Alkaline Silicates Testing Method in Soap

This method determines the Alkaline Silicates ( Sodium Silicates ) in the soap sample. This testing procedure applies to all Soap and soap Products containing synthetic detergents.

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Platinum evaporating dish, capacity 100 ml.

Platinum crucible 30 ml.

Watch glasses. 

Filter paper, ash-less grade small side 30 ml.


Hydrochloric Acid (AR, LR) specific Gravity 1.190 

Hydrofluoric Acid (A.R) Grade 48% HF.

Sulfuric Acid (AR, LR) specific Gravity 1.840


(A) When the sample contains no mineral matter that is insoluble in Water.

1.0 Weight a size of the sample such that it will contain no more than 0.20 gm of SiO2 into a platinum evaporating dish and ignite over a Muffle furnace or a Burner below a dull red heat ( 360°C  - 450°C ) until the mass is charred. Add distilled water and filter with filter paper. Wash the dish and residue on the filter paper 2-3 times with distilled water to extract all water-soluble material.

2.0 Return the filter paper containing the charred residue to the platinum dish and continue the ignition at a bright red heat (850°C - 950°C) until all carbon is removed. Unite the residue and all of the water extract and neutralize so carefully with a Hydrochloric acid and then add form 5.0 - 10.0 ml of HCl in excess. keep the dish covered with a watch glass during this addition to prevent loss from spattering.

3.0 Evaporate to dryness on a steam or water bath or on a hot plate at a temperature not exceeding 120°C. Cool the residue sample and moisten it with HCl let stand for 5 - 10 minutes and then break up all lumps with a stirring rod.

4.0 Add 25 ml of hot distilled water to the residue and heat for a few minutes and filter through a small ash-less filter paper. Repeat the above treatment, C, 3, and 4, beginning with ''Evaporator to dryness .........'' Filter with a second filter paper.

5.0 Place the two filter papers in a tared platinum crucible and ignite carefully and first at a low temperature until the paper is consumed and then over a burner or in a muffle at a bright red heat (850°C - 950°C). Cool to room temp. in a desiccator and weight. Repeat until constant weight is obtained.

Alkaline Silicates Testing Method in Soap

6.0 For the most accurate result moisten the weighed contents of the crucible with water. Add 10.0 ml of Hydrofluoric Acid, and 4.0 drops of Sulfuric Acid (H2SO4) and evaporate to dryness over a low flame. Ignite as directed in paragraph 5.0. Cool to room temp. in a desiccator and weight.

(B) When the sample contains mineral matter insoluble insoluble in water or when a determination of the highest accuracy is not required.

1.0  Use a portion of the solution after titrating the matter insoluble in alcohol but soluble in water, such that it will contain no more than 0.2 gm of SiO2.

2.0 Add 5 to 10 ml of HCL and proceed from here on as directed in (A) beginning with paragraph 3.0.


a. When SiO2 is weighed.

Sodium silicate (1Na2o : 3.25 SiO2) % = Weight of residue (from C,a 5)X131.8 / Weight of Sample

b. When SiO2 is Volatilized With Hydrofluoric Acid.

Sodium silicate % = (W1 - W2)X131.8 / Weight of sample


W1 = weight of crucible and contents from C (A) 5.

W2 = weight of crucible and contents from C (A) 6.

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