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9.5 Industrial Chemistry: 5. Saponification

Syllabus reference (October 2002 version)
5. Saponification is an important organic industrial process	Students learn to: describe saponification as the conversion in basic solution of fats and oils to glycerol and salts of fatty acids describe the conditions under which saponification can be performed in the school laboratory and compare these with industrial preparation of soap account for the cleaning action of soap by describing its structure explain that soap, water and oil together form an emulsion with the soap acting as an emulsifier distinguish between soaps and synthetic detergents in terms of: the structure of the molecule chemical composition effect in hard water distinguish between anionic, cationic and non-ionic synthetic detergents in terms of chemical composition uses	Students: perform a first-hand investigation to carry out saponification and test the product gather, process and present information from secondary sources to identify a range of fats and oils used for soap-making perform a first-hand investigation to gather information and describe the properties of a named emulsion and relate these properties to its uses perform a first-hand investigation to demonstrate the effect of soap as an emulsifier solve problems and use available evidence to discuss, using examples, the environmental impacts of the use of soaps and detergents

Extract from Chemistry Stage 6 Syllabus (Amended October 2002). © Board of Studies, NSW.

[Edited: 25Jun 08]

Prior Learning: HSC modules: 9.3.5 , 9.4.5

Background information: Revise the meaning of the following terms in Chemistry 9.3.5

homologous groups, functional groups, alkanol, alkanoic acid, ester, double bonds, triple bonds, saturated compound, unsaturated compound, esterification, alkyl group.

Glycerol is an alkanol with 3 hydroxy groups and the formula CH₂OHCHOHCH₂OH. Its systematic name is 1,2,3-propanetriol.

Esters are carbon compounds with the general formula RCOOR^' where R and R' are alkyl groups. Esters can be made by the reaction of an alkanol and an alkanoic acid.

alkanol   +   alkanoic acid                     ester   +    water 

Fats and oils are esters made from glycerol (1,2,3-propanetriol) and long chain fatty acids such as stearic acid (CH₃(CH₂)₁₆COOH). Different acids combined with glycerol produce different fats and oils

gather, process and present information from secondary sources to identify  a range of fats and oils used for soap-making

• Gather information by looking at web sites and in books and magazines.

  For an idea of the range of fats and oils that can be used, and some ideas to try yourself, look at Walton Feed, Montpelier, Idaho, USA.

  Most soap is made from vegetable oils, especially olive, palm and coconut oils. Some is made from animal fats, called tallows.

  When you have enough raw information, you could organise it in a table such as:

  Fat/ oil	Type of soap made

  
  
  
• Present the information on a chart, an overhead transparency or as a power point presentation.

perform a first-hand investigation to carry out saponification and test the product

• There are lots of ways to carry out this experiment. The following suggestion is so simple you could even do it at home.

  What you will need:

  • glass container with lid, about 200 mL size
  • 100 mL warm water
  • about 40 g washing soda (Na₂CO₃.10H₂O)
  • about 25 mL oil e.g. olive oil, coconut oil, vegetable cooking oil
  • safety goggles.

  Safety

  You are using an alkaline solution, about pH 12, so you must wear eye goggles and you must work near a tap so that a supply of running water is available. If alkali is splashed on the skin or eyes, you must wash it off immediately and continue washing for 15 minutes.

  Method

  1. Pour 100 mL warm water into the container.
  2. Add washing soda.
  3. Put lid securely on container and shake until it dissolves.
  4. Add 25 mL oil. Observe and record observations.
  5. Replace the lid and shake for 1 minute. Observe and record observations.
  6. Replace the lid and shake for 5 minutes. Observe and record observations.

  You have produced a mixture, do the components settle into layers on standing?

  Can you identify the components? The test for soap is quite simple - it forms a lather when you shake it with water.

  Can you see any problems with your soap? How could you improve on this method?

describe  saponification as the conversion in basic solution of fats and oils to produce glycerol and salts of fatty acids

• Saponification is the conversion in basic solution, of fats and oils to produce glycerol and salts of fatty acids. This is one way of making soap.

  Fat or oil+conc. NaOHglycerol+sodium salt of a fatty acid (soap)
  Fat or oil+conc.KOH glycerol+potassium salt of a fatty acid (soap)

  One naturally occurring fat is glycerol tristearate. When this is heated with a base such as sodium hydroxide, conversion occurs forming glycerol and a salt that is soap.

  

  The University of Sydney Web site that you used for polymers has good, relevant information on this topic.

describe the conditions under which saponification can be performed in the school laboratory and compare these with industrial preparation of soap.

