Chemistry 32A
Cover sheet for
OSMOSIS & DIALYSIS
Student name (please print)
____________________________________________
Lab section, day and time
____________________________________________
Date report submitted
____________________________________________
Provide your assessment of the experiment, some suggestions:




What did you learn from the experiment?
Were instructions clear and/or easy to follow, sufficient time for completion?
Highlights and/or lowlights?
Suggestions for improvement?
Background for the Experiment
1
In this experiment we study properties of a semipermeable membrane, called osmosis and dialysis. The
trnsport of a solvent such as water across the membrane is referred to as Osmosis, while size-selective
movement of molecules across the membrane is called dialysis. The membrane is a “molecular sieve”,
analogous to everyday examples such as a coffee filter or tea strainer. The filter screens out materials
larger than the size of the pores in the filter, letting the smaller size materials pass through.Your body
depends on such membranes in its cells and organs.
OSMOSIS results from the natural tendency towards mixing of fluids. If pure water is mixed with sea
water (about 3% salt), molecular motion mixes the two. If a membrane which passes only molecules of
water size is placed between a sample of sea water and pure water, the pure water will migrate towrds the
sea water side, diluting the salt water. The levels of water and sea water will change, with the height
difference reflecting what is termed “osmotic pressure”. At this point the mechanical pressure pushing
water from the salty side to the pur water side equals the pressure caused by concentration difference. If
enough pressure is applied to the salt water side, it will exceed the osmotic pressure, and water will flow
backwards towared the pure water side. This application is termed “reverse osmosis” and is a common
way to purify water for industrial use and to provide drinking in parts of the world where there is plenty of
sea water but little or no fresh water.
In our experiment we will fill one bag (which is a semipermeable membrane) with salt water, and another
with pure water. Each bag will then be placed (after weighing it) in a beaker filled with fluid of the
opposite type (e.g. salt water bag in pure water, pure water bag in salt water) and observe what happens. If
the bag (or your cells) have the same salt concentrtion as the surrounding fluid, there is no osmotic
movement since there is no concentration gradient or pressure difference. This is termed “isotonic”, and
intrvenious saline solution is adjusted to have the same salt content as your cells. If the bag or cell is
“saltier” than surroundings, fluid will move into the bag trying to equalize concentration, termed a
“Hypotonic” situation, whereas if the bag (or your cells) have less salt than the surroundings it is termed
“Hypertonic”
2
DIALYSIS utilizes a semipermeable membrane to separate solute molecules by size. Your kidneys
ordinarily remove waste products (urine with salts and small molecules) which pass through the filter,
while blocking large molecules such as blood cells and proteins.
Dialysis works on the principles of diffusion; whereby substances in water move from an area of high
concentration to an area of low concentration. For a person with kidney failure using dialysis, blood flows
by one side of a semi-permeable membrane and waste products of salt and water pass through the
membrane, replicating the action of a kidney to produce urine. Hemodialysis removes wastes and water by
circulating blood outside the body through an external filter. The blood flows in one direction and the
dialysate flows in the opposite. The counter-current flow of the blood and dialysate maximizes the
concentration gradient of solutes between the blood and dialysate, which helps to remove more urea and
creatinine from the blood. The concentrations of solutes (for example potassium, phosphorus, and urea)
may be undesirably high in the blood, but absent in the dialysis solution, and constant replacement of the
dialysate ensures that the concentration of undesired solutes is kept low on this side of the membrane. The
dialysis solution has levels of potassium and calcium that are similar to their natural concentration in
healthy blood. For another solute, bicarbonate, dialysis solution level is set at a slightly higher level than
in normal blood, to encourage diffusion of bicarbonate into the blood, to act as a pH buffer.
When about 25% excess glucose is found in urine, the condition is termed “diabetes”. Although glucose is
a “food” for cells, an excess can cause damage to organs such as kidneys nd eyes.
