Saponification is a process of cleavingesters intocarboxylate salts andalcohols by the action of aqueousalkali. Typically aqueoussodium hydroxide solutions are used.[1][2] It is an important type ofalkaline hydrolysis. When the carboxylate is a long chain, its salt is called asoap. The saponification ofethyl acetate givessodium acetate and ethanol:
Vegetable oils andanimal fats are the traditional materials that are saponified. These greasy materials, triesters calledtriglycerides, are usually mixtures derived from diverse fatty acids. In the traditional saponification, the triglyceride is treated withlye, which cleaves the ester bonds, releasing fatty acid salts (soaps) andglycerol. In one simplified version, the saponification ofstearin givessodium stearate.
This process is the main industrial method for producing glycerol (C3H5(OH)3).
Some soap-makers leave the glycerol in the soap. Othersprecipitate the soap bysalting it out withsodium chloride.
Fat in acorpse converts intoadipocere, often called "grave wax". This process is more common where the amount offatty tissue is high and the agents ofdecomposition are absent or only minutely present.
Thesaponification value is the amount of base required to saponify a fat sample.[3] Soap makers formulate their recipes with a small deficit of lye to account for the unknown deviation of saponification value between their oil batch and laboratory averages.
The hydroxide anion adds to the carbonyl group of the ester. The immediate product is called anorthoester.
Expulsion of the alkoxide generates a carboxylic acid:
The alkoxide ion is astrong base so the proton is transferred from the carboxylic acid to the alkoxide ion, creating an alcohol:
In a classic laboratory procedure, the triglyceridetrimyristin is obtained by extracting it fromnutmeg withdiethyl ether. Saponification to the soap sodium myristate takes place using NaOH in water. Treating the soap withhydrochloric acid givesmyristic acid.[4]
The reaction of fatty acids with a base is another main method of saponification. In this case, the reaction involves neutralization of thecarboxylic acid. The neutralization method is used to produce industrial soaps, such as those derived from magnesium, transition metals, and aluminium. This method is ideal for producing soaps derived from a single fatty acid, resulting in soaps with predictable physical properties, as required by many engineering applications.
Depending on the nature of the alkali used in their production, soaps have distinct properties.Sodium hydroxide (NaOH) produces"hard" soaps; hard soaps can also be used in water containing Mg, Cl, and Ca salts.[5] By contrast,potassium soaps (derived usingKOH) are"soft" soaps. The fatty acid source also affects the soap's melting point. Most early hard soaps were manufactured usinganimal fats and potash (KOH) extracted fromwood ash; these were broadly solid. However, the majority of modern soaps are manufactured frompolyunsaturated triglycerides such asvegetable oils. As with the triglycerides they are formed from,[6] the salts of these acids have weaker inter-molecular forces and thus lower melting points.
Lithium 12-hydroxystearate and otherlithium-based fatty acids are important constituents of lubricating greases. Inlithium-based greases, lithiumcarboxylates are thickeners. "Complex soaps" are also common, these being combinations of more than one acid salt, such asazelaic oracetic acid.[7]
Fires involvingcooking fats and oils (classified asclass K (US) or F (Australia/Europe/Asia)) burn hotter than most flammable liquids, rendering a standardclass B extinguisher ineffective. Such fires should be extinguished with awet chemical extinguisher. Extinguishers of this type are designed to extinguish cooking fats and oils through saponification. The extinguishing agent rapidly converts the burning substance to a non-combustible soap.
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Saponification can occur inoil paintings over time, causing visible damage and deformation. Oil paints are composed of pigment molecules suspended in an oil-bindingmedium. Heavy metal salts are often used as pigment molecules, such as inlead white,red lead, andzinc white.[8] If those heavy metal salts react withfree fatty acids in the oil medium, metal soaps may form in a paint layer that can then migrate outward to the painting's surface.[9]: 12–19
Saponification in oil paintings was described as early as 1912.[10]: 151 It is believed to be widespread, having been observed in many works dating from the fifteenth through the twentieth centuries; works of different geographic origin; and works painted on various supports, such as canvas, paper, wood, and copper. Chemical analysis may reveal saponification occurring in a painting's deeper layers before any signs are visible on the surface, even in paintings centuries old.[9]: 16
The saponified regions may deform the painting's surface through the formation of visible lumps or protrusions that can scatter light. These soap lumps may be prominent only on certain regions of the painting rather than throughout. InJohn Singer Sargent's famousPortrait of Madame X, for example, the lumps only appear on the blackest areas, which may be because of the artist's use of more medium in those areas to compensate for the tendency of black pigments to soak it up.[9]: 12–13, 15 The process can also form chalky white deposits on a painting's surface, a deformation often described as "blooming" or "efflorescence", and may also contribute to the increased transparency of certain paint layers within an oil painting over time.[9]: 16, 19
Saponification does not occur in all oil paintings and many details are unresolved.[9]: 19 At present, retouching is the only known restoration method.
... and although Petit declares this theory false, it is none the less on it and on its data that he bases his system of manufacture of hydrated white zinc, of which he is the inventor that is to say, the saponification of the oil, or the formation of metallic salts, dissolved therein.
