Water Insoluble Matter Testing Procedure

This test method determines that water is an insoluble matter. Applicable to all soaps and soap products containing synthetic detergents.


(I) Beakers or Erlenmeyer flasks, 250 ml.

(II) Gooch crucible or filter paper. The Gooch crucible or filter paper is prepared with a pad of asbestos fiber. The pad was washed with alcohol and ether, distilled waters, and then dried to constant weight at 105°C +/- 5°C cooled to room temperature in a desiccator, and weighed. 

(III) Watch glass. 

(IV) Air oven maintained at 105°C+/- 5°C.

(V) Filter flask, 500ml.


1.0 Ethyl alcohol, 96% and absolute. (Add a little or a few drops of phenolphthalein indicator solution. And neutralize with 0.1N Sodium hydroxide solution. To faint pink color just before using. 

2.0 (VI) Phenolphthalein indicator solution, 1% in 96% alcohol. 

Procedure Insoluble matter

Weight a sample of such size that it will yield 0.1 to 1.0 gm. of alcohol-insoluble matter. Place in 250 ml. flask or beaker, add 200 ml. of alcohol, cover with a watch glass, and heat on a steam bath with frequent stirring until the soap or sample dissolved. 

Filter into a filter flask through a counterpoised filter paper through a tared Gooch crucible with the aid of suction retaining in the beaker as much of the residue as possible. Repeat this extraction and decantation 3 times using 26 ml. hot alcohol / hot water each time. 

Evaporate any alcohol remaining in the beaker after decantation on a steam or water bath and dissolve the residue in the smallest possible amounts of distilled water 05 ml is usually sufficient. 

Add 50 ml of absolute alcohol slowly, stirring vigorously, to reprecipitate the insoluble matter. Heat to boiling on a steam or water bath and filter through the paper or crucible. 

Wash all of the residues into the filter and wash the filter several times with hot 96% alcohol to remove all of the alcohol-soluble matter. 

Water insoluble matter procedure

After the residue is thoroughly washed with alcohol change the receivers wash the residue with five 50ml portions of distilled water at 60°C.( if protein or starch is present use ice water 0-10°C.) 

Dry the filter paper or crucible containing the water-insoluble residue in an air oven for three hours at 105+- 5°C. Cool at room temperature in a desiccator and weight. 


Water-insoluble matter formula% =

Weight of insoluble matter X 100

Weight of sample

Alcohol Soluble and insoluble Matter formula

Combine the alcohol Filtrate and washing in a tared 250 ml beaker and evaporate on a steam or water bath. Dry to constant weight in an oven at 105°C +- 2°C. Cool to room temperature in a desiccator and weight. 

Alcohol soluble matter % = weight of residue in the beaker X 100

  Weight of sample

Alcohol Insoluble matter % = weight of insoluble matter X 100

    weight of sample

Free alkali formula (as NaOH)% = B. R X Normality of Acid X 4.0

      Weight of Sample 

Free Acid formula(as oleic Acid)% = B.R. X Normality of alkali X 28.25

        Weight of Sample

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