Determination of Total Fatty Matter of Soaps

This test method determines the total anhydrous soap and oils. This method applies to soaps soap products and synthetic detergents.


(a) Beakers 400 ml, and Soxhlet flask 500 ml.

(b) Separating Funnels 500 ml

(c) Extraction cylinder or Stokes flask

(I) Extraction cylinder 250 ml capacity and glass stopper 39 mm. diameter and 355 mm height.

(II) The Stokes flask is a 100 ml round bottom flask sealed onto a 150 ml Erlenmeyer flask.  

(d) Siphon a 2-hole silicon rubber stopper fitted with small diameter glass tubing constructed to permit removing the top layer from the extraction cylinder or Stokes flask.

Total Anhydrous Soap


I. Petroleum ether (AR) 20:80

II. Ethyl alcohol 96% ( Neutralize)  

III. Hydrochloric Acid (1:1). Dilute 50 ml of HCL ( Sp. gravity 1.19) with 50 ml of distilled water.

IV. Sodium hydroxide (NaOH) Solution of 1.0 Normality with standardized.

V. 1.0 % of Phenophthalein  Indicator in 96% alcohol.

VI. 0.1 % Methyl Orange indicator in distilled water.

VII. Sodium Chloride saturated solution in distilled water.


Weight a sample of such size that it will yield 1.0 to 5.0 gm of fatty acids. The sample may be placed directly in the Stokes flask, extraction cylinder, or into a volumetric flask. If the latter, make up to volume with distilled water, mix and transfer with a pipet a quantity such that it will yield 1-5 gm of fatty acid material into extraction cylinder or stokes flask.

Now add an equal volume of distilled water and 96 % alcohol to bring the total volume to 75 ml. Warm to promote solution but do not exceed a temp. of 60 °C.

Cool to sample at room temperature ''near about 20 to 25 °C''. Add HCL (1:1) until the solution is just acid to methyl orange and then add 10 additional ml.

Now add a sufficient quantity of saturated NaCl solution to make 160 ml. in the cylinder or to bring the upper level within the constricted portion of the stokes flask.

Add 50 ml of petroleum ether without inverting or mixing. Allow to stand until the acids have completely dissolved in the ether.

Remove the upper layer with the siphon and collect the ether extract in a 500-separating funnel. Shake vigorously and allow to separate. Again remove the ether layer with the siphon. 

Add 10 ml of 1.0 N NaOH Solution to the separating funnel and shake Allow to separate and draw off the lower layer. Repeat 3 more times.

Add 10-15 ml of distilled water to the funnel and shake. Allow to separate and draw off the lower water layer. repeat until the washing is neutral to phenolphthalein.

Draw the ether solution into a tared 400 ml beaker or 500 ml Soxhlet flask. Evaporate the petroleum ether in a water bath until the odor of petroleum ether is no longer perceptible. Dry in a hot air oven for 30 min. at 105 ± 5 °C. Cool to room temp. in a desiccator and weight.  

Calculation Of Total Anhydrous Soap %:-

                                      Weight of extract X 100
                                            Weight of Sample

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