Question

How was excess NaOH avoided in the separated solid soap?

Answer 1

Two procedures for soap making are given, a “cold” and a “hot” process. The cold process, suitable for homemade soaps, produces soap bars which retain the glycreine by-product, and if the amount of alkali employed in the saponification is limited, an excess of fat (superfatted). The hot process, more suitable for laboratory or industrial preparation, yields a more chemically pure powder; by-products and excess starting materials are separated.

Hot process

weigh a 10g of fat to conical flask. In a 150ml beaker prepare 100 ml of a 50-50 solution of alcohol-water, and add 10g of NaOH pellets to the beaker In the hood, mix the NaOH with 36 ml of the of the 50-50 alcohol-water solution . Stir the mixture until a transparent solution is formed.

Pour the lye solution into the conical flask containing the fat and mix well heat the flask on the boiling-water bath and allow it to cook for at least 30 minutes. While the mixture boils, some foam will form. The reaction is complete when oil globules are no longer visible when the reaction mixture is stirred.

Half fill beaker with 300 ml of clear (filtered) saturated salt (NaCl) solution and 50 ml of water. Pour the still hot reaction mixture containing soap, glycerine, NaOH, and alcohol into the salt solution. Stir the resulting mixture and allow to stand for 5-10 minutes. The soap will collect as a white layer on the surface of the salt water in the beaker.

filter and when most of the aqueous layer in the beaker has been drawn off, filter the remaining mixture of soap and liquid through a Buchner funnel Allow the product to suck dry for 10-15 minutes.

To avoid excess of NaOH use in process you must perform a continueos process where the unknown quantity of fatty acid use and react with caustic solution as per your process or as above process. after the completion of reaction isolate product and the recover unconsumed fatty acid from filtrate and same can be used in next cycle where you may get a better quality of soap insead of batch process.