10.2 × F N/10 NaOH (0.996) = Factor N/40 stearic acid
∴Factor N/40 stearic acid = 1.016.
Third—About 5 grams of soap are weighed and dissolved in 100 cubic centimeters of 50 per cent. neutralized alcohol in a 250 cubic centimeter Erlenmeyer flask over a water bath and connected with a reflux condensor. When completely dissolved, which takes but a few moments, it is cooled by allowing a stream of running water to run over the outside of the flask.
Fourth—The soap is precipitated with 15 to 20 cubic centimeters of the 10 per cent. barium chloride solution.
Fifth—After the addition of 2 to 5 cubic centimeters of α naphtholphthalein solution the solution is titrated with N/40 alcoholic stearic acid. α naphtholphthalein is red with an excess of stearic acid. To mark the color changes it is advisable to first run a few blanks until the eye has become accustomed to the change in the indicator in the same way. The change from green to red can then be carefully observed.
Let us presume 5 grams of soap were taken for the analysis and 20 cubic centimeters of N/40 stearic acid were required for the titration then to calculate the amount of NaOH since the stearic factor is 1.016.
20 × 1.016 = 20.32 N/40 stearic acid really required.
1 cubic centimeter N/40 stearic acid = 0.02 per cent. NaOH for 5 grams soap.
Δ 20.32 cubic centimeters N/40 stearic acid = 0.02 × 20.32 per cent. NaOH for 5 grams soap.
Hence the soap contains 0.4064 per cent. NaOH.