Unformatted text preview: EXPERIMENT 4 Preparation of Soap
Objective: Synthesize soap. Compare the properties of soap and a commercial detergent. Introduction: Soap has been made and used by people for ages, although the purposes to which it was put are not known. Inscriptions on excavated clay cylinders indicate that the Babylonians had devised a method for making soap as early as 2800 BC. The Egyptians were making as early as 1500 BC. Biblical evidence suggests that the Israelites were also making soap. Whatever its original purposes, the use of soap for personal hygiene had definitely occurred by the second century AD. The Alexandrian physician Galen (130 200 AD) mentions its use for washing the body. Soap was known during Roman times, but was seemingly unpopular as a personal hygienic. The Romans preferred to use perfumed oils, which they scraped off the skin with a specialized piece of metal. The soap making process known to the Romans was the same process as was used by previous civilizations: the boiling of animal fats and vegetable oils with ashes. Presumably, the unpleasant appearance and aroma of these early soaps accounted for their unpopularity. Little is known about the use of soap during the Dark Ages. Like so many things during the Dark Ages, knowledge of its value was probably lost and its use declined. The manufacture of soap in Europe and the Mediterranean region had reemerged by the end of the first millennium. Soap-makers guilds guarded their trade secrets closely, although the basic process was unchanged from earlier times, except for the addition of fragrance. A major step toward large-scale commercial soapmaking occurred in 1791 when the French chemist Nicholas Leblanc patented a process for making soda ash (sodium carbonate) from common salt. Soda ash is the alkali in ashes that reacts with fats and oils to form soap. The large quantities of good quality, inexpensive soda ash available because of the Leblanc process allowed soap to be made on a larger scale and at a more affordable cost. Soapmaking was put on a more scientific footing about 20 years later when the French chemist Michel Eugene Chevreul discovered the chemical nature of and relationship between fats, glycerine, and fatty acids. The combination of these scientific discoveries with the increasing industrialization of society made soap broadly available, changing it from a luxury item to a necessity. The manufacture of soap remained essentially unchanged from Leblanc's time until 1916, when the first synthetic detergent was developed in Germany in response to a World War Irelated shortage of fats. Detergents are made from petroleum-derived hydrocarbons. The greater availability and number of ways petroleum-derived hydrocarbons can be chemically combined to make detergents further increased the production and availability of detergents, so that today detergents are produced and used in much greater numbers than is soap. However, soap is still used in considerable amounts for personal hygiene and other applications.
1 41 Experiment 4: Preparation of Soap Soaps are sodium or potassium salts of long-chain carboxylic acids (fatty acids):
H2 C H3C C H2 H2 C C H2 H2 C C H2 H2 C C H2 H2 C C H2 H2 C C H2 H2 C C H2 H2 C C H2 O C O Na Hydrocarbon chain Carboxylate group Figure 4.1: A sodium carboxylate. Each soap molecule consists of two parts: a hydrocarbon chain and a carboxylate group. The hydrocarbon chain is compatible with dirt because dirt contains oily and greasy materials composed of hydrocarbon-like molecules. When soap molecules encounter a dirt particle, the hydrocarbon chains of the soap molecules dissolve in the dirt, leaving the carboxylate groups to cover the surface of the dirt particle. Because carboxylate groups are compatible with water, the entire structure--dirt particle surrounded by soap molecules--can be washed away with water:
H O C O H H Na+ O H
H O O C O O C H O O C Na+
O Na+ O C O Na
O + Dirt Na
+ O C H2O
O C O H H O C O O H H O Na+ Dirt Skin Na+ Skin Figure 4.2: The cleaning action of soap. So-called hard water limits the effectiveness of soaps. Hard water contains Ca2+, Mg2+, and similar metal-ion salts. These salts react with soap to form insoluble precipitates (e.g., bathtub rings):
O O O 2 H3C (CH2)16 C Na (aq) + Ca2+(aq) H3C (CH2)16 C O
2 Ca2+ (s) + 2 Na (aq) ! Water softeners prevent the formation of these precipitates by removing Ca2+ and Mg2+ from domestic sources of water. 42 Experiment 4: Preparation of Soap In this experiment, soap will be synthesized via a variant of the traditional process. Vegetable oil will be reacted with sodium hydroxide (an alkali). Animal fats and vegetable oils are rich in compounds called glycerol esters, such as glycerol tristearate. Most glycerol esters found in nature contain more than one type of carboxylate component (i.e., the 16 in the (CH2)16 will vary). When a glycerol ester is heated with an alkali solution, such as aqueous NaOH, it is converted into glycerol and salts of fatty acids. This process is known as saponification. An example of saponification involving glycerol tristearate follows:
H H C O O C O H H C C H O O C (CH2)16 CH3 O C (CH2)16 CH3 (CH2)16 CH3 O H H C C C H OH OH OH + 3 NaOH 3 H3C (CH2)16 C O Na + H H Glycerol tristearate Sodium stearate (soap) Glycerol Glycerol is soluble in water. The long-chain sodium carboxylates (soap) are much less soluble in water than is glycerol and can be readily precipitated by adding a concentrated sodium chloride solution to the reaction mixture. The soap can then be separated from the solution by filtration and purified by washing with cold water. Safety Precautions: Safety goggles must be worn at all times while you are in the laboratory. 95% ethanol is flammable. Make sure that there are no open flames in the laboratory. Work in the fume hood. In the event of a fire, notify your instructor. A fire in a small container can be smothered by covering the vessel with a watchglass. Use a fire extinguisher to eliminate a larger fire. If the fire is burning over too large an area to be extinguished easily, evacuate the area and activate the fire alarm. If your clothing should catch on fire, use the safety shower or a fire blanket to extinguish the flames. 2.5 M NaOH is caustic. Wear gloves while handling 2.5 M NaOH. Thoroughly wash off any sodium hydroxide spilled on your skin. If you get it into your eyes, immediately flush your eyes with water for at least 15 minutes. If you spill it on your clothing, remove the affected clothing before rinsing with water for 15 minutes. Notify your lab instructor, who will assist you. 43 Experiment 4: Preparation of Soap Procedure: Part A: Preparation of Soap Place about 10 g of oil or fat in a 400 mL beaker. Determine and record the mass of oil or fat. Add 20 mL of 95% ethanol, measured with a gradated cylinder, to the oil or fat in the beaker. Swirl the mixture until it becomes homogeneous. Record the appearance of the resulting mixture. CAUTION: The following reaction uses 2.5 M NaOH, which is caustic. Wear gloves while handling 2.5 M NaOH. Add 25 mL of 2.5 M NaOH, measured with a graduated cylinder, to the mixture in the 400 mL beaker. Record the appearance of the mixture. On the outside of the beaker, use a grease pencil to mark the liquid level of the mixture. CAUTION: The following reaction involves heating 95% ethanol, which is flammable. Make sure that there are no open flames in the laboratory. The reaction must be performed in the fume hood. Perform the following steps in the fume hood. Place a magnetic stirbar in the 400 mL beaker, and place the beaker on a stirrer-hotplate. Gently stir the mixture in the beaker. Heat the mixture to a gentle boil. If foam rises in the beaker, the mixture is being over heated. Heat the mixture at a gentle boil for at least 20 minutes. Watch the solution closely; it spatters easily. Using your wash bottle, occasionally squirt small amounts of distilled water around the inside of the beaker to rinse adhering particles into the mixture and to replace some of the liquid lost by evaporation. Maintain the liquid level in the beaker between 25 mL and its original value. Prepare an ice-water bath in a 600 mL beaker. After the mixture has gently boiled for at least 20 minutes, examine the liquid level. It should be no more than 75% of its original value. If the liquid level is more than 75% of its original value, continue boiling until evaporation has reduced the liquid level to less than 75% of its original value. Using beaker tongs, carefully lift the 400 mL off the stirrer-hotplate and place it in the ice-water bath. Turn the stirrer-hotplate off. Let the mixture in the 400 mL beaker cool to below room temperature. If necessary, add more ice to the ice-water bath. Record the appearance of the mixture in the 400 mL beaker. After removing the 400 mL beaker from the ice-water bath, insert a filled wash bottle in order to cool the distilled water. Add 50 mL of saturated NaCl solution, measured by graduated cylinder, to the mixture in the 400 mL beaker. The soap will precipitate. The excess sodium hydroxide and glycerol will remain in solution. Stir the mixture thoroughly but gently. Break apart chunks of soap floating in the mixture by pressing them against the wall of the beaker with a spatula, otherwise excessive amounts of sodium hydroxide and glycerol will be trapped in the soap and will interfere with subsequent tests to be performed with the soap. Assemble a vacuum filtration apparatus (see Figure 4.3). Insert a piece of filter paper in the Buchner funnel, turn on the aspirator, and wet the filter paper with distilled water. Decant the supernatant through the funnel. Then transfer the soap into the funnel using a glass stirring rod equipped with a rubber policeman and squirts of cold distilled water from a wash bottle, as necessary. After the filtration is complete, continue to pull air through the soap for 10 minutes. While the soap is drying, proceed with the next step. 44 Experiment 4: Preparation of Soap Figure 4.3: Vacuum filtration assembly. To check for residual sodium hydroxide in the soap, place a pea-sized sample of the soap