Sodium hydroxide (NaOH), also called caustic soda or lye, is the workhorse alkali of modern industry. It is a strong base that neutralises acids, saponifies fats, dissolves amphoteric oxides and precipitates metal hydroxides, which is why pulp mills, water utilities, alumina refineries, textile plants and pharmaceutical plants all depend on it. Understanding the commercial grades and the chemistry behind each use helps buyers select the right form for a given process.
Chemical Identity and Commercial Grades
Caustic soda is sold as a solid or as a liquid, and the choice of form is driven by handling capability, freight cost and the water balance of the end process. Solid material is hygroscopic and absorbs carbon dioxide from air, converting slowly into sodium carbonate, so packaging and storage discipline directly affect the delivered assay.
| Parameter | Typical specification |
|---|---|
| Chemical name | Sodium hydroxide (caustic soda, lye) |
| CAS number | 1310-73-2 |
| Molecular formula | NaOH |
| Molecular weight | 39.997 g/mol |
| Commercial forms | Flakes 98-99%, pearls 98-99%, solution 32% and 50% |
| Appearance | White, odourless, strongly hygroscopic solid; clear liquid solution |
| Melting point | 318 °C (anhydrous) |
| Boiling point | 1388 °C |
| Density | 2.13 g/cm3 (solid) |
| Solubility | About 109 g per 100 mL of water at 20 °C, with strong heat release |
| Storage | Sealed HDPE or lined steel; keep away from aluminium, zinc, tin and copper alloys |
Pulp, Paper, Textiles and Soap Production
In the kraft process, sodium hydroxide is the cooking chemical that dissolves lignin and releases cellulose fibre from wood chips, and it is used again in the bleaching and extraction stages. Textile mills use it for mercerising cotton, where the fibre swells, gains lustre and takes dye more evenly, and for scouring and finishing greige fabric. Soap manufacture consumes caustic soda in the saponification of fats and oils, a reaction that also yields glycerine as a valuable by-product. Detergent and surfactant plants use neutralised caustic streams, and chlorine bleach production relies on the reaction of chlorine with sodium hydroxide.
Water Treatment, pH Control and Environmental Applications
Municipal and industrial water treatment is one of the most consistent demand centres for liquid caustic. It raises the pH of acidic water and wastewater, neutralises spent acid streams, precipitates dissolved heavy metals as hydroxides for settling or filtration, and regenerates ion exchange resins. Flue gas scrubbers use caustic solutions to capture acidic gases, and many chemical plants use it for caustic scrubbing of process vents before discharge.
Dosing is controlled by pH and, where metals are present, by the hydroxide solubility curve of each metal.
Over-dosing can re-dissolve amphoteric metals such as aluminium and zinc, so set points are tight.
Liquid 50% caustic freezes near 12 °C and must be kept warm in cold climates.
Alumina Refining, Metals and Energy Systems
The Bayer process is the single largest metallurgical use. Bauxite is digested with hot sodium hydroxide solution, alumina is dissolved selectively, and the clarified liquor is precipitated to give aluminium hydroxide for calcination into smelter-grade alumina. Caustic is also used in metal cleaning and pickling lines, in electroplating baths, in petroleum refineries for treating and sweetening hydrocarbon streams, and in oilfield drilling muds where it controls viscosity and protects drill pipe from corrosion. Energy applications include electrolytes in fuel cells and alkaline battery systems.
Food, Pharmaceutical and Consumer Applications
Food-grade caustic soda is used for peeling fruit and vegetables, for de-bittering olives, for adjusting the pH of cocoa during chocolate processing, and in the production of caramel colour. Pharmaceutical manufacturing uses it in the synthesis of aspirin, anticoagulants and cholesterol-lowering medicines, as well as in purification steps. Consumer products such as oven cleaners, drain openers and chlorine bleach formulations are built around its ability to dissolve grease and protein. In food and drug production the material must meet the relevant food-chemical purity specification and be accompanied by a certificate of analysis covering heavy metals and other trace contaminants.
Frequently Asked Questions
Q: What is the CAS number of sodium hydroxide?
Sodium hydroxide is registered under CAS 1310-73-2 and is also known as caustic soda or lye.
Q: What is the difference between flake, pearl and liquid caustic soda?
All three are the same chemical. Flakes and pearls are 98-99% solids convenient for batch plants, while 32% and 50% solutions suit continuous processes and reduce dissolution heat.
Q: Why is caustic soda used in water treatment?
It neutralises acids, raises pH, precipitates heavy metals as hydroxides and regenerates ion exchange resins used in demineralisation.
Q: How is caustic soda used in alumina refining?
It digests bauxite in the Bayer process, selectively dissolving alumina so that it can be separated from iron and silica residues and later precipitated.
Q: Can sodium hydroxide be stored in aluminium or zinc containers?
No. It attacks aluminium, zinc, tin and copper alloys and releases flammable hydrogen, so sealed HDPE, polypropylene or lined steel vessels must be used.
Q: Does caustic soda lose strength in storage?
Yes, if it is exposed to air. It is hygroscopic and absorbs carbon dioxide, forming sodium carbonate, so containers must stay sealed and stock should be rotated.