• In the laboratory

  Soap is made by heating a mixture of an oil, such as olive oil, with sodium hydroxide solution.

  The soap produced can be precipitated by adding concentrated sodium chloride solution, then washed to remove the glycerol and excess sodium hydroxide. Dilute hydrochloric acid may be used to neutralise the excess alkali.

• Industrial preparation of soap

• Industrially, soap can be prepared in 1 stage (Kettle Boiled Batch Process) or 2 stages (hydrolysis then neutralisation). This can also occur in the school laboratory. The University of Sydney’s Key Centre for Polymer Colloids (KCPC) site describes the industrial process and how it differs from school laboratory procedures.

  A good way to compare processes like this is to draw up a table showing similarities and differences between them. You could use the above link to do this.

perform a first-hand investigation to gather information and describe the properties of a named emulsion and relate these properties to its uses

• An emulsion is a mixture of two liquids that are dispersed and suspended in one another. Neither liquid will dissolve in the other.
  
  
• Examples of emulsions are:

  Emulsion	Contents - emulsions of:
  milk	fat droplets in water. (Proteins are the natural emulsifiers in milk.  Additional emulsifiers can be added to milk to help keep the fat suspended and prevent it floating to the top as a cream layer.)
  mayonnaise	oil, water and vinegar, with egg added to prevent it separating into layers.
  cosmetic creams	oil and water (other chemicals added for perfume and colour).
  paints	pigments, solvents and polymers.

  
  
  
• Make your own emulsion, you will find information on the making of emulsions and some tips on how to make them in the Senior Science Module 9.2.2.

  Choose an emulsion, study its properties and relate these to its uses.

• Safety Issues

  Before carrying out your investigation you must identify all safety issues and record the safe work practices you intend to use.

  Physical hazards here will depend on the emulsion you decide to make or test. They could include the use of heaters, blenders and boiling water. With emulsions that are to be eaten or used on the skin you must prevent the growth of microbes - consider cleanliness and preserving ingredients.

• Properties of the emulsion

  Select properties that are related to the use of the emulsion. For many emulsions you will want to look at how long it lasts (without settling out into layers), aesthetic properties such as its appearance, colour, creaminess and whether it feels greasy, sticky or oily. You may also like to look at whether a dye, such as food colouring, spreads evenly through the emulsion. A water soluble dye will spread through an oil in water emulsion, but not through a water in oil emulsion as you can see at the KCPC site.

perform a first-hand investigation to demonstrate the effect of soap as an emulsifier.

	Method 1	Method 2
Method	Add 1 mL oil to 5 mL water in a test tube. Stopper the test tube. Shake for 10 seconds. Stand for 10 minutes.	Add 1 mL oil to 5 mL water in a test tube. Stopper the test tube. Add 5 mL soap solution. Shake for 10 seconds. Stand for 10 minutes.
Result

Describe the effect of the soap. Did it help to keep the oil dispersed through the water?

account for  the cleaning action of soap by describing its structure.

explain  that soap, water and oil together form an emulsion with the soap acting as an emulsifier.

These two outcomes can be considered together.

The cleaning action of soap can be explained by its structure which allows it to act as an emulsifier. Look again at the KCPC , Key Centre for Polymer Colloids, University of Sydney, NSW. website.

• Most dirt is non-polar. Grease consists mostly of long chain, non-polar hydrocarbons. However, water is polar, so it will not dissolve this non-polar dirt and grease.
  
  
• When soap dissolves in water, the ions making up the soap dissociate:

  RCOO^–Na⁺ (s)           RCOO^–(aq)   +   Na⁺ (aq)

  The negative fatty acid ion is a surfactant (surface acting agent). The positive ion plays no part in cleaning.

• Surfactants lower the surface tension of water, by disrupting hydrogen bonds between water molecules, and thus increase its ability to wet a surface.

  Water does not wet grease very well.	Water with surfactants spreads out over the grease, wetting it.

  
  
  
• The fatty acid anions (surfactants) in soaps have a long, non-polar tail, consisting of a hydrocarbon chain, and a polar, anionic (negatively charged) head.

  	The non-polar tail is hydrophobic, which means that it prefers to be away from water. The polar head is hydrophilic, which means that it is attracted to water.

  
  
  
• When surfactants are added to water, they do not spread evenly through the water, instead they clump together, with the negative heads pointing outwards. The negative ends interact with polar water molecules and the whole clump stays suspended in the water, forming an emulsion rather than a solution.