3
THE EXPERIMENT
In our dialysis experiment we will use a semi-permeable membrane in the form of tubing which has a pore
size large enough to pass small molecules, but not large ones such as starch. The membrane is calibrated
by approximate molecular hole size, in our case “1000”. Thus molecular sizes up to 1000 in molecular
weight can pass, but none much larger. We will utilize pure honey and a solution of 50% honey in water.
Honey contains simple sugars including fructose and glucose, you will calculate (or look up) the moleculr
weights to predict if they will pass the membrane. We will also add about 10 drops of starch solution to
see if it can pass the membrane.
4
Procedure
The processes can be done in any order. To save time you may want to start the osmosis 20 minute
soak while preparing the dialysis bags.
1. Place 4 lengths of dialysis tubing about 7 inches long in water for 10 minutes to soften and
hydrte the membrane. Pat dry and weigh each piece of tubing after hydration.
2. Tie off one end of each tubing with a knot, tight enough to prevent any leakage
3. Fill the tubing open end with the following items, then tie a knot at the end, and rinse off the
tubing with distilled water to remove any contamination beyond the knots.
a. About 5 mL of 100% honey + 5 drops starch solution
b. About 5 mL of 50% honey + 5 drops starch solution
c. About 5 mL of salt water
d. About 5 mL of distilled water
4. Weigh each bag after filling and rinsing. Pat dry with a paper towel. Use a watch glass or other
convenient rigid carrier to hold the tubes (will need to subtract the carrier weight).
5. Place the 4 bags into separate beakers with sufficient liquid to completely cover each bag
a. 100% honey & starch into beaker covred with water and 5 drops of iodine solution
b. 50% honey & starch into a beaker covered with water nd 5 drops of iodine solution
c. Bag with saturated salt water placed in beaker covered by distilled water
d. Bag with distilled water placed in beaker, covered by sturated salt water
6. Let dialysis and osmosis occur for 20 minutes and observe any size or color change to the bag or
the solution surrounding the bags.
7. Remove and re-weigh each bag after rinsing with distilled water patting dry. If you use the same
watch glass or other carrier, you can use one weighing for that item. In the data sheet you will
calculate change of mass for each bag.
8. A test for sugar is “Benedict’s test”, to see if ANY sugar passed through the dialysis bag. A
sample of water taken from that surrounding the honey bags is placed in a test tube nd heated in
a water bath. When a sugar containing solution is added to the test tube, it will turn from blue to
an ornge color indicating presence of sugar. We will also “test the test” with a known “blank”
test tube with no sugar, and a “known positive” using a drop of honey in distilled water.
a. 1 mL water from water surrounding 100% honey + 1 mL Benedict reagent
b. 1 mL water from water surrounding 50% honey + 1 mL Benedict reagent
c. 1 mL water + 1 drop honey + 1 mL Benedict reagent (positive verification test)
d. 1 mL water + 1 mL Benedict reagent (no honey, negative verification test)
9. A test for starch is Iodine. Iodine in solution with potssium iodide has a faint yellow-brown
color, whicch turns blue when starch is present. Verify that the starch turns blue by adding a
few drops of starch solution to a test tube followed by one drop of iodine solution. Did the water
surrounding honey bags test positive for starch?
5
PRELAB
1. Look up (or calculate) molecular weights for the following materials, plus provide structural
formulas (showing bonds) for these 4 materials.
a. Fructose C6H12O6
Molecular weight____________ grams/mole
Structural Formula:
b. Glucose C6H12O6
Molecular weight ___________ grams/mole
Structural Formula:
c. Vitamin “C” or ascorbic acid C6H8O6
Molecular weight____________ grams/mole
Structural Formula:
d. Starch is a polymer of glucose (C6H10O5)*n , where “n” may exceed 1000. Show a basic
unit of the polymer, with parenthesis around it, and an estimate of molecular weight.
2. Fructose and Glucose share the same empirical formula, but are not the same. Describe in a
few words the differences you see in the structural formulas.
3. From your understanding of osmosis and dialysis, predict the movement of solvent (material
with large concentration and usually lower molecular weight e.g. water) and solute (material
with lower concentration, e.g. honey) in this experiment. Both can occur.
