Saponification of Triglycerides - Lab Report Example

Saponification of Triglycerides I. Introduction Triglycerides, or triacylglycerols, are composed of three fatty acids linked to glycerol by fatty acyl esters (-O-CO-R). The fatty acids, straight-chain monocarboxylic acids, may be saturated (no C=C double bonds) or unsaturated. Fats and oils are trigylcerides. They are distinctive from the fact that at room temperature fats are solid and oils are liquid. In other words, liquid triglycerides are oils, while solid triglycerides are fats (Fessenden et al., 1998). Soapis produced by the saponification (hydrolysis) of a triglyceride. In this process the triglyceride is reacted with a strong base such as sodium or potassium hydroxide to produce glycerol and fatty acid salts (Whitten et al., 2007). The saponification of glyceryl tristearate is illustrated in Figure 1. CH2O2C(CH2)16CH3 CH2OH CH2O2C(CH2)16CH3 + 3 NaOH CH2OH + 3 CH3(CH2)16CO2-Na+ CH2O2C(CH2)16CH3 CH2OH glyceryl tristearate glycerol sodium stearate (soap) Figure 1. Saponification of triglyceride. (Fessenden et al., 1998: 962) The commonest soaps are the fatty-acid salts of sodium and potassium. Hard soaps are sodium salts while soft soaps are potassium salts. The fatty-acid salts of ammonium are also sometimes used for cleansing. Only a few other soaps are of practical importance, for example lead soaps which are used in medicinal plasters, zinc soaps which are used in ointments, and aluminum soaps which are used in waterproofing. Very few of the salts of fatty acids have the properties of common soap. Most of them are but slightly soluble in water, and therefore do not yield suds and have little or no detergent (i.e., cleansing) action. All are nevertheless termed soaps by chemists (Lewkowitsch, 1904). The saponification number (S) is the number of milligrams of potassium hydroxide required to convert one gram of the fat completely into glycerine and potassium soap. It gives information concerning the character of the fatty acids of the fat and in particular concerning the solubility of their soaps in water. Table 1 lists the saponification number of the common fats and oils. Table 1. Saponification Numbers of Common Fats and Oils. Fat or Oil Saponification No. Fat or Oil Saponification No. Rapeseed oil 170 - 179 Mutton tallow 192 - 195.5 Menhaden oil 190.6 Peanut oil (arachis) 190 - 196 Corn oil 188 - 193 Horse oil 195 - 197 Olive oil 185 - 196 Beef tallow 193.2 - 200 Soy bean oil 193 Palm oil 196 - 205 Cacao butter 193.55 Butter 220 - 233 Linseed oil 192 - 195 Palm kernel oil 242 - 250 Cottonseed oil 193 - 195 Coconut oil 246 - 260 Lard 195.4 (Lewkowitsch, 1904: 400) Table 1 shows that butter ranks with palm kernel oil and coconut oil as having a very high saponification number. This is due to the fact that its triglycerides contain appreciable quantities of fatty acids (myristic acid and small quantities of lauric acid) which when they form soap combine with relatively more sodium or potassium than the more common acids of fats. These acids occur in undecomposed butter in chemical combination as triglycerides. Their sodium or potassium soaps are quite soluble in water. The high saponification number of coconut oil and palm kernel oil is due to the large proportion of fatty acids (lauric acid and myristic acid) that they contain. In this laboratory exercise, saponification as the hydrolysis in basic solution of fats and oils to produce glycerol and salts of fatty acids, and determination of saponification number of sample fat and oil were evaluated. II. Method Triglyceride (2 g) was accurately weighed and was placed in a dry round bottom flask. The prepared ethanolic potassium hydroxide solution (50 cm3) was added to the flask using a pipette. Few anti bumping granules were added to the stirring solution. After setting up the reflux condenser, the solution was boiled for at least 30 minutes. A blank experiment with identical volume as the sample solution was also carried out. After refluxing, the solution was allowed to cool. After cooling, a few drops of phenolphthalein indicator solution were added to both flasks. The blank and test solutions were titrated with 0.5 M HCl. The titration values and the weight of triglyceride used were recorded. The molar difference between the amount of 0.5 M HCl required to neutralize the blank and the amount of HCl required to neutralize the sample equals the amount of 0.5 M KOH used in the saponification process. III. Results The saponification value (S) of fat and oil were calculated using Equation (1) given below: Saponification value (S) = (B-T) x 0.5 x 56 mg/g fat (1) W where B is the blank value in cm3, T is the test value in cm3, and W is the weight of fat/ oil in g. The average molar mass of fat (MM) was computed using Equation (2): Molar mass of fat/oil (MM) = 3 x 56 x 1000 (2) S Table 2. Saponification and molar mass values of fat and oil samples. Sample Weight (g) B (cm3) T (cm3) S (mg/g fat) MM (g/mol) Fat 2.03 38.7 23.6 208.28 806.61 Oil 2.05 38.7 26.2 170.73 984.01 IV. Discussion and Conclusion Triglycerides or triaclyglycerols (TAGs) in fats and oils have no ionic charges, and are nonpolar and hydrophobic. TAGs can be hydrolyzed into their component fatty acids and alcohols - that is, they are broken down by the addition of a water molecule. In the body, enzymes known as hydrolases carry out this hydrolysis. In laboratory process, as in the one performed in the experiment, hydrolysis can be carried out through saponification - where the hydrolysis is carried out in the presence of a strong base. In the case of the experiment, we used ethanolic KOH to facilitate the hydrolysis. Hydrolysis of esters is a reaction having nucleophilic acyl substitution. This process can be divided into basic and acid related reactions. The acid catalyzed mechanism is the reverse of Fischer esterification, wherein esters are obtained by refluxing the parent carboxylic acid with the appropraite alcohol with an acid catalyst. The experiment showed the hydrolysis of esters based on alkaline hydrolysis of esters from the fat and oil samples. The steps of the mechanism are detailed as follows: (Carey 2000: 246) The calculated values of the saponification number and molar mass of fat and oil were derived using Equations (1) and (2). The saponification numbers of the sample fat and oil are 208.28 and 170.73 mg/g, respectively. With reference to Table 1, it can be assumed that the oil is rapeseed oil which has S range of 170-179. The sample fat can either be beef tallow (S = 193.2-200) or butter (S = 220-233). The saponification number indicates the fatty acid chain length of a triglyceride. In order for saponification to occur, a certain amount of KOH is needed to complete the hydrolysis of one gram of fat or oil. The value is just a measurement of the KOH amount. From Table 2, the molar mass values of fat and oil are 806.61 and 984.01 g/mol. Triglycerides with long fatty acids have more mass. As the mass increases, the saponification number decreases (an inverse relationship). So, a given mass of a triglyceride will have larger number of moles at low molecular weight, and accordingly consume a larger number of moles KOH. Triglycerides containinglong fatty acidswill have alowersaponification number than triglycerides with shorter fatty acids. Therefore it is expected that the experimental fat sample will contain longer fatty acids than the oil sample. V. References Carey, F.A. 2000. Organic Chemistry, 4th edn. New York: McGraw-Hill. Fessenden, R.J., Fessenden, J.S., Logue, M. 1998. Organic Chemistry, 6th edn. California: Brooks/Cole Publishing Company. Lewkowitsch, J.1904. Chemical Technology and Analysis of Oils, Fats, and Waxes. London: Macmillan. Whitten, K.W., Davis, R.E., Peck, M.L, and Stanley G.G. 2007. Chemistry 8th edn. California: Brooks/Cole Publishing Company. Read More