FREE ALKALI OR ACID.
(a) Alcoholic Method.
Test a freshly cut surface of the soap with a few drops of an alcoholic phenolphthalein solution. If it does not turn red it may be assumed free fat is present; should a red color appear, free alkali is present. In any case dissolve 2 to 5 grams of soap in 100 cubic centimeters of neutralized alcohol and heat to boiling until in solution. Filter off the undissolved portion containing carbonate, etc., and wash with alcohol. Add phenolphthalein to the filtrate and titrate with N/10 acid and calculate the per cent. of free alkali as sodium or potassium hydroxide. Should the filtrate be acid instead of alkaline, titrate with N/10 alkali and calculate the percentage of free fatty acid as oleic acid.
The insoluble portion remaining on the filter paper is washed with water until all the carbonate is dissolved. The washings are then titrated with N/10 sulfuric acid and expressed as sodium or potassium carbonate. Should borates or silicates be present it is possible to express in terms of these. If borax is present the carbon dioxide is boiled off after neutralizing exactly to methyl orange; cool, add mannite and phenolphthalein and titrate the boric acid with standard alkali.
(b) Bosshard and Huggenberg Method.[14]
In using the alcoholic method for the determination of the free alkali or fat in soap there is a possibility of both free fat and free alkali being present. Upon boiling in an alcoholic solution the fat will be saponified, thus introducing an error in the analysis. The method of Bosshard and Huggenberg overcomes this objection. Their method is briefly as follows:
Reagents.
1. N/10 hydrochloric acid to standardize N/10 alcoholic sodium hydroxide.
2. Approximately N/10 alcoholic sodium hydroxide to fix and control the N/40 stearic acid.
3. N/40 stearic acid. Preparation: About 7.1 grams of stearic acid are dissolved in one liter of absolute alcohol, the solution filtered, the strength determined by titration against N/10 NaOH and then protected in a well stoppered bottle, or better still connected directly to the burette.