Sodium hydroxide

Sodium hydroxide is an alkali which is also known as caustic soda. Caustic means "burning" and caustic soda takes its name from the way it can burn the skin. It has the chemical formula of NaOH.

Sodium hydroxide pellets on a glass dish

It is a base, meaning it has a high pH. It is the most common thing used to raise the pH of solutions; for example, to neutralize an acid.

As a solid, the alkali is deliquescent and readily absorbs moisture and carbon dioxide from the air. It dissolves easily in water, and makes the water warm when this happens. Sodium hydroxide is used as a solution called lye to make soap. Lye is also used to unclog drains. Its dissolving properties and its ability to easily break surface tension are fundamental for its uses. It works in two ways. First, it combines with grease to make soap. Second, it dissolves hair (which is soluble in any basic solution).

Making sodium hydroxide

Sodium hydroxide can be made (with chlorine and hydrogen) using the chloralkali process. A solution of sodium chloride is electrolyzed and sodium hydroxide is made around the cathode, where water is reduced to hydrogen gas and hydroxide ion. The hydrogen is released and the hydroxide bonds with the sodium to make sodium hydroxide. Special care is required to prepare a solution of sodium hydroxide or NaOH in water because considerable heat is liberated by the exothermic reaction. The solution may splatter or boil.

2Na⁺ (from salt) + 2H₂O → 2NaOH + H₂ at cathode

2 Cl^- (from salt) → Cl₂ at anode

Uses

See also: Hydrodesulfurization

Sodium hydroxide is a common strong base used in industries. Sodium hydroxide is used to make sodium salts and detergents, controlling pH levels, and making organic compounds. It is most often handled as an aqueous solution when there is a lot of it, since solutions are cheaper and easier to handle.

Sodium hydroxide is used commonly where increasing alkalinity or reducing acidity is needed.

Sodium hydroxide can remove sulfurous impurities in crude oil called caustic washing. Sodium hydroxide reacts with weak acids such as hydrogen sulfide and mercaptans to make sodium salts which would not explode so it can be removed. The waste which is formed is toxic and hard to deal with so the process is not allowed in many countries.

Other common uses of sodium hydroxide is:

• making soaps and detergents. Sodium hydroxide is used to make hard bar soap, while potassium hydroxide is used for liquid soaps. Sodium hydroxide is used more often than potassium hydroxide because it is cheaper and less of it is needed.
• as drain cleaners that convert fats and grease into soap, which dissolves in water stopping it from clogging in drains.
• for making artificial textile fibres such as rayon.
• making paper. Around 56% of sodium hydroxide made is used by industry, 25% of which is used in the paper industry.
• making bauxite ore purer. This is known as the Bayer process.
• removing grease, oil and impurities in metals
• oil refining
• making dyes and bleaches
• in water treatment plants for pH regulation
• to treat bagels and pretzel dough, giving the distinctive shiny finish

Chemical pulping

See the main article: Pulp (paper)

Sodium hydroxide is also commonly used in pulping of wood for making paper or regenerated fibers. Along with sodium sulfide, sodium hydroxide is one of the compounds in the white liquor solution used to separate lignin from cellulose fibers in the kraft process. It also plays a role in several later stages of the process of whitening the brown pulp resulting from the pulping process. These stages include oxygen delignification and simple extraction, all of which need a strong alkaline surrounding with a pH > 10.5 at the end of the stages.

Tissue digestion

Similarly, sodium hydroxide is used to destroy tissues. This process involved placing a dead body into a closed chamber, then adding mixed sodium hydroxide and water (which breaks the chemical bonds that keep the flesh intact). This turns the body into a liquid with a dark brown color, and the only solids after that are bone hulls, which can be crushed between one's fingertips.

Sodium hydroxide is commonly used in the process of breaking down roadkill in landfills. Due to it being easy to find and low cost, it has been used by criminals to dispose of dead bodies.

Sodium hydroxide is dangerous because it is able to hydrolyze protein. Even if it is mixed with water, when it is spilled on the skin, burns may result if the area is not washed thoroughly with running water for a few minutes. Splashes in the eye can lead to blindness.

Dissolving amphoteric metals and compounds

Strong bases attack aluminium. Sodium hydroxide reacts with aluminium and water to make hydrogen gas. The aluminium takes an oxygen atom from sodium hydroxide, which in turn takes an oxygen atom from water, and makes two hydrogen atoms. This thus produces hydrogen gas and sodium aluminate. In this reaction, sodium hydroxide make the solution alkaline, which aluminium can dissolve in.

2 Al + 2 NaOH + 2 H
2O → 2 NaAlO
2 + 3 H
2

Sodium aluminate is an inorganic chemical that is used to make aluminium hydroxide. Pure sodium aluminate (anhydrous) is a white solid having many formulas given as NaAlO
2, Na
3AlO
3, Na[Al(OH)
4], Na
2O · Al
2O
3 or Na
2Al
2O
4. Making sodium tetrahydroxoaluminate(III) or hydrated sodium aluminate is given by:

2 Al + 2 NaOH + 6 H
2O → 2 Na[Al(OH)
4] + 3 H
2

This reaction can be useful in etching, removing anodizing, or turning a polished surface to a satin-like finish, but without further passivation such as anodizing or alodining the surface may become worse, either under normal use or in extreme atmospheric conditions.

