label a 600 mL beaker with a symbol that is meaningful to
you. Then add 400 mL of tap water in ONE of the following set-ups and allow it
to come into temperature equilibrium with its environment:
a. Use cold tap water and place the beaker into ice in an ice bucket on your
lab bench.
b. Use hot tap water and place it in the hot water bath provided on a side
counter in the lab.
c. Use room temperature water and place it on your lab bench with no
container.
2. Turn on and calibrate the spectrophotometer nearest you. Make sure it is set at
550 nm. Keep a cuvette of tap water handy to recalibrate it from time to time
throughout the experiment.
3. Prepare a permeable “cell” of dialysis tubing as follows: twist and double back
one end of a length of cellulose tubing and tie it very tightly with string. Place 4
mL of the 10 g/L KMn04 provided into the bag, and tie off the open end in a
similar manner.
4. Record the temperature of the water in your beaker. Then, at a time that you
identify as being t = 0, drop the bag into your beaker. Record the time in a table
you have prepared and count all time from this moment.
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5. Take samples from your beaker in the following manner:
a. About 10 seconds before taking your sample, use a disposable plastic
pipet to carefully stir the water in the beaker WITHOUT touching the bag.
b. At the designated time, use the pipet to quickly transfer some of the
outer liquid from the beaker into a cuvette. Be sure the cuvette is at least
half full so it will encounter the light beam in the spectrophotometer.
c. Since time doesn’t particularly matter now, slow down. Use the pipet to
knock any bubbles out of your sample. Wipe the outside of the cuvette,
and then place it into the spectrophotometer chamber with the label facing
you. It is best if you use the same cuvette and the same manner of
placing it into the chamber throughout the experiment.
d. Measure the absorbance of the liquid to the correct number of significant
digits. (3 up to abs of about 0.6; then 2 from 0.6 to 1.5). Record the data
in a table similar to the one below:
Time (minutes)
Absorbance 550nm
e. Pour the sample back into the beaker from which you took it. This is
necessary to keep the total volume constant throughout the experiment.
f. Repeat taking samples at various times throughout the next hour and a
half. Suggested times for sampling are 1, 3, 5, 7, 10, 15, 30, 45, 60 and
75 minutes after placing the bag in the water.
6. Prepare a graph showing the absorbance of KMn0 4 in the outer solution versus
time for your sample. This is Figure 1. (Notice that it would be possible to
convert the absorbance measurements to concentration of KmnO4 using the
standard curve you produced in the first lab at the beginning of the semester.
However, since absorbance and concentration are directly proportional, using the
absorbance without conversion gives the same relative information).
Part B of Experiment 1: The effect of time on Rate of Diffusion.
Measure the slope of the line you produced for Figure 1 over several 2-minute timeperiods chosen from throughout the time span of the experiment. Produce at least eight
data points: six from the first 15 minutes and two from the later parts of the experiment.
Record these in a table similar to the one below, and use them to produce Figure 2.
Time (minutes)
[Use (t1 + t2)/2]
Rate of Diffusion
(Δ abs/2 minutes)
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Logan, 2003
Part C of Experiment 1: The effect of Temperature on the Rate of Diffusion.
Measure the slope of the line you produced for Figure 1 over the time period from 2 to 5
minutes. Report this datum together with the initial temperature of your sample on the
board and obtain similar data from the other students in the lab. Record the data points in
a table similar to the one shown below and use them to produce Figure 3.
Temperature (oC)
Rate of Diffusion
(Δ abs/3 minutes)
Experiment 2: Diffusion of dyes into a colloidal medium.
Your instructor will place 20 drops of each of the following substances (one per tube) on
the surface of the starch colloid contained in some large test tubes.
Substance (all are 0.1 M)
Potassium dichromate
Malachite green
Methylene blue
Gentian violet
Eosin Y
Congo Red
Molecular Weight (Daltons)
294
364
374
484
691
695
At the end of 48 hours, (or some other period of time as directed by your instructor)
measure the distance each dye has diffused through the colloid to the nearest millimeter.
Prepare a graph (Fig. 4) of distance diffused versus molecular weight.
