How many grams of potassium hydroxide are needed to react with 1 gram of tri-

oleine?

Q: How many grams of potassium hydroxide are needed to react with 1 gram of tri-

stearine?

Q: How many grams of potassium hydroxide are needed to react with 1 gram of tri-

palmitine?

Q: How many grams of potassium hydroxide are needed to react with 1 gram of tri-

oleine?

The answer to such a problem, the ratio of alkali to fat, is called a saponification value and it is important to get this number right. If you use less than the stoichiometric amount of alkali, not all of your fat will be saponified. Not only will you get less soap than you might have, but that soap will have to dissolve the un-saponified fat in addition to that on your pots and pans. If you use more than the stoichiometric amount of alkali, not only will you have wasted alkali, but the leftover alkali will saponify your hands. If you are to get the most soap for your money, if you are to get the mildest soap for your delicate skin, it is important to get the saponification value, as Goldilocks might have said, just right.

The problem is that you are unlikely to find tri-oleine, tri-palmitine, or tri-stearine in the grocery store; fats and oils generally contain a variety of fatty acids, chiefly oleic, palmitic, and stearic acids but including a dozen or so other less abundant acids as well. How could you determine the saponification value for a fat of unknown composition? You could make a series of soaps, varying the ratio of alkali to fat, and then testing the finished soaps for excess alkali using pH test paper. The best ratio of alkali to fat would be the one which completely saponified the fat without leaving excess alkali. This optimum alkali/fat ratio would be the saponification value for the unknown fat.

Table 19-1 lists saponification values for a variety of common fats and oils. Compare your calculated saponification values for tri-oleine, tri-palmitine, and tri-stearine to those in the table. In practice, fats and oils vary in composition and it is better to risk having excess fat rather than excess alkali. The saponification values are generally discounted by 5% or so; simply multiply each saponification value by a factor of 0.95. You'll get a little less soap than you might have, but that's a small price to pay for soap that doesn't eat your skin off.

Material Safety

Locate an MSDS for sodium hydroxide (CAS 1310-73-2). Summarize the hazardous properties, including the identity of the company which produced the MSDS and the NFPA diamond for this material.[1]

Your most likely exposure will be eye or skin contact. In case of eye contact flush them with cold water and call for an ambulance. In case of skin contact wash the affected area with plenty of water until your skin no longer feels slippery.

You should wear safety glasses and gloves while working on this project. Leftover sodium hydroxide can be flushed down the drain with plenty of water. Leftover fat or oil can be thrown in the trash.

Research and Development

So there you are, studying for a test, and you wonder what will be on it.

• Study the meanings of all of the words that are important enough to be included in the index or glossary.

• You should know all of the Research and Development points from Chapter 14 and Chapter 15.

• You should know the reactions of Equation 19-1.

• You should know the meaning of the adage "Like dissolves like."

• You should know why fatty acid salts dissolve in both water and in oils and fats.

• You should know the hazardous properties of sodium and potassium hydroxide.

• You should know how soap emulsifies fat.

Notes

[1] The NFPA diamond was introduced Section 15.2. You may substitute HMIS or Saf- T-Data ratings at your convenience.

19.3.

At the dawn of the twenty-first century there remains in the world a small but dedicated sub-culture of people who are not afraid to experience the joys of manufacturing; a hardy folk for whom the word chemical is not equated with "evil poison foisted on unsuspecting innocents by heartless multi-national corporations;" a people who recognize that something as good and wholesome as soap requires for its manufacture something as caustic and poisonous as caustic soda. I speak, not of industrial or laboratory chemists, but of soapers—crafters who in an age of pre-packaged conveniences indulge in the simple pleasure of making soap from scratch.

The first choice of the soap-maker is that of which fat to saponify. You can use any fat or oil derived from animal or vegetable sources; mineral and motor oils are not triglycerides and so are not useful for making soap. Among animal fats, you may choose tallow rendered from beef, lard from pork, or suet from goat. Butter and bacon grease make perfectly functional soaps, though they will win no prizes for their aromas. Among vegetable oils, olive oil has been the traditional choice for fine soap. Palm oil is renowned for its rich lather. Other vegetable oils are often used in combination with tallow or lard rather than by themselves. Even vegetable shortening and margarine can be used, though the soft, whipped, or tub margarines contain excess water, complicating the calculation for the amount of caustic soda to be used. Each fat lends its own particular qualities to the finished soap and experienced soapers blend different oils to produce the qualities they desire, but for a first soap I recommend using any of those listed in Table 19-1.

The next choice of the soap-maker is that of which alkali to use. Potassium hydroxide will produce a soft or liquid soap, while sodium hydroxide will produce the familiar solid bar. Either of these alkalis might be manufactured from scratch, using potash or soda from Chapter 8 and lime from Chapter 10 to produce caustic soda or caustic potash as described in Chapter 15. Recall from Table 8-1, however, that 1000 pounds of wood provide only about a pound of potash. Consider as well that caustic soda is sold as a drain opener, and so it is frequently available in grocery stores and hardware stores. For these reasons, store-bought caustic soda (lye) is the choice of most soapers.

You want 100% sodium hydroxide; name-brand drain openers contain perfumes or other ingredients which may interfere with saponification. Many grocery stores have removed lye from their shelves in the mistaken belief that it is more dangerous than the name- brand drain openers. And while lye deserves all of the dire warnings on its label, you will find the same warnings on the name brands because they are almost universally either sodium hydroxide or sulfuric acid. You would do the world of soapers a service by pointing this out to grocery store managers when the opportunity arises; we all benefit from cheap and convenient sources of lye.

The traditional method for making soap, the hot process, involves leaching ashes to get alkali, causticizing the alkali with lime to make caustic soda, and boiling the resulting lye with fat to make soap. It turns out that the boiling step has less to do with saponification than with removing bejeezical water to concentrate the lye. We can skip the boiling step if we start with solid caustic soda. This cold process is the method preferred by the majority of modern amateur soap-makers.

For your first soap, I recommend a cold process using that container of choice for caveman chemists, the 2-liter pop bottle, which will accommodate between one and two pounds of fat. Weigh out your fat on a balance and record the weight in your notebook. Place it in a beaker or a saucepan and warm it on a hot-plate until it melts (if solid) or until it is warm to the touch (if liquid). While your fat is warming, use UFA and Table 19-1 to answer the following question: