Determination of Free Alkali Content in Soap

Outline of the Method: (Free Carbonated Alkali) The matter insoluble in alcohol in the soap is dissolved in water and filtered. The water-insoluble carbonates are estimated by absorbing carbon dioxide gas evolved by reacting with the acid and expressed as sodium carbonate.

APPARATUS

1. Volumetric Carbonate Apparatus

2. Glass Beads

REAGENTS

1. Hydrochloric Acid-0.5N, accurately Standardized

2. Dilute Hydrochloric Acid-1:2(v/v)

3. Alkaline Absorbent solution: Mix equal volumes of 1.0 N sodium hydroxide (Carbonate-free) solution and 0.1 N barium chloride solution. Allow settling overnight, filter & preserve in well-stoppered bottles.

4. Magnesium Chloride Solution 20 percent (m/m)

5. Phenolphthalein Indicator - Dissolve 1g in 100ml of 95 percent rectified spirit.

6. Methyl orange indicator: Dissolve 0.1 g in 100 ml of water.

7. Trichloro benzene: 1:2:4 isomer, Boiling point 213 Deg. C and Relative Density 1.47.

Free alkali Procedure

Weigh accurately 2 to 10g of the sample and digest with 200 ml of freshly boiled ethyl alcohol in a convened vessel on a steam bath until the soap is dissolved.

Filter into a filtering flask through a counterpoised filter paper neutral to phenolphthalein, or through a weighted Gooch or sintered crucible with suction, protecting the solution from carbon dioxide and other acid fumes during the operation by covering it with a watch glass.

Wash it several times with hot ethyl alcohol to remove all the alcohol soluble. After filtering and washing the residue thoroughly with hot ethyl alcohol, change the receiver, and extract the residue with successive portions of water at about 60 Deg. C and wash the residue thoroughly on the filter paper or in the crucible.

Weigh a sufficient quantity of the sample into the sample flask to yield about 0.2g of carbon dioxide. Add about 400ml of un-boiled water to which have been added 2 ml of an alkaline absorbent solution to prevent the loss of carbon dioxide. Heat the flask in a steam bath until the soap is dissolved and cool until the flask is only slightly warm to the hand.

Add 30 ml of Magnesium chloride solution and a few glass beads to prevent bumping. Start the water through the condenser. Apply suction to the side tube to evacuate the system and reduce the pressure to 65 to 85 mm as indicated by the manometer.

Determination of free alkali content in soap

CAUTION: Maintain a properly reduced pressure and do not allow air to enter the system at any time during the test. Add dilute Hydrochloric acid containing a few drops of methyl orange indicator through the dropping funnel until the mixture in the flask is acidic (See Note). Avoid a large excess of acid. Add trichlorobenzene through the dropping funnel in the proportion of about 1ml to every 2g of the sample.

NOTE: Shake the conical flask at frequent intervals from the time the acid is added until the sample flask and

The condenser has been filled with water.

Place the small flame of a burner immediately in contact with the bottom of the sample glass and heat continuously for 30 minutes. after this, discontinue heating and pour boiled and cooled (carbon dioxide-free) water at 50oC through the condenser tube to fill the flask and the condenser to just below the side of the arm of the condenser.

Disconnect the conical flask, add one milliliter of phenolphthalein indicator, and titrate with 0.5N hydrochloric acid with vigorous agitation until the pink color disappears. Add the acid drop by drop. If it is not possible to titrate immediately, stop the flask tightly to guard the entrance of air.

Conduct a blank determination in order to establish the equivalent of the alkaline absorbent solution in terms of 0.5N Hydrochloric acid and also to correct for any carbon dioxide in the reagents.