80 SOAPS AND PROTEINS 



of the system) remains uncolored while the contents of this frame- 

 work (the soap-in-water portion) turns bright red. A drop of 

 phenolphthalein solution dropped upon a 10 percent sodium 

 stearate/ water gel remains uncolored. If, however, the gel is 

 slightly squeezed (which breaks the encircling hydrated sodium 

 stearate film and squeezes out the enclosed solution of soap-in- 

 water) the spot turns bright-red. Any other solid soap/ water 

 system behaves in similar fashion. 



Another variant of the experiment may be made by warming 

 a concentrated sodium oleate or other soap solution which at 

 ordinary temperature fails to color to phenolphthalein. Such a 

 mixture, on being warmed, turns pink. While it is ordinarily 

 said that under such circumstances the hydrolysis of the soap is 

 increased, it is equally true that such a temperature change marks 

 a displacement in the system from a solution of the solvent in the 

 soap to one of the soap in the solvent. 



To make sure, for experimental purposes, of definitely " acid " 

 or " alkaline " soaps we have added to our chemically " neutral " 

 soaps (like molar sodium oleate) known and large surpluses of free 

 fatty acid or alkali. When free fatty acid is added it emulsifies 

 readily in the soap, yielding a mixture more viscid than the origi- 

 nal soap gel and practically as transparent as the original sodium 

 oleate. Phenolphthalein added to the mixture remains colorless. 

 Still, when water is added to such an obviously " acid " soap, 

 the mixture turns pink or bright red as the added water is 

 increased. The opposite type of experiment may be made by 

 adding an excess of sodium hydroxid to the sodium oleate. 

 Under such circumstances the mixture may assume a pinkish 

 tinge, but this is because the excess of sodium hydroxid is hydrated 

 and separates out in emulsified form in the chemically " neutral " 

 sodium oleate. 1 It is not the hydrated soap but the hydrated 

 sodium hydroxid which turns pink. When instead of sodium 

 hydroxid, sodium chlorid is used, such pinking of the system does 

 not follow. Hydration of the neutral salt occurs and the mixture 

 becomes more viscid, just as when sodium hydroxid is added, but 

 since sodium chlorid is neutral and since the soap is not soluble in 

 the salt-water no change in the color of the indicator becomes 

 manifest. 



1 See page 93 on the salting out of soaps. 



