Labs 6 & 7
Diffusion and Osmosis
Diffusion
• dialysis tubing filled with
0.15mg/ml KMnO4
• tubing placed into a beaker
of water
• one beaker kept at into
room temp water, the other
placed into ice cold water
bath
Diffusion
0.06
0.05
Absorbance (545nm)
• KMnO4 particles should
diffuse out of the dialysis
bag and into the surround
beaker water
• take a sample of water from
each beaker at defined
times and measure its
absorbance at Abs 545nm
0.04
0.03
Room temperature
0.02
ice cold
0.01
0
0
5
10
20
40
60
Time (minutes)
– detecting the presence of
KMnO4 particles
• because you did a standard
curve – you could plot this
as concentration vs. time
• diffusion should be higher
at room temperature vs.
cold
15
bath temp
room temp
Time (minutes)
ice cold
Abs 545
0
0
0
5
0.025
0.01
10
0.045
0.015
15
0.048
0.024
20
0.041
0.034
40
0.039
0.036
60
0.034
0.035
KMnO4 Standard Curve and Unknown
Sample # Abs545nm
1
2
3
4
5
6
7
8
9
10
Column3
Column4
Column5
slope = 0.284 – 0.044
0.0156-0.002
slope = 17.554
molar absorptivity = 17.554
[KMnO4]
mg/ml
0.992
0.5
0.284
0.13
0.075
0.044
slope
unknown
1
0.5
0.25
0.125
0.0625
0.03125
0.015625
0.0078125
0.00390625
0.001953125
KMnO4 Standard Curve
1.6
1.4
0.108 unknown
17.55412522
0.108
Absorbance 545nm
Column1 Column2
1mg/ml
KMnO4
1.2
1
0.8
0.6
0.4
0.2
0
concentration
0.006152444 mg/ml
KMnO4 concentration (mg/ml)
Diffusion
KMnO4 concentration (mg/ml)
• you have already calculated
the molar absorptivity for
KMnO4 (see spec lab)
• but you did a standard
curve and you could recalculate it
• for each absorbance value
– divide by the molar
absorptivity to get the
concentration
• the units are the same as
the standard – which was
mg/ml
• plotting concentration vs.
time looks just like
absorbance vs. time
0.003
0.0025
0.002
0.0015
room temperature
0.001
ice cold
0.0005
0
0
5
10
15
20
40
60
Time (minutes)
Time (minutes)
0
5
10
15
20
40
60
room temp ice cold
mg/ml
0
0
0.00142418
0.00057
0.00256352 0.000855
0.00273442 0.001367
0.00233565 0.001937
0.00222172 0.002051
0.00193688 0.001994
Diffusion
• place a crystal of KMnO4 in a petri dish of water
and measure its spread through the water over
time
– diffusion results in the increasing diameter of the
KMnO4 “cloud” in the water
• place a drop of NaOH in an agar dish and
measure its diffusion through the agar
– agar is a colloid and will slow the diffusion of NaOH vs.
water
• same concept at the dialysis tubing study
– high concentration to low concentration
Osmometer
• based on saturated sucrose solutions
• submerged in pure water – i.e. hypotonic
solution (high [water], low [solute])
– low osmotic pressure
– water will move from high [water] to low
[water]
– volume of the sucrose solution increases
– sucrose solution moves up the tube
• SO: water movement into the osmometer
pushes the sucrose solution up
– you measured distance over time as a way of
measuring osmotic pressure
• many osmometers are attached to pressure
transducers that measure the pressure
physically “pushing” the sucrose solution up
Osmometer
• measured the distance the red
sucrose solution travelled up
the pipette and plotted it
versus time
• the slope of the graph is
essentially the osmotic
pressure of the water (or
sucrose solution)
– i.e. the driving pressure that
causes water to move
Potatoes and Turgidity
• immersed pieces of potato in increasing molarities of sucrose
– 0.0M sucrose = pure water
– up to 1.0M sucrose
• each sucrose solution has a defined number of sucrose
particles and free water molecules
– the higher the concentration – the higher the osmotic pressure
of the solution
• the osmotic pressure of the potato is defined also
– it has to be compared to the OP of the surrounding sucrose
solution in order to figure out which way water will flow
– hypotonic solutions – water flows out of solution into cells
– hypertonic solutions – water flows into solutions from the cells
• increased water movement into the potato slices will
increase its weight
Potatoes and Turgidity
Change in Mass Over Time @ Various Sucrose Concentrations
– i.e. where the line crosses
the 0 axis
0.4M Sucrose
0.6M Sucrose
0.8M Sucrose
1.0M Sucrose
1.3
1.1
Potato Mass (g)
0.9
0.7
0.5
0.3
0.1
-0.1 0
5
10
15
20
25
30
-0.3
-0.5
Time (min.)
Total Percent Change in Mass vs. Sucrose Concentration
40
y = -43.459x + 26.688
R² = 0.9479
30
% Change
• graphing the weight
change vs. time can show
you the rate of osmosis
• graphing the final weight
change vs. sucrose
molarity tells you what
[concen.] is isotonic
0.0M Sucrose
20
10
0
0.0
0.2
0.4
0.6
-10
-20
[Sucrose] (M)
0.8
1.0
1.2
• go to this website and try the self quizzes at
the ends of the exercises:
http://www.phschool.com/science/biology_
place/labbench/lab1/intro.html