in a 100 mL beaker and add 10 mL of 95% ethanol. CAUTION: The following step involves heating 95% ethanol. Perform this step in the fume hood and extinguish any flames. In the fume hood, gently heat the mixture on a hotplate. Stir with a glass stirring rod. Continue to stir and heat until the soap has dissolved. Remove the beaker from the hotplate with tongs and let the solution cool to room temperature. Add three drops of phenolphthalein indicator solution. If the solution turns bright pink, significant amounts of sodium hydroxide are present. Wash the soap with cold water and filter it again to remove the sodium hydroxide. Repeat this procedure until the solution produces only a pale pink color or no color at all after addition of phenolphthalein. Determine and record the mass of a paper weighing cup. Remove the soap and filter paper from the Buchner funnel. Scrape the soap off the filter paper and into the weighing cup. Determine and record the combined mass of the soap and weighing cup. Determine the mass of the soap by difference. 45 Experiment 4: Preparation of Soap Part B: Properties of Soaps Wash your hands with some of the soap you just prepared. Rinse your hands free of lather and soap. Record your observations. Coat your hands with a small amount of oil. Try to rinse the oil off with tap water alone. Then use soap followed by rinsing with tap water. Record your observations. Rub chalk dust on your hands. Use soap and water to wash your hands. Record your observations. Prepare a suspension of your soap by placing about 0.2 g of soap in a culture tube and adding distilled water until the culture tube is three-quarters full. Place your finger over the open top of the culture tube and shake to dissolve as much soap as possible. Set the culture tube in a small beaker to keep it upright and allow the undissolved soap to settle to the bottom of the culture tube. Prepare a second suspension of solid detergent in the same way the soap suspension was prepared. Half-fill two culture tubes with CaCl2 solution (40 mg Ca2+ per liter of water). Set the culture tubes in a small beaker to keep them upright. Using a clean Pasteur pipet, add three drops of the soap suspension to one of the CaCl2 solutions. Record your observations. Mix the culture contents by tapping near the bottom of the culture tube. Record your observations. Using a second, clean Pasteur pipet, add three drops of the detergent suspension to the other CaCl2 solution. Record your observations. Mix the culture contents by tapping near the bottom of the culture tube. Record your observations. Waste Disposal: All liquid chemical waste is to be flushed down the drain with plenty of water. Rinse the culture tubes with tap water and place in the glass waste can. 46 Experiment 4: Preparation of Soap Name: _____________________________ Date: _________________________________ Lab Instructor: ______________________ Lab Section: ___________________________ EXPERIMENT 4 Preparation of Soap Pre-laboratory Questions: 1. Write the balanced equation for the reaction between Mg2+ and sodium stearate (soap)? 2. What are the products of the reaction between KOH and the triglyceride shown below?
H H C O O C O H H C C H O O C (CH2)15 CH3 O C (CH2)14 CH3 (CH2)17 CH3 3. What safety precaution must be observed when heating 95% ethanol? Why must this precaution be observed? 4. What safety precaution must be observed when using 2.5 M NaOH? Why must this precaution be observed? 47 Experiment 4: Preparation of Soap Name: _____________________________ Date: _________________________________ Lab Instructor: ______________________ Lab Section: ___________________________ EXPERIMENT 4 Preparation of Soap Results/Observations: Part A: Preparation of Soap 1. Mass of oil or fat: 2. Appearance of oil or fat: 3. Volume of 95% ethanol added: 4. Appearance of the resulting mixture: 5. Volume of 2.5 M NaOH added: 6. Appearance of the mixture after adding 2.5 M NaOH: 7. Appearance of the mixture after cooling in an ice-water bath: 8. Volume of saturated NaCl solution added: 9. Appearance of soap after vacuum filtration: 10. Appearance of soap in 95% ethanol solution after addition of phenolphthalein indicator: Trial 1 11. Mass of a paper weighing cup: 12. Combined mass of soap and weighing cup: 13. Mass of soap calculated by difference: 48 Trial 2 Trial 3 Experiment 4: Preparation of Soap Part B: Properties of Soaps 14. Observations on washing your hands with the soap: 15. Observations on washing oil off your hands with tap water: 16. Observations on washing oil off your hands with soap and tap water: 17. Observations on washing chalk dust off your hands with soap and water: 18. Appearance of soap suspension: 19. Appearance of detergent suspension: 20. Observations on adding the soap suspension to a solution of CaCl2: 21. Observations on adding the detergent suspension to a solution of CaCl2: 49 ...
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This note was uploaded on 10/06/2008 for the course CHEM 2090 taught by Professor Zax,d during the Fall '07 term at Cornell.
- Fall '07