In the Bayer process, sodium hydroxide is used to remove impurities in alumina containing ores (bauxite) to make alumina (aluminium oxide) which is the used to produce aluminium via the electrolytic Hall-Héroult process. Since the alumina is amphoteric, it dissolves in the sodium hydroxide, leaving impurities less likely to dissolve in water at high pH such as iron oxides behind in the form of an alkaline red mud.

Other amphoteric metals are zinc and lead which dissolve in pure sodium hydroxide solutions to give sodium zincate from zinc and sodium plumbate from lead.

Esterification and transesterification reagent

Sodium hydroxide was used in soap making (cold process soap, saponification). It was made for a hard surface rather than a liquid because it was easier to store and transport.

In the making of biodiesel, sodium hydroxide is used as a catalyst for the transesterification of methanol and triglycerides. This only works with anhydrous sodium hydroxide, because combined with water the fat would turn into soap, which would have methanol in it. NaOH is used more often than potassium hydroxide because it is cheaper and less is needed. Due to costs, NaOH, which is made using common salt is cheaper than potassium hydroxide.

Skincare ingredient

Sodium hydroxide is used in some skincare and cosmetic produces, such as facial cleansers, creams, lotions, and makeup. It is typically used in low amounts as a pH balancer, due its highly alkaline nature.

Food preparation

Food uses sodium hydroxide for washing or chemical peeling of fruits and vegetables, processing chocolate and cocoa, making caramel coloring, processing poultry by putting it in hot water, processing of soft drink, and making ice cream thicker. Olives are often soaked in sodium hydroxide to soften it; pretzels and German lye rolls have a sodium hydroxide solution put of it before baking to make them crisp. Because of the difficulty in getting food grade sodium hydroxide in small amounts for home use, sodium carbonate is often used instead of sodium hydroxide. It is known as E number E524.

Cleaning agent

See the main article: Cleaning agent

Sodium hydroxide is commonly used as a cleaning agent where it is often called "caustic". After It is added to water and heated, it can used to clean process equipment, storage tanks, etc. It can dissolve grease, oils, fats and deposits made with protein. It is also used for cleaning pipes for removing waste under sinks and drains. Surfactants can be added to the sodium hydroxide solution in order to stop redeposition. A sodium hydroxide soak solution is used as a degreaser on stainless steel and glass bakeware. It is also commonly found in oven cleaners.

A common use of sodium hydroxide is to make parts washer detergents. Parts washer detergents made with sodium hydroxide are some of the strongest washer cleaning chemicals. The detergents made with sodium hydroxide include surfactants, rust inhibitors and defoamers. A parts washer heats water and the detergent in a closed space and then sprays the heated sodium hydroxide and hot water using pressure against dirty parts to degreasing it. Water and sodium hydroxide detergent-based parts washers is often thought to be better for the environment than the other solvent-based cleaning methods.

Hardware stores grade sodium hydroxide to be used as a type of drain cleaner.

Paint stripping with caustic soda

Sodium hydroxide is used in the home as a type of drain openers to unblock clogged drains, usually in the form of dry crystal or as a thick liquid gel. It dissolves greases to produce water soluble products. It also hydrolyzes proteins, such as those found in hair, which may block water pipes. These are sped by the heat generated when sodium hydroxide and the other chemicals in the cleaner dissolve in water. Such alkaline drain cleaners and their acidic versions are highly corrosive and should be handled with great caution.

Relaxer

Sodium hydroxide is used in some relaxers to straighten hair. However, because of the high chance of chemical burns and high intensity of the burn, people who make relaxers use other alkaline chemicals to sell to consumers. Sodium hydroxide relaxers can still be brought, but they are used commonly by people who know how to use it correctly.

Paint stripper

A solution of sodium hydroxide in water was used as the most common paint stripper on wooden objects. Its use has become less common, because it can damage the wood surface and could stain the colour.

Water treatment

Sodium hydroxide is sometimes used in water purification to raise the pH of water supply . Increased pH makes the water less corrosive to the pipes and reduces the amount of lead, copper and other toxic metals that can dissolve into the drinking water.

Historical uses

Sodium hydroxide has been used for detecting carbon monoxide poisoning, with blood samples of such patients turning into a vermilion color when a few drops of sodium hydroxide was added. Today, carbon monoxide poisoning can be detected by CO oximetry.

In cement mixes, mortars, concrete, grouts

Sodium hydroxide is used in some cement mix plasticisers. This helps homogenise cement mixes, preventing sand and cement from separating which decreases the amount of water required in a mix and increases the easiness of mixing, placing, strengthening and finishing the cement product, be it mortar, render or concrete.