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Soap Making Archive
[This is where we store technical data that has been previously published in our soap making webpage.
Technical and Scientific Information
C : Carbon, atomic weight 12.010; O : oxygen, atomic weight 16.0000
H : hydrogen, atomic weight 1.0080 K : potassium, atomic weight 39.096
Na : sodium, atomic weight 22.997 OH : hydroxide
KOH : potassium hydroxide, molecular weight 56.104
NaOH : sodium hydroxide, molecular weight 40.005
Lye: a hydroxide dissolved in water; typically NaOH but it could be a hydroxide of another metal
Saponification number is the amount of potassium hydroxide, in grams, required to make soap with a 1,000 grams of a given fatty acid or natural fat or oil. A more useful measure is the sodium hydroxide saponification number which is the amount of sodium hydroxide required to make soap with 1,000 grams of a given fatty acid or natural fat or oil. Since natural fats and oils are mixtures of fatty acids, it becomes an academic exercise to calculate the saponification number for the component fatty acids. The saponification numbers are offered below and are based upon common fats and oils.
Fat or Oil Sodium Saponification No. (K) Saponification No.
Almond (Sweet) oil 137.3 192.5
Apricot kernel oil 135.5 190
Avocado oil 133.7 187.5
Beef tallow 140.5 197
Babassu oil 176.1 247
Castor oil 128.6 180.3
Coconut oil 191.1 268
Jojoba oil 69.5 97.5
Kukui nut oil 135.5 190
Lard (oil) 138.8 194.6
Olive oil 135.3 189.7
Palm oil 142 199.1
Palm Kernel oil 156.8 219.9
Peanut oil 137.0 192.1
Shea butter (African karite butter) 128.3 180
Soybean oil 135.9 190.6
Waste Kitchen fat* 135.5 190
Wheatgerm oil 131.9 185
*This is a saponification number based on an estimated mixture of waste fats. If you have knowledge of the composition of the mixture, you can adjust the number to suit. Waste kitchen fat is a combination of used cooking oils, drippings from roasting and broiling, and rendered fat from frying and usually contains salt, sugar, sodium phosphate, sodium nitrite, etc. A glance at the ingredient list on the label of prepared meats will give you an idea of the contents of the waste fat. This mixture needs to be cleaned up and that is accomplished by straining, settling and skimming plus mixing with hot water. Boiling the water-fat mixture is not necessary but diluting out the salts from the fat with water works best at higher temperatures. Changing the water several times is required. [The writer uses three changes of water and allows the debris to settle to the bottom of the water or float to the top (of the water at the interface between the fat and water). By allowing the fat to solidify by cooling, the debris can be removed by scraping. Store the fat, which is both freshly rendered and cleaned, in the freezer, otherwise, it will become rancid and the odor will carry over into soap made from rancid fat.
(C17 H35COO)3 C3H5 + 3NaOH => 3C17H35COONa + C3H5(OH)3 [This is the reaction for stearin, an ester of stearic acid; beef tallow contains 75% palmitin and stearin and 25% olean. A weight balance calculation reveals a ratio of 7.4278etc. of stearin to 1 of sodium hydroxide or if reversed, a ratio of NaOH of .1346289etc. to 1 of stearin. If this latter ratio is multiplied by 1,000 we get the sodium hydroxide saponification number of 134.63, which compares closely with the sodium hydroxide saponification numbers offered above for similarly mixed fats. The products of the chemical equation, above, are sodium stearate and glycerin, respectively. Sodium stearate is a soap.]