Caustic soda (scientific name sodium hydroxide) is a soluble strong base. Soda ash (scientific name sodium carbonate) is actually a salt, because it undergoes hydrolysis in water to make the solution alkaline, and because it has some similar properties to caustic soda, it is juxtaposed with caustic soda, which is called "two alkalis"in industry.
Both caustic soda and soda ash are easily soluble in water, are strongly alkaline, and can provide Na+ ions. These properties make them widely used in soap making, textile, printing and dyeing, bleaching, paper making, refined petroleum, metallurgy and other chemical industries.
Ordinary soap is the sodium salt of higher fatty acid, which is generally prepared by saponification of oil under the action of a slight excess of caustic soda. If fatty acid is used directly as raw material, soda ash can also be used instead of caustic soda to make soap. In the printing and dyeing and textile industries, a large amount of lye should also be used to remove the grease on cotton yarn and wool. The production of man-made fibers also requires caustic soda or soda ash. For example, to make viscose fiber, firstly, 18-20% caustic soda solution (or soda ash solution) should be used to impregnate cellulose to make it into alkali cellulose, then the alkali cellulose is dried and pulverized, and finally the sulfonic acid is treated with dilute alkali solution. The acid salt dissolves, and a viscose solution is obtained. After filtering and vacuuming (removing air bubbles), it can be used for spinning.
Refined petroleum also uses caustic soda. In order to remove the gum in the petroleum fraction, concentrated sulfuric acid is generally added to the petroleum fraction so that the gum becomes an acid residue and separates out. After pickling, the petroleum also contains acidic impurities such as phenol, naphthenic acid and excess sulfuric acid, which must be washed with caustic soda solution and then washed with water to obtain refined petroleum products.
In the paper industry, chemical treatment is first used to cook raw materials containing cellulose (such as wood) and chemicals into pulp. The so-called alkaline pulping is to use caustic soda or soda ash solution as a cooking liquid to remove lignin, carbohydrates and resins in the raw materials, and neutralize the organic acids therein, so as to separate the cellulose.
In the metallurgical industry, the active ingredients in the ore are often converted into soluble sodium salts to remove insoluble impurities. Therefore, it is often necessary to add soda ash (which is also a flux) and sometimes caustic soda. For example, in the smelting process of aluminum, soda ash and caustic soda are used in the preparation of cryolite and the treatment of bauxite. For another example, when smelting tungsten, powdered tungsten is obtained by first roasting concentrate and soda ash into soluble sodium tungstate, and then undergoing acid precipitation, dehydration, reduction and other processes.
In the chemical industry, caustic soda is used in the production of metallic sodium and electrolyzed water. The production of many inorganic salts, especially the preparation of some sodium salts (such as borax, sodium silicate, sodium phosphate, sodium dichromate, sodium sulfite, etc.) requires caustic soda or soda ash. Caustic soda or soda ash are also used in the synthesis of dyes, pharmaceuticals and organic intermediates.
