Inorganic compound sodium hydroxide (NaOH) is also known as caustic soda or lye. It is an opaque, odorless, hygroscopic white solid that absorbs air moisture and is deliquescent and corrosive. This white solid ionic compound contains sodium cation Na+ and hydroxide anions OH−. It has a bitter taste and a soapy feel. Water dissolves it easily and reacts exothermically, producing heat. In this article, we will discuss methods for the preparation of caustic soda in detail. It aims to provide a detailed understanding of the preparation of caustic soda and its various applications, promising an informative experience for the reader.
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Caustic Soda and Its Unique Properties
Before moving on to the preparation of caustic soda, let’s look closer at its common properties.
| Molar mass | 39.9971 g/mol |
| Melting point | 323 °C (613 °F; 596 K) |
| Boiling point | 1,388 °C (2,530 °F; 1,661 K) |
| Solubility in water | 418 g/L (0 °C), 1000 g/L (25 °C), 3370 g/L (100 °C) |
| Acidity (pKa) | 15.7 |
| Solubility | Soluble in glycerol, slowly soluble in propylene glycol, insoluble in ether, and negligible in ammonia |
| Crystal structure | Orthorhombic |
| Density | 2.13 g\cm3 |
| pH | 13 (highly alkaline) |
This compound exhibits protonation, as you can see. It has the following related compounds:
| Other anions | Sodium oxide, sodium hydride, sodium hydrosulfide |
| Other cations | The compounds in question are lithium hydroxide, potassium hydroxide, caesium hydroxide, rubidium hydroxide, and francium hydroxide. |
| Related compounds | Both sodium deuteroxide and sodium chloroxide are present. |
As explained in these tables, this substance holds various chemical and physical properties. Still, some industries tend to use caustic soda alternatives in place of this substance because it has some hazards, too. Its most-used alternatives are:
magnesium hydroxide (Mg(OH)₂)
- sodium carbonate (Na2CO3)
- sodium bicarbonate (NaHCO3)
- sodium citrate (Na₃C₆H₅O₇)
Manufacture of Caustic Soda
Now that we know much about caustic soda and its characteristics, it is time to have a look at different methods for preparing caustic soda.
Caustic soda is produced or prepared during the electrolysis of sodium chloride (NaCl) solution, often known as brine—water that has been heavily salted—the process used to extract NaOH. NaCl undergoes this reaction, resulting in sodium hydroxide, hydrogen gas (H2), and chlorine gas (Cl2). Na+ and OH- are the two ions found in NaOH. Caustic soda formation is a simple process. To make brine, they first infuse water with salt. In the next stage, they run an electrical current across the brine. English scientist Michael Faraday, who also made contributions to electromagnetic induction, diamagnetism, and electrolysis, discovered this process, known as electrolysis. As previously mentioned, the process of producing electrolytic caustic soda also removes hydrogen and chlorine gases in addition to caustic soda (NaOH).
Castner-Kellner Process
In the Castner-Kellner process, a brine solution (NaCl) electrolyzes. An ebonite-lined rectangular steel tank forms the Castner-Kellner cell. The cathode is a layer of mercury at the bottom, and the anode is titanium. During the caustic preparation process, brine ionization occurs, leading to the formation of caustic soda, also known as the caustic soda preparation equation:
During the industrial preparation of caustic soda, the accumulation of sodium ions at the mercury cathode creates a sodium amalgam. Chlorine ions travel out of the cell and toward the anode. After that, we move the amalgam to a different chamber known as the denuder and treat it with water to create a solution of sodium hydroxide. The process of making caustic soda culminates in the formation of solid sodium hydroxide upon evaporation. Caustic soda can be produced commercially by heating milk of lime (Ca(OH)₂) in an iron tank to treat an aqueous solution of sodium carbonate (Na₂CO₃). The following is the chemical equation used to prepare caustic soda:
Membrane Technology
The first membrane process, the above-explained procedure, is done by membrane technology. This process involves the use of brine and electricity, along with a proton exchange membrane divider between electrodes, to extract three compounds from the final cycle. Membrane cell technology is known for its efficiency and environmental sustainability. The anode and cathode, which are the two electric connection points of each chlorine production cell, are split apart by an ion-exchange membrane in the membrane cell process.
This membrane selectively permits the passage of sodium ions and a limited quantity of water as they migrate toward the negatively charged cathode. At the cathode, water is divided, resulting in the creation of hydrogen gas, which bubbles out and is collected. About 30% of the acidic fluid (sodium hydroxide) that is still present exits the cell. Often, the concentration shifts 50% further away from the cell.
At the anode, chlorine gas bursts out on the other side of the membrane, chlorine gas bursts from the anode. Prior to purification, one can use an ion exchanger to saturate the “spent” brine with extra solid salt. Out of the three methods, the membrane cell process uses the least amount of electricity, and it requires less steam—less than one tonne for every tonne of caustic soda—to concentrate the soda.
Nelson’s Cell Method
Commercial manufacturers usually use the diaphragm cell, also known as the Nelson cell, to manufacture sodium hydroxide (caustic soda). The Nelson-Diaphragm Cell method’s chemical reaction is as follows:
2NaCl (salt) + 2H₂O (water) + electricity → Cl₂ (chlorine) + H₂ (hydrogen) + 2NaOH (sodium hydroxide).
This process splits water into hydrogen and caustic soda (sodium hydroxide) at the cathode and produces chlorine at the anode by oxidizing chloride ions. The diaphragm lets the diluted caustic brine escape the cell by stopping the caustic soda and chlorine reaction. Commercial caustic soda solutions typically range in weight from 49.50% to 51.00% in laboratory preparation. We maintain a maximum of 100 parts per million (ppm) of chloride (Cl), 50 parts per million (ppm) of sulphate (SO), 40 parts per million (ppm) of iron (Fe), and 4.0 parts per million (ppm) of other impurities.
In the industrial preparation of caustic soda, we offer caustic soda class 10, even though this matter is available in different forms, such as caustic soda pearls, caustic soda flakes, and caustic soda lye (liquid).
Loewig’s Process
Loewig’s process involves creating sodium ferrate (Na2FeO4), which water then demolishes. Following the mixing of the soda liquors with ferric oxide, a revolving furnace typically dries them and heats them to a brilliant red temperature.
Final Words
Numerous industrial processes employ the highly corrosive inorganic compound caustic soda, also known as sodium hydroxide (NaOH). The main method for preparing caustic soda is electrolysis of sodium chloride, which produces sodium hydroxide, hydrogen gas, and chlorine gas. The Nelson’s cell method and the membrane cell procedure are the two primary industrial ways of preparing it; both rely on electricity to speed up the chemical processes.