DIFFUSION LAB REPORT
This second lab report is intended to build on skills you practiced in lab report one and to
add some new ones. You will write more statements that obey the rules for an hypothesis
when you formulate separate conclusions for the four graphs you prepare. You will
continue to analyze error, but by estimating its magnitude rather than calculating the
standard deviation. (The proper procedure is to repeat data until you have a reproducible,
statistically significant result, but we don’t have time to do this on every experiment.) You
will learn to manipulate data in order to obtain additional information from it. You will
again analyze all graph shapes to seek the underlying natural mechanism that can be
deduced from it. And you will inductively draw a composite conclusion from three related
graphs in order to understand their relationship to each other.
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Logan, 2003
The issue of this lab report is: What are the forces that control the rate of diffusion.
Figures 2, 3, and 4 all show data concerning the rate and will provide the principle
material for the discussion. (Figure 1 provides raw data and doesn’t directly represent the
rate of diffusion. It has a smaller role in discussion).
The report will consist of a cover page plus Results, Discussion, and Conclusion sections
(and Literature Cited if you cite any). The Results section will consist of the four figures,
each with complete title and caption, plus the error analysis for each graph (CAN you
accept this line shape?). The Discussion section will discuss Figures 2, 3 & 4 separately
and draw an individual conclusion (within the discussion) about the relationship that it
shows (mechanism, WHY is the line this shape?). Finally you will prepare a single
Conclusion section that presents a composite statement about the forces that control
diffusion rate (the subject of Figures 2, 3, and 4).
General information about the preparation of the three parts is presented below.
A. Results:
Each graph must have a suitable (yet concise) caption below the X-axis. The captions,
as always, include a title that describes the central concept of the figure or table,
followed by text that includes a brief description of the data shown, the source of the
data, its purpose, and any relationship it may have with other tables or figures in the
report. It must contain only objective information; no discussion or conclusions are
allowed in a caption.
Each graph must have a text error analysis section for the purpose of determining that
the interpretation you have made of the graphed data (the line you have drawn) is
appropriate for the data.
1.
2.
3.
4.
5.
Describe the appearance of the data and propose a line shape for it.
Describe the pattern of the data points. Are most on the line? Describe the
positions and magnitude of distance from the line (this can involve several
data points collectively or a single data point, depending on whether your
discussion can be applied to a collection of points.)
Propose possible sources for the variability in the data patterns that you see.
Remember that these ought to be variations that you would have gotten had
you repeated the data collection many times.
ANALYZE your proposals by comparing the data to the source: do they
match in pattern? In magnitude (distance from the line)? Do you have
separate observations or written records that support or conflict with your
proposal?
Use the above reasoning to decide whether to accept the line shape you
proposed. If you have accounted for the position of the points as error, you
may accept your interpretation of the data (your line). Otherwise you must
reinterpret the data since error analysis would have shown that the position
of these points was NOT probably due to error.
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Logan, 2003
6.
Conclude this section by stating the line shape and relationship that you are
accepting (use the form of an hypothesis statement).
B. Discussion
Discuss the meaning of the three rate graphs: this is the central task of a scientific report.
Introduce each section by briefly describing the figure related to it, the shape of the curve,
and a general sense of the information content. (Remember, these have already been
covered in the Results section. Don’t repeat anything unnecessarily).
Analyze each graph by relating the shape of the curve to an underlying mechanism that
might govern the phenomenon being studied. For instance, say you discover in Results
that the relationship between rate of diffusion and time is an inverse proportion (that is, the
shape of the curve is an accepted fact). You must discuss factors involved in diffusion
that might cause this curve shape to be true.
Consider such factors as the molecular movement of molecules, their kinetic energy, the
relative concentrations of KmnO4 and water in different parts of the system, the nature of
any barriers that might be present. (Diffusion is a physical process, so you don’t have to
consider chemical reactions that might take place).
Propose how some of these or other factors might be involved in the process you
measured. Many elements of these discussions involve INFERRANCES since they
involve factors or characteristics not directly measured in the experiment. If you cite
references (you are not expected to) do so properly. Show how you reason through and
TEST each idea by discussing how parts of the curve match or fail to match your
proposals. State a specific conclusion at the end of each section.
C. Conclusion
The Conclusion section for this report is an overall conclusion making a conceptual
statement about factors that control the rate of diffusion. You must take into account the
results and your discussion about the last three figures in the report (Figures 3, 4, and 5).
Remember that the issue is to discover how rate of diffusion is controlled, and use
inductive reasoning to combine the ideas from all three graphs to develop a coordinated
concept. Consider the factors that you thought might affect the rate in each individual
case to see how and whether these same factors are present in the other two cases.
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