Earlier, the alkalis used in soap manufacturing process, were obtained from the ashes of plants, but now days, they are produced commercially. Today, the term alkali describes a substance, which is a base (chemically) and which reacts with an acid to neutralize it. The most common alkalis used now days in soapmaking is sodium hydroxide (NaOH), which is also known as caustic soda; and potassium hydroxide (KOH), also known as caustic potash.
Potassium based soaps form a more water-soluble product than a sodium-based soap, and hence they are known as "soft soaps." Soft soaps, alone or in combination with sodium-based soaps, is generally used to make shaving creams and other water-soluble products.
When oil is mixed with an alkali, it forms glycerin and a sodium salt of fatty acid. The fatty acids that are required to manufacture soaps are supplied by tallow, grease, fish oils, and vegetable oils. The characteristics of soap, such as hardness, lathering qualities and transparency may vary depending on the combinations of alkalis and fats used as ingredients. Alkaline cleaners can also be used to remove oil from an oil-based paint, by drying it and causing it to peel or crack. Alkali products also have a property to darken aluminum surfaces
Alkaline substances vary in their strength. While most of the alkaline substances are toxic, some may be corrosive and some may irritate skin and eyes. The stronger alkalis can cause burns and, in the case swallowed, may cause death or internal injuries. On the basis of strength, alkalis can be -
- Mild - An example of mild alkali is baking soda (sodium bicarbonate)
- Moderate - Common examples of mild alkalis include - household
ammonia, borax and trisodium phosphate (TSP)
- Very Strong - Washing soda (sodium carbonate) and lye (caustic soda) are most common strong alkalis
In earlier days, rainwater was filtrated through hardwood ashes in Africa, to produce potassium hydroxide solution. Bar soaps was made using sodium hydroxide, which is what you get when you run electricity through salt water. Today, potassium hydroxide is made using a similar process.
Along with its wide known use in soapmaking, additional alkalis, such as sodium carbonate, hydroxide or trisodium phosphates are used dishwashing products to aid in handling greasy food soils.
Use in Soapmaking
Acidic oils and alkaline solutions are prime ingredients in soapmaking. To produce a perfect bar of soap, oil and alkalis should be in perfect balance. The unsaponified oils are known as "Free fatty acids", which add to the moisturizing characteristics of high quality soaps. However if these oils are used in excess, the soap will not lather and will have a shortened shelf life. The use of excess alkali in a soap make it harsh and drying to the skin.
Acid V/s Alkali & Soapmaking
|Normal pH is 7.0
Water should be 7.0
|Acids are chemicals that have a pH lower than "neutral". An acid can be mild or extremely caustic.||Alkaline substances have a pH more than 7.0. They can be mild or corrosive.|
|The type of acid, which is used to make soap is a fatty acid, obtained either from animal fat or vegetable oil.||The alkali, which is used to cause the chemical reaction with the fatty acids, is either made from potash (lye water) or sodium hydroxide.|
a base reacts with oil or fat, fatty acids get separated from the
glycerin and the potassium of sodium component of the alkali bonds with
fatty acids. The product, which is formed by the sodium or potassium and
the fatty acid, is a salt. In technical terms, soap is a salt. Glycerin
(also called glycerol) is a by-product, which has its own cleansing
properties. It is hydroscopic, i.e. it moisturizes as it attracts water
from the air.