Question

4. Tristearin and stearic acid are insoluble in water. However, sodium stearate (soap) is soluble. Why is there this difference in solubility? 5. Use of concentrated NaOH (lye or caustic soda) in soap preparations can produce a very basic product. A good soap tests between pH 7 and 9; pH 10 is rather harsh; a pH of 11 means something is wrong. How does your soap compare with these values? How could you reduce the pH of your preparation?

for number 5. my soaps ph is at 11

Answer 1

4. Tristearin and stearic acid are insoluble in water. Sodium stearate is soluble.The difference is solubility is due to

Structure of Stearic acid

As it is a fatty acid, it is mainly a long hydrocarbon chains consists of carboxyl group and methy group in opposite sides. Chain length range will be 3 to 24. Major hydrogenated vegetable and animal oils contains fatty acids of length C₁₆ - C₂₀. But there are no double bonds found between the neighbouring carbon atoms, this show it is a saturated acid. This leads to the flexibility of the hydrocarbon chain for rolling into a ball and stretching into a long zig zag. Here the tail is found as hydrophobic which neglects water. The tail alkane immediately dissolves in oil, fat and grease.But it does not dissolve in water.

OH stearic acid

Structure of tristearin

Fat mixture from the cow fat which is treated by steam is called tallow. From the Extraction of tallow tristearing is produced. Tristearin is the one which has glycerol molecule attached with three stearic acid molecules.

Tristearin

Structure of Sodium Stearate

Stearic acid on heating with Sodium hydroxide leads to the formation of Sodium Stearate.

o Nat

A Structure with long hydrocarbon tail and carboxylic acid head group shows it is an instance of soap or detergent. This Structure indicates the polarisability in the head carboxyl group, gives way for the dissolution in water which is hydrophilic(attracts water). In reverse it does not dissolve in grease or oil.

5. Soap

Soap is a disinfecting agent and a blurred material obtained from the reaction of fatty acids with alkali. It contains mainly Sodium or Potassium salts of acids. Usually the capabality of cleansing is improved by increasing the fatty things. The bathing bars are considered to be less active than the toilet soaps, because the fatty material content is about 60 to 80% in the toilet soaps. But the bathing bars consists of only 40 - 60%. Toilet soap's cleansing activity is more in addition, it will not cause damage to the skin more over it soften the skin. Mainly toilet soaps are produced from vegetable oils. In contrast animal fats are also used in soap making, they are referred as oil soaps. A process called Saponification, in which comes from the reaction of the alkaline solution. Here the term 'lye' points out the soap which is made of Sodium Hydroxide.

I am using Hamam for about 35 years. I am free from skin problems like pimples, allergy, marks and rashes. TFM level is 68%, and it is mentioned as grade 3 soap. As a good sanitizer, it has a good aroma. It consists of some herbs and neem.

The pH value of the Soap is very important to determine its quality. But most of the soap brands regret to print the pH value in the label. Standard value of the pH for the bathing soap must be in the range of 7.5 to 9. Neutral value will be 7. pH of 8 is tolerable. When it exceeds above 12 years the skin gets damaged. pH of 14 is completely restricted for usage.

Partial alkalinity should be maintained. Complete Saponification process required for a pure Soap. Acceptable alkaline range will be 0.05%. Depending upon our Skin quality, our environment, soaps can be chosen.

How can the pH of the soap be reduced during preparation?

Reducing the pH value can be carried out by neutralisation. Neutralising can be by the addition of some amount acids such as acetic acid, citric acid etc., From these citric acid is more efficient. By using indicator we can get the accurate pH value of our soap. Weigh small amount of the sample correctly and measure the pH value. By adding a quarter teaspoon of the acid, with vigrous stirring and then check the pH again. Repetition should be done and mark the values for each quarter teaspoon of acid. The reaction should be continued until the pH value attains 7 +/- 0.5. After it reaches stop repeating and weigh the sample. For bulk preparation, the amount of acid to be added can be calculated by subtracting the initial weight, and by deriving the ratio of the bulk sample to the initial sample.