Experiment 15 Preparation and Properties ofColloidal 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.
Fe(OH)3+ HCl = FeOCl+ 2H2O
FeOCl =FeO+ + Cl-
The 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
H2S (excess) H+ + HS-
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 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 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
①Preparation 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.
②Purification 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:
①Take 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.
② Mix 2mL of ferric hydroxide sol with 2mL Sb2S3 sol in a small testtube. Observe the phenomenon and explain.
③ Put 2mL Sb2S3 sol into a small test tube. Heat the tube. Explain observed phenomenon.
(5) Protection of macromolecular solution to sol
Take two large test tubes, pour 2mL distilled water into the first tube and add 2mL of3% 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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基础化学实验（英）