Summary
The pH of the laurates, myristates, palmitates, and stearates of sodium and potassium at 25° and 50°C. was determined by use of a glass electrode. pH:concentration curves were constructed, and hydroxyl ion activity was calculated. In general, the concentration of hydroxyl ion ranges from 0.001 to 0.0001 N; for the less dilute solutions of the higher soaps, the upper limit is exceeded by several fold, and in the more dilute solutions the concentration may fall beneath the lower value. Solubilized hexane reduced the pH of potassium and sodium laurate but very slightly. Potassium or sodium chloride reduced the pH of soap solutions over a certain range but caused a slight increase in a narrow intermediate range.
Using the pH values obtained, the actual concentration of fatty acid in the soap solutions was calculated, and was found to beless than the saturation concentrations obtained by conductivity measurements through the entire range investigated. Free fatty acid therefore never separates as such from pure soap solutions unless acted upon by excess of acid such as carbon dioxide.
Percentage hydrolysis was calculated and curves were constructed. In general, the potassium soaps are hydrolyzed more than the corresponding sodium soaps; the difference is slight for the laurates but is considerably greater for the myristates, palmitates, and stearates in higher concentrations at 25°C. The difference is appreciably smaller at 50° than at 25°. Percentage hydrolysis for the laurates is very much less than for the higher soaps.
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McBain, J.W., Laurent, P. & John, L.M. The hydrolysis of soap solutions. III. Values of pH and the absence of fatty acid as free liquid or solid. J Am Oil Chem Soc 25, 77–84 (1948). https://doi.org/10.1007/BF02579731
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DOI: https://doi.org/10.1007/BF02579731