Experiment 15 Preparation and Properties of Colloidal Systems
Purposes
1. Understand the preparation method of colloid.
2. Verify the properties of colloidal system.
3. Observe the protective action of macromolecular solution to colloid.
Principles
1. Preparation of the colloid
The colloid is a dispersion system of matter. When the particles of 1nm～100nm of matter
disperses in the another medium, it will become a dispersed system of colloid. The dispersed
system of colloid mainly includes two types: a sol and a macromolecular solution. For the
preparation method of colloid, there is a condensation method and a dispersion method. Matter
can be put into the colloidal state by means of dispersion methods, in which large pieces of the
substance are broken up into particles of colloidal size, and condensation methods, in which
molecules or ions or atoms are made to cluster together to form particles of the desired size. The
preparation of colloid is preceded usually with some chemical reaction or the physical
condensation.
(1) Chemical reaction method
Preparation Fe（OH）3 sol by hydrolyzation of FeCl3: A dark red colloidal suspension of iron
(Ⅲ) hydroxide may be prepared by mixing a concentrated solution of iron (Ⅲ) chloride with hot
water.
boiling
Fe(OH)3 + 3HCl
FeCl3+3H2O
Fe(OH)3+ HCl = FeOCl+ 2H2O
FeOCl =FeO+ + ClThe colloidal Fe(OH)3 selectively adsorbs FeO+ ions，it becomes a positively charged colloid.
A colloidal suspension of antimony (Ⅲ) sulfide is produced by the reaction of hydrogen
sulfide with antimony potassium tartrate dissolved in water.
2(SbO)K(C4H4O6) + 3H2S = Sb2S3 + 2KHC4H4O6 + 2H2O
H+ + HS-
H2S (excess)
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The colloidal Sb2S3 selectively adsorbs HS- ions，and it becomes a negatively charged sol.
(2) The physical condensation method
An insoluble sulfur sol is prepared with the change solvent method: The solubility has
different properties because the substance is in the different solvents. Add sulfur saturated solution
of ethanol to water. Because the sulfur is insoluble in water, sulfur atoms gather each other to
form a sulfur sol.
2. Optical and electrical properties of the colloid
When a strong beam of light is passed through a colloid, the beam becomes visible because
the colloidal particles reflect and scatter the light. This phenomenon is called the Tyndall effect.
But when a strong beam of light passes through a solution, no Tyndall effect is observed because
the solution particles are too small to scatter the light. Thus, the Tyndall effect is one property that
distinguishes colloidal dispersions from solutions.
One of the most important properties of dispersed colloidal particles is that they are usually
electrically charged. The charge of a colloid may be determined by placing it in a U tube
containing two electrodes. When a current passes through the U tube, each electrode will attract
particles of opposite charge. A negative colloid will begin to accumulate around the positive
electrode and a positive colloid around the negative electrode. The movement of electrically
charged suspended particles toward an oppositely charged electrode is called electrophoresis. If an
iron (III) hydroxide sol is placed in an electrolytic cell, the dispersed particles move to the
negative electrode. This is good evidence that the iron (III) hydroxide particles are positively
charged. Most hydroxides of metals have positive charges, while most sulfides of metals form
negatively charged colloidal dispersions，which move to the positive electrode.
3. Purifying of the colloid
Because the presence of excess ions gradually brings about the coagulation of colloids, the
removal of some ions is necessary if the dispersion is to be kept for any length of time. Water,
ions, and small molecule flows can across a semipermeable membrane, but the colloidal particles
cannot, which can be used to separate and purify the colloid. The membrane is called a dialyzing
membrane. This process is called dialysis.
4. Coagulating of the colloid
The major reason for the stabilization of colloids is owing to charges absorbed by colloidal
particles. You might expect these very small crystals to aggregate into larger crystals when adding
ions of opposite charge because the aggregation would bring ions of opposite charge into contact.
There are several methods of bringing about the coagulation of colloidal dispersions. The most
effective way of coagulating colloidal dispersions of the sol is by adding an electrolyte. This
introduces a very large number of ions that remove the adsorbed ions, so that the colloidal
particles no longer repel one another but coalesce rapidly into larger particles. The choice of
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electrolyte is dependent on the type of adsorbed ions to be removed. For the iron (III) hydroxide
sol can be made to aggregate by the addition of an ionic solution, particularly if the solution
contains anions with multiple charges (such as phosphate ions, PO43-). Colloidal the iron (III)
hydroxide particles are positively charged, so the greater of the negative charge, the more effective
is the coagulation. On the other hand, colloidal Sb2S3, because the absorbed ions are negative ions,
the most effective coagulating electrolytes are those which have positive ions of high charge. For
this reason, aluminum chloride (AlCl3) is more effective than an equivalent quantity of sodium
chloride (NaCl) in coagulating Sb2S3 sols.
The mixing of two colloidal dispersions whose particles are oppositely charged causes both
to coagulate. In many cases, heating is also a method of bringing about the coagulation of
colloidal dispersions.
