Experiment 9: Preparation of Soap PDF

EXPERIMENT 9 Preparation of Soap Objectives 1. To prepare soap from olive oil 2. To investigate some properties of soap Introduction A soap is the sodium or potassium salt of a long-chain fatty acid. The fatty acid usuall...

EXPERIMENT 9 Preparation of Soap Objectives 1. To prepare soap from olive oil 2. To investigate some properties of soap Introduction A soap is the sodium or potassium salt of a long-chain fatty acid. The fatty acid usually contains 10 to 20 carbon atoms. Solid soaps usually consist of sodium salts of fatty acids, whereas liquid soaps consist of the potassium salts of fatty acids. Soap, such as sodium stearate, consists of a polar end that is soluble in water, and a nonpolar end (the hydrocarbon chain of the fatty acid) that is insoluble in water. O O- Na+ nonpolar, hydrophobic polar, hydrophilic Soap consists of a molecule containing a polar group, normally ionized, which is water soluble, and a nonpolar hydrocarbon portion that is water insoluble. When very small amounts of soaps are put into water, the molecules do not dissolve, but rather, become concentrated at the water surface with their polar ends immersed in the water and hydrocarbon chains protruding from the surface. It is this arrangement of soap molecules that accounts for the lowering of surface tension of water and the production of bubbles. If the concentration of soap is increased to a certain point, the mixture becomes turbid because of the formation of colloidal spherical micelles. A micelle is a submicroscopic grouping of molecules forming a tiny liquid droplet with the charged groups on the outside and hydrocarbons chain on the inside (Fig 9.1). 55  Na H O Na Fig. 9.1 A soap micelle solvating a droplet of oil Na H H O Na head Na H H Na tail O H H Na Na O H Na The importance of micelles is their ability to act as solvents for oil-soluble substances. When a soap micelle dissolved in water comes in contact with oil molecules, it absorbs the oil from an aqueous phase into interior by van der Waals interactions in much the same way that distribution of an organic molecules occurs in a separatory funnel. In this way, soap micelles in effect make oil and greases soluble in water and allow them to be washed from skin and clothing. To be effective in solubilizing oils and greases, micelles must be composed of soaps containing 10 to 20 carbons. Below 10 carbons there is insufficient van der Waals interaction to solubilize fats, and above 20 carbons the soap is too insoluble in water to form a sufficiently concentrated colloidal suspension. Synthetic detergents have largely replaced soap during the last two decades because soap has two serious drawbacks. The first disadvantage is that soap becomes ineffective cleanser in hard water, which contains appreciable amounts of Ca 2+ or Mg2+. When soap is used in hard water, the insoluble calcium salts of the fatty acids, and other precipitates are deposited as curds (referred to as bathtub rings). 2C17H35CO2-Na+ + Ca2+  (C17H35CO2)2- Ca2+(s) + 2Na+ soap curd The other is that, in an acidic solution, soap is converted to free fatty acid and therefore loses its cleansing action. C17H35CO2-Na+ + HCl  C17H35CO2H(s) + NaCl Water softeners are added to soaps to help remove the troublesome hard-water ions so that the soap will remain effective in hard water. Sodium carbonate or sodium phosphate or borax (Na2B4O7) will precipitate the ions as the carbonate or phosphate. 56 Unfortunately, the precipitate may become lodged in the fabric of items being laundered, causing a grayish or streaked appearance. Ca2+ + CO32-  CaCO3(s) 3 Ca2+ + 2PO43-  Ca3(PO4)2(s) Na2B4O7 + Ca2+  CaB4O7(s) + 2Na+ Soap Making Common soaps are made from a variety of fats and oils by a process called saponification (hydrolysis). This involves boiling the oil or fat in basic solution such as lye (NaOH) or potash (KOH), until the hydrolysis is complete. The products of this reaction are glycerol and the soap (the salt of a long-chain fatty acid). In general, the reverse of esterification (making ester) is called hydrolysis, and hydrolysis of fatty acid ester (fat or oil) is called saponification. O CH2 O C C17 H OH 35 CH2 O O CH O C C17 H 3NaOH O 35 CH OH + C17 H 35 C O Na CH2 O C C17 H sodium stearate 35 CH2 OH a soap tristearin glycerol Optional ingredients that are added to bar soaps include zinc oxide to make the soap whiter, lanolin to act as an emollient, pumice to help remove heavy soil by abrasion, and perfumes. Apparatus & Chemicals Hot plate Vegetable oil Ice cubes 95% ethanol Büchner funnel saturated NaCl Filter flask (500 mL) 25% NaOH Filter paper 5% FeCl3 PH paper 5% CaCl2 Boiling chips 5% MgCl2 Glass rod Graduate cylinder (50 mL) 57 Experimental Procedure I. Preparation of soap 1. Place 23 mL of a vegetable oil into a 250 mL Erlenmeyer flask. 2. Add 25 mL of 95% ethanol (a solvent) and 20 mL of 25% NaOH solution. 3. Heat the flask with its contents gently in a boiling water bath while stirring the mixture constantly with a glass rod. 4. After 20 min heating, the odor of alcohol will disappear indicating the completion of the reaction. A pasty mass containing a mixture of the soap, glycerol, and excess sodium hydroxide is obtained. 5. Place the flask with its contents in an ice-water bath 6. Add about 150 mL of saturated NaCl solution to the soap mixture while stirring vigorously to precipitate or salt out the soap. This process increases the density of the aqueous solution; therefore, soap will float out from the aqueous solution. 7. Filter the precipitate soap with the aid of suction and wash it with 15 mL of ice-cold water. II. Properties of soap Emulsifying Properties 1. Place 5 mL of water in a small test tube 2. Add 5 drops of the vegetable oil and shake the tube. A temporary emulsion of tiny oil droplets in water will be formed. 3. In a new test tube, place 5 mL of water and add a small piece of the soap you have prepared before shaking. 4. Allow both tubes to stand a while. Compare the appearance and the relative stabilities of the two emulsions. 5. Record your observations in you report. 58 Hard Water Reactions 1. Place 25 mL of water in a 50 mL beaker. 2. Add about one-third spatula-full of the prepared soap. 3. Warm the beaker with its contents to dissolve the soap. 4. Label 5 clean test tubes and place them in a test tube rack. 5. Put about 5 mL of the warm soap solution into each test tube. 6. Put a small amount of each of the following into the tubes as indicated: Tube # 1: 2 drops of 5% calcium chloride solution Tube # 2: 2 drops of 5% magnesium chloride solution Tube # 3: 2 drops of 5% iron (III) chloride solution Tube # 4: 2 drops of tap water Tube # 5: will be used later for a basicity test. Shake the first four test tubes and record your observations in your report. Alkalinity (Basicity) Test the soap solution in tube # 5 with a wide-range pH paper. Record the results in your report. 59