  Surfactants clump together and stay suspended in water.	Non-polar grease molecules are taken into the non-polar centre of the clump.

  
  
  
• Surfactants help to remove the dirt.

  	The tails dissolve in the greasy dirt and the heads dissolve in water, drawing water onto the dirt and fabric. As the water is swirled around it pulls the grease out of the fabric.

  
  
  
• Surfactants keep the grease suspended in the water.

  	Keeping the grease suspended means it can be carried away by the water. Soap, water and grease together form an emulsion, with the soap acting as an emulsifier, suspending the normally incompatible grease in the water.

distinguish between  soaps and synthetic detergents in terms of:

• the structure of the molecule
• chemical composition
• effect in hard water

The word detergent means a cleaning agent. Detergents, like soaps, contain surfactants(surface acting agents) which help to clean.

• Soaps and synthetic detergents both have water soluble and oil soluble ends and both clean in the same way (see above). They can be distinguished by the structureof their molecules, their chemical composition and their effect in hard water.
  
  

  	Soaps	Detergents
  Made from	fatty acids in animal and vegetable oils	hydrocarbon chain from petroleum
  Composition	sodium or potassium salts of long chain (alkanoic) fatty acids	usually hydrocarbons with a sulfate or sulfonate end
  Structure	ionic or polar head & long, non-polar hydrocarbon tail. anionic	similar structure to soap - head & non-polar hydrocarbon tail may be anionic, cationic or non-ionic
  Manufacture	saponification - heating fats or oils (esters) with NaOH or KOH< - precipitation with sodium chloride	alkanol from petroleum is reacted with H2SO4 to form sulfonic acid this is reacted with NaOH to form sodium sulfonate
  Reaction with hard water	do not lather well in hard water soap anions form precipitates with cations e.g. Ca2+ and Mg2+ in hard water This forms a scum in the water and on clothes, making clothes dull and grey	lather in hard water do not precipitate mineral salts in hard water
  Biodegradability	biodegradable	biodegradable if hydrocarbon chain is straight. non-biodegradable if branched chain.
  Phosphates	no phosphates	may be mixed with phosphates that pollute the environment.
  Other	cheaper to make not very soluble deteriorate with age.	more expensive soluble in water do not deteriorate with age, very stable.

distinguish between anionic, cationic and non-ionic synthetic detergents in terms of

• chemical composition
• uses

Look at the University of Sydney’s Key Centre for Polymer Colloids ( KCPC ) site

• Surfactant molecules in detergents can be anionic, cationic or non-ionic.

  Anionic	Cationic	Non-ionic

  Anionic surfactants are the most widely used detergents. They are used in dishwashing liquids and laundry detergents. Their particles have a negatively charged head. The most common ones have a long hydrocarbon end, obtained from petroleum, and the ionic end is a sulfate (SO₄^2–) ion or a sulfonate (SO₃^–) ion. The hydrocarbon end has a special ring structure made of 6 carbon atoms, called a benzene ring, so they are called alkyl benzene sulfonates or sulfates. Anionic surfactants are highly sudsing and have excellent cleaning properties, especially for fabrics that absorb water readily e.g. cotton, wool and silk.

  

  Cationic surfactants are detergents made of particles with a positively charged head. They are usually ammonium compounds. They are used as cleaners, fabric softeners (their positive charge adheres to fabrics that usually carry negative charges, reducing static) and as germicides (ammonium ions disrupt the cell walls of some pathogenic bacteria) in mouthwashes, nappy washes and antiseptic soaps. They are not used in dishwashers as glass has a negatively charged surface, which attracts the positive heads, leaving the tails to make the glass slippery.

  

  Non-ionic surfactants have a hydrophilic end with many oxygen atoms that form hydrogen bonds with water. They do not ionise in water and are low sudsing. They are used as detergents for the laundry, for automatic dishwashers and for washing cars. They are also used in cosmetics and froth flotation.

  You might like to summarise this information in the form of a table.

solve problems and use available evidence to discuss, using examples, the environmental impacts of the use of soaps and detergents.

Information from module 9.4.5 might help you here.

Points you should mention in your discussion include:

• Biodegradability (soap most biodegradable, anionic detergents usually fairly biodegradable and can precipitate out with cations, non-ionic detergents are low-sudsing and usually biodegrade, cationic detergents can kill some of the microbes that biodegrade the detergent so are the least biodegradable).
• Presence of phosphates.
• Entry into and effects on waterways including eutrophication.
• Measures to overcome problems.