5. The protective action of macromolecular solution to sol
If a macromolecular solution is placed in a sol, the macromolecular compounds can prevent
the colloidal particles from coalescing. The macromolecular solution, acting as a stabilizing agent
for particles of a colloid, is called a protective colloid, because colloidal particles are coated with
these macromolecular compounds.
Apparatus and Reagents
Apparatus: U-shaped tube (a device for electrophoresis), an instrument for Tyndall effect, a
magnetic stirrer, an alcohol lamp, beaker (100mL×5), graduated cylinder (10mL×4, 20mL×3), test
tube, a watch glass, medicine dropper, pH indicator papers, triangle and glass rod.
Reagents: 3% of ferric hydroxide, 0.4% of antimony potassium tartrate, saturated hydrogen
sulfide, a saturated solution of sulfide in ethanol, saturated NaCl solution, 0.01mol•L-1NaCl,
0.01mol•L-1CaCl2, 0.01mol•L-1AlCl3, 0.1mol•L-1NH3•H2O, 0.05mol•L-1I2, 0.1mol•L-1KSCN,
0.05mol•L-1AgNO3, 3% of gelatin, 2% of CuSO4, celloidin and starch solution.
Procedure
1. Preparation of colloid
(1) Preparation of the iron (III) hydroxide sol: Boil about 30mL distilled water in a small
beaker. While the water is boiling, add 3mL of 3% ferric chloride solution drop by drop, and
continue boiling for 2min～3min. Colloidal ferric hydroxide forms. Retain this colloid.
(2) Preparation of the antimony (III) sulfide sol: Measure out about 20mL of 0.4% antimony
potassium tartrate into a small beaker. With stirring add about 10mLH2S solution drop by drop
until the solution changes orange. Insoluble antimony (III) sulfide in the colloidal state is thus
produced. Retain this colloid.
(3) Preparation of the sulfur sol by change solvent: Add about 20mL distilled water to a
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small beaker, then add 2 mL～3 mL sulfur saturated solution of ethanol with stirring. Observe
this prepared mixture immediately and insoluble sulfur sol is thus prepared. Retain this colloid.
2. Properties of colloidal systems
(1) Tyndall effect: Take above three types of solution, shine a light through each liquid.
Observe the Tyndall effect. For comparison，examine distilled water and 2% of CuSO4 solution in
the same light beam.
(2) Electrical properties of colloidal particles—electrophoresis: Place the colloidal antimony
(III) su1fide in a U-tube (FigureⅡ- 4-1) and in an electrophoresis apparatus. Observe the dispersed
particles. Which electrode will they move to? Estimate the charge of the colloidal particles.
(3) Purification of the colloid—dialysis
① Preparation of the dialysis bag: Place proper amount of celloidin in a small clean beaker
and swirl. Wait for a minute and then turn the beaker upside down. The material should be solid.
Cautiously cut out a piece and get a dialysis bag.
② Purification of macromolecular solution: Add proper amount of starch solution in a
dialysis bag and 2 drops of saturated sodium chloride solution, which in turn is placed in a small
beaker of distilled water. Take the little liquid outside of the bag after 10 minutes and examine
chloride ions (silver nitrate test). Then test the little liquid inside and outside the colloidal starch
suspension for starch (iodine solution test). Record your results. Explain your experimental
results.
③Purification of Fe(OH)3 sol: Place the colloidal ferric hydroxide in the dialysis bag. Be
careful that none of the liquid contaminates the outside of the dialysis bag and close it tightly. If so,
wash it with distilled water. Suspend the dialysis bag in a beaker of distil1ed water. Then change
the water at intervals of 10 minutes. Test the liquid outside the colloidal ferric hydroxide for
chloride ions (silver nitrate test) and ferric ions (KCNS test). Record your results.
(4) Coagulation of the colloid:
① Take three test tubes and place 2mL Sb2S3 sol respectively, add 0.01mol•L-1NaCl in the
first tube, then add 0.01mol•L-1CaCl2 in the second tube and 0.01mol•L-1AlCl3 in the third tube
drop by drop. Count the drops and shake the tube after the addition of each drop or so. When the
precipitation begins to persist, record the drops you added. Compare and explain.
②Mix 2 mL of ferric hydroxide sol with 2mL Sb2S3 sol in a small test tube. Observe the
phenomenon and explain.
③Put 2mL Sb2S3 sol into a small test tube. Heat the tube. Explain observed phenomenon.
(5) Protection of macromolecular solution to sol
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Take two large test tubes, pour 2mL distilled water into the first tube and add 2mL of 3%
gelatin solution into another tube, then add 4mL Sb2S3 sol in each test tube respectively. Swirl the
tube gently. Finally add the saturated NaCl solution to each test tube after setting for 3min.
Observe the difference between the two test tubes.
Questions
1. Why is FeCl3 solution dropped into the boiling water when we prepare the ferric hydroxide sol?
2. Why can the sol be stabilized by an added gelatin?
3. Try to explain the reason of Tyndall effect.
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