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[Type text][Type text][Type text] Lab 5: Synthesis of Soap Background: For centuries, soaps have been made from animal fats (tallow and lard) and lye (sodium hydroxide, NaOH) extracted by filtering hot water through wood ashes. Animal fats (and vegetable oils) are esters of long chain carboxylic (fatty) acids and the tri-alcohol glycerol (1,2,3-propanetriol) known as triglycerides. The process of breaking these esters dow in the presence of a strong base such as lye is referred to as saponification. Saponification breaks these esters into liquid glycerol and solid carboxylate salts which we call soap (Figure 1). Figure 1: Saponification of a triglyceride by sodium hydroxide. Fats are rarely as simple as the “tripalmitin” shown in Figure 1 because the triglyceride may contain three different fatty acid residues. For example, beef tallow generally contains 50% stearic acid (C18), with the majority of the remaining acids being myristic (C14), palmitic (C16), and oleic (C18) unsaturated acids. Furthermore, tallow may be blended with either coconut, palm, or olive oils from vegetables to produce a wide variety of soaps. Softer soaps are made in much the same way as mentioned above, but use potassium hydroxide instead of sodium hydroxide to give the potassium salts. Soaps that we commonly use for bathing have been carefully washed free of any base that may remain from the saponification so as to not burn our skin. Soap molecules are considered amphipolar. One region of the soap molecule is said to be hydrophobic (water hating or grease loving) and the other end is hydrophilic (water loving or grease hating). Figure 2: Representation of a soap molecule's dual nature of polarity. When you use soap, the hydrophobic tail is attracted to the dirt (nonpolar material) and the hydrophilic head is attracted to water. Clusters of soap particles called “micelles” are formed with all the tails to the center and all the heads on the surface of the micelle and the dirt can be washed away. In hard water the carboxylate “Heads” interact with Ca2+, Fe3+, or Mg2+ ions that replace the sodium or potassium cations of the soap molecules to form insoluble salts we call “soap scum” (Figure 3). Figure 3: Example of a water insoluble carboxylate soap known as soap scum. Detergent molecules are similar to soaps in that they are also amphipolar. But, unlike soaps they are not derived from naturally occurring fats or oils. The major structural difference between soaps and detergents is found in the polar head. Soap has a carboxylate salt in the polar region, while detergents have sulfate or phosphate salts, as shown in Figure 4. In general, detergents do not form insoluble salts with hard water and leave no “scum”. Figure 4: Structure of a common detergent, sodium laurylsulfate. In this lab, we will synthesize soap from a fat or oil source of your choosing and investigate the properties of soaps and detergents. SAFETY 1. When you prepare the soap you will need to boil the mixture until most of the water is evaporated. This may lead to splattering so be careful. 2. Sodium and potassium hydroxide is corrosive and can burn your skin. Be careful! Pre-lab Assignment for Lab 4: Synthesis of Soap 1. Why is determining an accurate theoretical yield of soap impossible? 2. Explain why detergents or soaps are needed to remove non-polar stains from clothes and dishes (water doesn’t work!). A drawing should be included. 3. Draw the structure and provide the name of the products from following saponification: 4. Make an outline of the procedure using pictures. Make sure you can complete the lab using only these pictures. Vegetable oil or canola oil is provided for your use in this lab to make soap. However, you may bring in a different oil/fat if you would like, e.g. olive oil, coconut oil, lard, etc. Procedure: 1. Preparation of soap: a. Prepare a mixture of 15 mL of 20% (5 M) sodium hydroxide and 10 mL of oil in a 250-mL beaker. b. Turn the hot plate on a medium and get the solution boiling, and then IMMEDIATELY switch to the lowest setting to maintain boiling. Be careful not to over boil, hot oil and base will burn your skin. c. Stir the mixture with a wooden tongue depressor as you are heating it to prevent explosive boiling of the hydroxide-oil mixture. Boil the mixture, observing the precautions listed above. The saponification is complete if a wax-like solid begins to form that, upon further cooling, becomes hard and somewhat brittle. If the mixture cools to a syrupy liquid, it might be advisable to add more (5 mL) 20% hydroxide