4. Do you expect to see any differences between the two solutions of honey, 100% honey versus
50% honey + 50% water)? Why or why not?
6
Part A: Osmosis
Hypotonic
Hypertonic
salt water inside bag distilled distilled water inside bag salt
water outside bag
water outside bag
BEFORE OSMOSIS, filled bag
salt water inside bag
distilled water inside bag
salt water inside bag
distilled water inside bag
a
Watch Glass with filled osmosis bag
grams
b
Watch Glass alone (no bag)
grams
c=a-b
Mass of bag & CONTENTS
grams
REACTION TIME
Start Time
minutes
End time
minutes
elapsed time
minutes
AFTER OSMOSIS, filled bag
distilled water inside bag
d
Watch Glass with filled osmosis bag
grams
e
Watch Glass alone (no bag)
grams
Mass of filled osmosis bag, after osmosis
grams
f = (d-e)
CHANGE IN MASS
g = (f-c)
mass difference = water gained or lost by osmosis
h=100*(g /c) % = (mass gain/loss *100 / before dialysis mass)
7
salt water inside bag
salt water inside bag
distilled water inside bag
grams
%
Part B: Dialysis
BEFORE DIALYSIS, filled bag
NO honey
NO honey
bag with 100% honey
bag with 50% honey
a
Watch Glass with filled Dialysis bag
grams
b
Watch Glass alone (no bag)
grams
Mass of filled dialysis bag, before dialysis
grams
c=(a-b)
DIALYSIS TIME
bag with 100% honey
bag with 50% honey
Start Time
minutes
End time
minutes
Elapsed time (about 20 minutes)
minutes
AFTER DIALYSIS, filled bag
bag with 100% honey
bag with 50% honey
d
Watch Glass with filled Dialysis bag
grams
e
Watch Glass alone (no bag)
grams
Mass of filled dialysis bag, after dialysis
grams
f=d-e
CHANGE IN MASS
g=f-c
bag with 100% honey
bag with 50% honey
mass difference, before & after soaking
grams
h=100*g/c % mass gain (mass change *100 / before dialysis mass)
%
Note: bag with 50% honey already had 50% water, so values represent the water gain and not the total water content
Part C: Testing for sugar passing through membrane
Benedicts reagent changes color in presence of reducing agents (sugars)
NEGATIVE result means NO honey went through bag, color stays a clear blue after heating
POSITIVE result means honey went through the bag into water solution, color goes from blue to brown.
color after heating
Positive or Negative result?
T.T. #1, 2 mL water surrounding 100% honey bag
T.T. #2, 2 mL water surounding 50% honey bag
T.T. #3, 2 mL DI water, NO honey (negative control)
T.T. #4, 2 mL DI water + 1 drop of honey (pos. cntrl)
Part D: Testing for transfer of Starch across membrane
Starch turns blue in presence of iodine, and vice versa
NEGATIVE result is no color change, therefore no starch present
POSITIVE result is blue color, indicates Starch in presence of iodine
Color observed
Negative control, note color of deionzed water sample + 5 drops iodine only
Postiive control, note color of water sample + 1 drop starch + 5 drop iodine
Add a small amount of iodine solution to water bath surrounding bag to see if starch came through
water around 100% honey
Color of solution outside bag, with Iodine added
Did Starch molecules come through the dialysis bag?
8
water around 50% honey
POST LAB QUESTIONS
9
1.
Assume the molecular weight cutoff of the dialysis tubing is 1,000. Honey is a mixture of glucose
and fructose. Are both of these small enough to fit through the semi-permeable dialysis membrane?
2.
Should starch molecular polymer molecules with n=1000 fit through the membrane?
3.
Explain the mass change of the dialysis bag after water immersion. Is this dialysis or osmosis?
4.
Explain the results of Benedict’s test. Does a positive result show dialysis or osmosis?
5.
Draw a picture that would demonstrate your results from questions 1 and 3, Show the direction of
component movements through the membrane.