JULY, 1948
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POTASSIUM SOAP--SOFT OR HARD? ARTHUR
r. NELSON
State Teachers College, St.
POTASSIUM soap is described in most chemistry texL7 used in high schools and for General Chemistry and Organic Chemistry in $alleges and universities as soft soap while in others it is stated that liquid soaps are usually potassium soaps or potassium soaps are dispensed as liquid soaps. While the melting points of many of the sodium salts of organic acids are higher than those of the corresponding potassium salts, the melting points of sodium stearate and sodium palmitate are so high that it would seem improbable that the potassium salts of these acids and other fatty acids found in soap would be liquid or even of soft consistency a t room temperature. Many liquid soaps are dissolved sodium soap yet not described as soft soaps. Potassium soaps were prepared from several common soaps, such as Lux, Ivory, Ivory Flakes, and Swan, and in all cases the hardness of.the resulting soap was comparable with the original soap. Potassium soaps are usually disp&nsed as liquid or soft soap, because it is more difficult to separate this soap from its soap lye than it is to separate the more common sodium soap from its lye. There are two reasons for this: first, potassium salts are, in general, more soluble than sodium salts and this is also true of the soaps, and second, it would require large quantities of potassium chloride, a salt less abundant and more costly than sodium chloride, to salt it out. To prepare a small amount of potassium soap, half a cake of any ordinary bar soap or half a box of soap flakes is dissolved in a liter of boiling water in a 2-liter beaker. If bar soap is nsed as the starting material it should be sliced into thin shavings. When the soap is dissolved, glass beads or boiling chips and a few drops of methyl orange are added, followed by concentrated hydrochloric acid in slight excess. The acid solution containing the fatty acids is boiled until the fatty acids form a clear layer on the surface of the water solution. The fatty acids are then separated from the major por-
Cloud, Minnesota
t'ion of the acid water solution. This may be accomplished conveniently by the use of a siphon made by bending a 20-inch length of glass tubing 6 to 10 mm. 0. D. and attaching to it a 20-inch length of rubber tubing. It is wise to place two short pieces of rubber tubing, one over the other, on the siphon to insure h'andling it without too great discomfort while siphoning off the hot acid solution. The fatty acids should be washed twice by boiling with hot water for several minutes, separated carefully from the final wash water, and placed in a 400-ml..beaker. The fatty pcids are now.neu5ralized with potassium hydroxide, the approximate amount of which may he calculated from the quantity of hydrochloric acid required for the liberation of the fatty acids. This amount should be increased by 20 per cent and dissolved in 95 per cent ethyl alcohol, 15 grams per 100 ml. The beaker containing the fatty acids is then placed in a boiling water bath and the alcoholic potassium hydroxide solution added gradually. After three fourths of it is added the alcoholic soap solution should be tested after each addition by taking a drop of it on a clean stirring rod and mixing it with two drops of phenolphthalein solution on a white porcelain color reaction plate. When the.test gives a slight pink coloration, the potassium hydroxide is present in sligh: excess. The heating of the alcoholic soap solution is continued with stirring to evaporate the alcohol until it is quite viscous, when it may be transferred to a mold of desired shape and size or left in the beaker to solidify on cooling. The soap prepared in this manner will be discolored due to the removal of the antioxidants from the soap and oxidation by atmospheric oxygen. If the neutralization of the fatty acids and evaporation of the alcohol is performed under vacuum, the soap will not be so discolored, but in any case, the potassium soap will be hard.
