(adapted from Blackburn et al., Laboratory Manual to Accompany World of Chemistry, 2 nd ed., (1996) Saunders College Publishing: Fort Worth) Purpose: To prepare a sample of soap and to examine its properties. Caution: Sodium hydroxide solution is extremely harmful to the skin and clothing. It is especially harmful to the eyes. Wear safety goggles. Any sodium hydroxide spilled on the skin should be washed off immediately with large quantities of water. Kerosene and ethanol are both flammable. Care should be taken when using these chemicals around flames. Introduction: A soap is the sodium or potassium salt of a long chain fatty acid. Most solid soaps are sodium salts of the type to be made in this experiment. The starting materials are fats or oils, which are the glycerol esters of the fatty acids. A typical cooking oil, cottonseed oil, and a concentrated solution of sodium hydroxide are the principal reactants. Ethanol is used in this experiment to serve as a common solvent for the reactants and hence speed up the reaction. The soap making reaction is called saponification. However, the saponification reaction is not used in the commercial soap making operations of today. The sodium salts of the long chain fatty acids are typical surface active agents. They have a polar end which is hydrophilic (water loving) and a long nonpolar chain which is hydrophobic (watr hating). As a consequence they can form emulsions by suspending oil in water when their nonpolar end is in the oil and their polar end is in the water. The emulsification of kerosene in water by means of soap is just such a process. When present, calcium, magnesium, iron, and some other metal salts can form insoluble precipitates with the long chain fatty acids. This kind of reaction is responsible for the problems which arise when soap is used with hard water (water containing appreciable amounts of dissolved calcium, magnesium, or iron salts). Revision SP11 Page 1 of 7
Equipment: 250 ml beaker sodium hydroxide (20% solution) 100 ml beaker ethanol wire gauze saturated solution of sodium chloride laboratory burner calcium chloride (5% solution) glass stirring rod magnesium chloride (5% solution) test tubes ferric chloride (5% solution) filter flask and Buchner funnel kerosene filter paper phenolphthalein indicator solution cooking oil graduated cylinder watch glass to extinguish possible ethanol flames Procedure: Measure 20 g of cooking oil into a 250 ml beaker. Add 20 ml of ethanol and 25 ml of 20% sodium hydroxide solution. Stir the mixture in the beaker. Place the beaker on a wire gauze supported on a ring stand and heat gently. CAUTION!! Heat this solution gently: keep the flame away from the top of the beaker to prevent the alcohol from catching on fire. If the vapors of the mixture do ignite, carefully slide the watch glass on to the top of the beaker to extinguish the flame. Continue to stir as the heating is continued. Cup your hand and waft the vapors to your nose. Do not let your hair or clothing get near the flame. The heating is continued until the odor of alcohol has disappeared and a pasty mass remains in the beaker. This pasty mass is a mixture of the soap and the glycerol freed in the mixture. Turn off the burner and allow the beaker to cool to room temperature on the ring stand. Once the beaker is cool, move it safely to the benchtop. Add 100 ml of saturated sodium chloride solution to your soap preparation and stir the mixture thoroughly with a glass stirring rod. This process is called salting out and is used to remove the soap from water, glycerol, and any excess sodium hydroxide present. After the mixture has been stirred and mixed completely, filter off the soap with a vacuum filtration apparatus and wash once with ice water. Get the soap as dry as possible. Cut up the soap with a spatula while air is being pulled through the Buchner funnel. Weigh your dried soap and record the weight (1). Washing Properties: Take a small amount of the soap and try to wash your hands with it. It should lather rather easily if soft water is used. If soft water is not available, use deionized water. If too much oil was used, the soap will feel greasy; if too much sodium hydroxide is still present, it will also make the hands feel slick but will roughen them. Record your observations (2). Revision SP11 Page 2 of 7
Emulsification: Put 5 to 10 drops of kerosene in a test tube containing 10 ml water and shake to mix. An emulsion or suspension of tiny oil droplets in water will be formed. Let this stand for a few minutes. What happens (3)? Prepare another test tube with the same ingredients and also add a small portion (1/2 g or so) of your soap. Shake to mix. Compare the relative stabilities of the two emulsions. Does the soap have any effect (4)? Hard Water Reactions: Take 1 g of your soap and warm it with 50 ml of water in a 100 ml beaker. When you have obtained a reasonably clear solution, pour about 15 ml into each of three test tubes. Test one of the three tubes with 10 drops of 5% CaCl2 solution, one with 10 drops of 5% MgCl2 solution and one with 10 drops of 5% FeCl3 solution. Let these solutions stand until you have finished the other tests. Then make your observations (5 7). Basicity: A soap with free alkali can be very damaging to skin, silk, or wool. Dissolve a small piece of your soap in 15 ml of ethanol and then add two drops of phenolphthalein. If the indicator turns red, the presence of free alkali is indicated. What happens with your sample (8)? Submit the rest of your soap sample according to the instructions of your lab instructor. Be sure to include your name, your lab partner s name, the product name and the yield of product in grams. Waste Disposal and Cleanup: Dispose of the kerosene solutions in the appropriate waste container. Ask you lab instructor for proper disposal of all other solutions. Thoroughly clean all glassware used and return them to their proper places. Revision SP11 Page 3 of 7
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DATA SHEET Name: Partner: I. Preparation (1) Yield of Soap: g II. Properties (please avoid the use of pronouns) (2) Washing Properties: III. Emulsification of oil and water (3) Behavior of kerosene and water mixture: (4) Behavior of kerosene water soap mixture: IV. HardWater Reactions (5) Results of test with CaCl 2 solution: (6) Results of test with MgCl 2 solution: (7) Results of test with FeCl 3 solution: V. Basicity (8) Behavior of soap alcohol solution with phenolphthalein: Revision SP11 Page 5 of 7
POST LAB QUESTIONS Name: Partner: 1. Why is it necessary to remove unreacted sodium hydroxide from the soap before using it on the skin? 2. What is hard water? What effect does it have on the cleansing action of soap? 3. Suppose in the experiment that some students used cottonseed oil, some used olive oil, and some corn oil. Would their results be similar? Explain. 4. State one difference between the soap you made and a bar of commercially available soap. Revision SP11 Page 6 of 7
PRE LAB ASSIGNMENT Name: 1. State one safety precaution to be taken when using: (a) Sodium hydroxide solution: (b) Ethanol: 2. Describe how to determine safely if all ethanol has evaporated from the reaction mixture. 3. What class of natural compounds serves as the starting material for the preparation of soap when reacted with NaOH? 4. How is soap recovered as a solid product from the solution containing the other products of the reaction? Revision SP11 Page 7 of 7