Science

General applications

Sodium hydroxide is the principal strong base used in the chemical industry. In bulk it is most often handled as an aqueous solution, since solutions are cheaper and easier to handle. It is used to drive chemical reactions and also for the neutralization of acidic materials. It can be used also as a neutralizing agent in petroleum refining. It is sometimes used as a cleaner.

Paint stripper

A solution of sodium hydroxide in water was traditionally used as the most common paint stripper on wooden objects. Due to its caustic nature and the fact that it can damage the wood surface raising the grain and staining the color, its use has become less common.

Gold pennies experiment

Sodium hydroxide has also been used in conjunction with zinc for creation of the famous "Gold pennies" experiment. When a penny is boiled in a solution of NaOH together with some granular zinc metal (galvanized nails are one source), the color of the penny will turn silver in about 45 seconds. The penny is then held in the flame of a burner for a few seconds and it turns golden brown. The reason this happens is that granular zinc dissolves in NaOH to form Zn(OH)42- (tetrahydroxozincate). This zincate ion becomes reduced to metallic zinc on the surface of a copper penny. Zinc and copper when heated in a flame form brass.

Alumina production - Bayer process

Sodium hydroxide is used in the refining of alumina containing ore (bauxite) to produce alumina (aluminium oxide) which is the raw material used to produce aluminium metal via the smelting process.

Use in chemical analysis

In analytical chemistry, sodium hydroxide solutions are often used to measure the concentration of acids by titration. Since NaOH is not a primary standard, solutions must first be standardised by titration against a standard such as KHP. Burettes exposed to NaOH should be rinsed out immediately after use to prevent "freezing" of the stopcock. Sodium hydroxide was traditionally used to test for cations in Qualitative Inorganic Analysis, as well as to provide alkaline media for some reactions that need it, such as the Biuret test.

Soap production

Sodium hydroxide was traditionally used in soap making (cold process soap, saponification). Persians and Arabs began producing soap in this way in the 7th century, and the same basic process is used today.

Paper making

Sodium hydroxide was also widely used in making paper. Along with sodium sulfide, NaOH is a key component of the white liquor solution used to separate lignin from cellulose fibers in the Kraft process. It also plays a key role in several later stages of the process of bleaching the brown pulp resulting from the pulping process. These stages include oxygen delignification, oxidative extraction, and simple extraction, all of which require a strong alkaline environment with a pH > 10.5 at the end of the stages.

Biodiesel

For the manufacture 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 be tainted with methanol. It is used more often than potassium hydroxide because it is cheaper and a smaller quantity is needed.

Aluminium etching

Strong bases attack aluminium. Sodium hydroxide mixes with aluminum to create hydrogen gas. This can be useful in etching through a resist, removing anodizing, or converting a polished surface to a satin-like finish, but without further passivation such as anodizing or alodining the surface may become degraded, either under normal use or in severe atmospheric conditions.

Food preparation

Food uses of sodium hydroxide include washing or chemical peeling of fruits and vegetables, chocolate and cocoa processing, caramel color production, poultry scalding, soft drink processing, and thickening ice cream. Olives are often soaked in sodium hydroxide to soften them, while pretzels and German lye rolls are glazed with a sodium hydroxide solution before baking to make them crisp. Due to the difficulty in obtaining food grade sodium hydroxide in small quantities for home use, sodium carbonate is often used in place of sodium hydroxide[8].

Specific foods processed with sodium hydroxide include:

    * The Scandinavian delicacy known as lutefisk (from lutfisk, "lye fish").
    * Hominy is dried maize (corn) kernels reconstituted by soaking in lye-water. These expand considerably in size and may be further processed by frying to make corn nuts or by drying and grinding to make grits. Nixtamal is similar, but uses calcium hydroxide instead of sodium hydroxide.
    * Sodium hydroxide is also the chemical that causes gelling of egg whites in the production of Century eggs.
    * German pretzels are poached in a boiling sodium carbonate solution or cold sodium hydroxide solution before baking, which contributes to their unique crust.
    * Most yellow coloured Chinese noodles are made with lye-water but are commonly mistaken for containing egg.

Domestic uses

Sodium hydroxide is used in the home as a drain cleaning agent for clearing clogged drains. It is distributed as a dry crystal or as a thick liquid gel. The chemical mechanism employed is the conversion of grease to a form of soap. Soap is water-soluble, and can be dissolved by flushing with water. Sodium hydroxide also decomposes complex molecules such as the protein that composes hair. Such drain cleaners (and their acidic versions) are highly caustic and should be handled with care.

Sodium hydroxide has been used as a relaxer to straighten hair. However, because of the high incidence and intensity of chemical burns, chemical relaxer manufacturers have now switched to other alkaline chemicals, although sodium hydroxide relaxers are still available, used mostly by professionals.

Tissue digestion

This is a process that was used with farm animals at one time. This process involves the placing of a carcass into a sealed chamber, which then puts the carcass in a mixture of sodium hydroxide and water, which breaks chemical bonds keeping the body intact. This eventually turns the body into a coffee-like liquid, and the only solid remains are bone hulls, which could be crushed between one's fingertips. It is also of note that sodium hydroxide is frequently used in the process of decomposing roadkill dumped in landfills by animal disposal contractors[citation needed].

Sodium hydroxide has also been used by criminals and serial killers to dispose of their victims' bodies.[9]

Illegal drugs

Sodium hydroxide is used in the process of making methamphetamine, dimethyltryptamine, and other illegal drugs. Contrary to popular media reports, it is not actually an "ingredient" in these drugs, but simply a strong base used to manipulate the pH at various points in a chemical synthesis.

Cleansing agent

Sodium hydroxide is frequently used as a cleaner in breweries, where it is simply called "caustic". It is added to water, heated, and then used to clean the large stainless steel tanks where beer is brewed, fermented, and stored. It can dissolve oils and protein-based deposits. A sodium hydroxide soak solution is used as a powerful degreaser on stainless and glass bakeware. It is also the most common ingredient in oven cleaners.

Atmospheric CO2 "scrubber"

Within contained atmospheres containing animal life (such as the International Space Station with its astronauts or within a submerged submarine with its crew) normal respiration by the occupants generates carbon dioxide. This can rise to high enough concentration to create a toxic atmosphere and so must be restrained to non-toxic levels. The property of sodium hydroxide to absorb and convert carbon dioxide has been noted above as an inconvenience, but is in this case applied as a solution to this problem. The contained atmosphere is circulated through a contacting medium containing either dry hydroxide or which is washed with a water solution of the substance.