solution and boil the mixture until its water is expelled. This should be complete in 30-45 minutes. d. While the mixture is heating, you should prepare a concentrated salt solution by dissolving 50 g of sodium chloride in 150 mL of distilled water in a 400-mL beaker. (Prepare this solution immediately so that it is ready when your soap is ready.) You should also prepare 50 mL of iced 5% NaHCO3 (sodium bicarbonate) solution (cool it with ice, don't add the ice to the solution). e. When the saponification reaction is complete, remove the flask from the heat. f. Pour the reaction mixture containing the soap quickly into the saturated salt solution (you may have to scrape the solid soap into the solution using a wooden tongue depressor). g. Stir the mixture thoroughly for ~2-3 minutes; then, collect the precipitated soap on a filter in a Buchner funnel. h. Wash the soap with 10 mL portions of ice-cold sodium bicarbonate solution. i. After you have collected and washed the soap and if you plan to take your soap home, continue to draw air through the soap for several minutes to help dry it. If you do not wish to take your soap home, it can be discarded in the trash. 2. Evaluation of Soap Two solutions labeled “soap” and “detergent” can be found in the hood. You will use these pre-made solutions to test the properties of soaps and detergents. Part 1: Emulsification (dissolving) of Oils a. Place 15 drops of mineral oil in each of three separate test tubes labeled A, B, C. b. Add 5 mL of distilled water to tube A, 5 mL of soap solution to tube B, and 5 mL of the detergent solution to tube C. c. Shake the tubes briefly and observe how well the oil is emulsified in each. How can you compare the emulsification? Record your observations in the table on the report sheet. d. Discard the solutions down the drain. Part 2: Reaction with metal ions a. Place 5 mL portions of soap solution in each of three clean, separate test tubes, labeled A, B, C. b. Add 2 mL of a 1% solution of CaCl2 to tube A, 2 mL of 1% MgCl2 to tube B, and 2 mL of 1% FeCl3 into tube C. c. Shake each tube and observe what happens. Note whether a precipitate forms on the report sheet. Discard the solutions into waste containers in hood. d. Repeat this experiment with the detergent solution. Part 3: Is soap basic or acidic? Phenolphthalein is an indicator which turns pink in basic solution, with pH greater than 7.0, but which is colorless in acidic solutions. a. Add a 5-mL sample of soap solution into an empty test tube labeled tube A and a 5 mL sample of the detergent solution to an additional empty test tube labeled tube B. b. Add one drop of phenolphthalein to each test tube. Note any observations. c. Add 3 M HCl dropwise to each test tube, A & B. If the color of the solution is still pink, due to the phenolphthalein, then keep adding acid until the pink color disappears. Record how many drops this requires. Data and Post-Lab Assignment for Lab 4: Synthesis of Soap Observations for the Evaluation of Soap and Detergents Part 1: Emulsification with Oil Observations- what did you see when the solutions were mixed with mineral oil? How well did the solutions dissolve the mineral oil? Tube #A: Water Tube #B: Soap Solution Tube #C: Detergent Solution Part 2: Hard Water Ions Observations- what color changes or precipitations did you observe? Did a reaction take place? Tube #A: Soap plus Ca2+ Tube #B: Soap plus Mg2+ Tube #C: Soap plus Fe3+ Tube #A: Detergent plus Ca2+ Tube #B: Detergent plus Mg2+ Tube #C: Detergent plus Fe3+ Part 3: Acid/Base Observations- What colors did you observe, how light/dark were they, when did the colors change? Tube #A: Soap plus phenolphthalein Tube #B: Detergent plus phenolphthalein Tube #A: Soap plus HCl (pink color is gone) Tube #B: Detergent plus HCl (pink color is gone) Post-Lab Questions (use complete sentences) 1. When hydrochloric acid is added to the soap solution a reaction takes place. What are the two products that are produced? 1. Old-fashioned lye soap was often said to be “harsh” on the skin (cause irritations and rashes). What would be the most reasonable source of this condition? 1. Answer the following questions based on the results the evaluation of the soap/detergent/ water solutions in the three tests. 2. What solution was the mineral oil most and least soluble in? Why do you think this is? 2. Which solution reacted with the metal ions? How could you tell a reaction took place? 2. What solution was more basic? Explain how you came to your conclusion. 8 O O Na Non-PolarTail (hydrophobic) PolarHead(hydrophillic) O O Ca 2+ O O S O O O Na O O KOH