Preparation of Soap

This is a favorite exercise of lab manuals. Many of the ones I've seen use ethanol to aid solution during saponification, which can be hazardous, and concentrated salt to precipitate the product, which is unnecessary. We use the procedure we found in a free pamphlet from Lewis Red Devil, who make grocery-store lye.

You start by making a fairly concentrated sodium hydroxide solution. According to the recipe, dissolve 340 g (12 oz.) NaOH in 1.18 L (2.5 pints) soft water, approximately 7 M. In our scaled-down prep, the student uses 12 mL of NaOH and 27 g of fat. The student warms the fat in a beaker set into a larger beaker containing water (a "double boiler") to avoid overheating, to a temperature appropriate to the fat used. For shortening such as Crisco, they use 45 degrees; for lard they use 35 degrees. This procedure also works with olive oil, for which heating isn't even necessary. Though the resulting soap is a little soft, it holds its shape well.

They then add the NaOH very slowly with steady stirring using a tongue depressor or popsicle stick. It takes at least ten minutes for the brew to thicken up. One drop of food coloring or fragrance is added here, and stirring is continued until the stuff piles up when dripped back into the beaker. It is then poured, or scraped, into a small plastic drinking cup and left in the drawer to cure for a week.

The write-up for this exercise is pretty subjective. My general aim is to mesh with stuff we are talking about in lecture, so I asked them (1) why we refer to soap as a "salt", (2) what kinds of molecular properties enable substances to dissolve in water, leading to (3) their guess as to whether the glycerol produced is soluble in water (apparently not obvious), and (4) to show the structures of the products expected from saponification of ethyl acetate (whose structure I give them; with the breadth of material to cover, I'm not too intense on nomenclature).

 

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Revised 8/31/06