Sodium hydroxide is a popular strong
base used in industry. Sodium hydroxide is used in the manufacture of sodium salts and detergents, pH regulation, and organic synthesis. In bulk, it is most often handled as an
aqueous solution, since solutions are cheaper and easier to handle. Sodium hydroxide is used in many scenarios where it is desirable to increase the
alkalinity of a mixture, or to neutralize acids. For example, in the petroleum industry, sodium hydroxide is used as an additive in
drilling mud to increase
alkalinity in
bentonite mud systems, to increase the mud
viscosity, and to neutralize any
acid gas (such as
hydrogen sulfide and
carbon dioxide) which may be encountered in the
geological formation as drilling progresses. Another use is in
salt spray testing where pH needs to be regulated. Sodium hydroxide is used with hydrochloric acid to balance pH. The resultant salt, NaCl, is the corrosive agent used in the standard neutral pH salt spray test. Poor quality
crude oil can be treated with sodium hydroxide to remove
sulfurous impurities in a process known as
caustic washing. Sodium hydroxide reacts with weak acids such as hydrogen sulfide and
thiols to yield non-volatile sodium salts, which can be removed. The waste which is formed is toxic and difficult to deal with, and the process is banned in many countries because of this. In 2006,
Trafigura used the process and then
dumped the waste in Ivory Coast. Other common uses of sodium hydroxide include: • for making soaps and detergents. Sodium hydroxide is used for 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 a smaller quantity is needed. • as drain cleaners that convert pipe-clogging fats and grease into soap, which dissolves in water • for making artificial textile fibres such as
rayon • in the manufacture of
paper. Around 56% of sodium hydroxide produced is used by industry, 25% of which is used in the paper industry. • in purifying
bauxite ore from which
aluminium metal is extracted. This is known as the
Bayer process. • de-greasing metals •
oil refining • making
dyes and
bleaches • in water treatment plants for pH regulation
Chemical pulping Sodium hydroxide is also widely used in pulping of wood for making paper or regenerated fibers. Along with
sodium sulfide, sodium hydroxide 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.
Tissue digestion In a similar fashion, sodium hydroxide is used to digest tissues, as in a process that was used with farm animals at one time. This process involved placing a carcass into a sealed chamber, then adding a mixture of sodium hydroxide and water (which breaks the chemical bonds that keep the flesh intact). This eventually turns the body into a liquid with a dark brown color, and the only solids that remain are bone hulls, which can be crushed between one's fingertips. Sodium hydroxide is frequently used in the process of decomposing
roadkill dumped in landfills by animal disposal contractors. In Mexico, a man who worked for drug cartels admitted disposing of over 300 bodies with it. Sodium hydroxide is a dangerous chemical due to its ability to hydrolyze protein. If a dilute solution is spilled on the skin, burns may result if the area is not washed thoroughly and for several minutes with running water. Splashes in the eye can be more serious and can lead to blindness.
Dissolving amphoteric metals and compounds Strong bases attack
aluminium. Sodium hydroxide reacts with aluminium and water to release hydrogen gas. The aluminium takes an oxygen atom from sodium hydroxide, which in turn takes an oxygen atom from water, and releases two hydrogen atoms. The reaction thus produces
hydrogen gas and
sodium aluminate. In this reaction, sodium hydroxide acts as an agent to make the solution alkaline, which aluminium can dissolve in. : Sodium aluminate is an inorganic chemical that is used as an effective source of
aluminium hydroxide for many industrial and technical applications. Pure sodium aluminate (anhydrous) is a white crystalline solid having a formula variously given as , , , or . Formation of sodium tetrahydroxoaluminate(III) or hydrated sodium aluminate is given by: : This reaction can be useful in
etching, 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. In the
Bayer process, sodium hydroxide is used in the refining of alumina containing ores (
bauxite) to produce alumina (
aluminium oxide) which is the raw material used to produce aluminium via the
electrolytic Hall-Héroult process. Since the alumina is
amphoteric, it dissolves in the sodium hydroxide, leaving impurities less soluble at high
pH such as
iron oxides behind in the form of a highly alkaline
red mud. Other amphoteric metals are zinc and lead which dissolve in concentrated sodium hydroxide solutions to give
sodium zincate and
sodium plumbate respectively.
Esterification and transesterification reagent Sodium hydroxide is traditionally used in soap making (
cold process soap,
saponification). It was made in the nineteenth century for a hard surface rather than liquid product because it was easier to store and transport. 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. Due to production costs, sodium hydroxide, which is produced using common salt, is cheaper than potassium hydroxide.
Skincare ingredient Sodium hydroxide is an ingredient used in some
skin care and
cosmetic products, such as facial cleansers, creams, lotions, and makeup. It is typically used in low concentration as a
pH balancer, due its highly alkaline nature.
Food preparation Food uses of sodium hydroxide include washing or chemical peeling of
fruits and
vegetables,
chocolate and
cocoa processing,
caramel coloring production,
poultry scalding,
soft drink processing, and thickening
ice cream.
Olives are often soaked in sodium hydroxide for softening;
pretzels and German
lye rolls are glazed with a sodium hydroxide solution before baking to make them crisp. Owing 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. It is known as
E number E524. Specific foods processed with sodium hydroxide include: • German
pretzels are poached in a boiling
sodium carbonate solution or cold sodium hydroxide solution before baking, which contributes to their unique crust. • Lye water is an essential ingredient in the crust of the traditional baked Chinese moon cakes. • Most yellow coloured
Chinese noodles are made with lye water but are commonly mistaken for containing egg. • One variety of
zongzi uses lye water to impart a sweet flavor. • Sodium hydroxide causes gelling of egg whites in the production of
century eggs. • Some methods of preparing olives involve subjecting them to a lye-based brine. • The Filipino dessert () called uses a small quantity of lye water to help give the rice flour batter a jelly-like consistency. A similar process is also used in the kakanin known as or except that the mixture uses grated
cassava instead of rice flour. • The
Norwegian dish known as
lutefisk (). •
Bagels are often boiled in a lye solution before baking, contributing to their shiny crust. •
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. Hominy is used to create
masa, a popular flour used in Mexican cuisine to make
corn tortillas and
tamales.
Nixtamal is similar, but uses
calcium hydroxide instead of sodium hydroxide.
Cleaning agent Sodium hydroxide is frequently used as an industrial
cleaning agent, where it is often called "caustic". It is added to water, heated, and then used to clean process equipment, storage tanks, and other things. It can dissolve
grease,
oils,
fats and
protein-based deposits. It is also used for cleaning waste discharge pipes under sinks and drains in domestic properties.
Surfactants can be added to the sodium hydroxide solution in order to stabilize dissolved substances and thus prevent redeposition. A sodium hydroxide soak solution is used as a powerful degreaser on
stainless steel and glass bakeware. It is also a common ingredient in oven cleaners. A common use of sodium hydroxide is in the production of
parts washer detergents. Parts washer detergents based on sodium hydroxide are some of the most aggressive parts washer cleaning chemicals. The sodium hydroxide-based detergents include surfactants, rust inhibitors and defoamers. A parts washer heats water and the detergent in a closed cabinet and then sprays the heated sodium hydroxide and hot water at pressure against dirty parts for degreasing applications. Sodium hydroxide used in this manner replaced many solvent-based systems in the early 1990s when
trichloroethane was outlawed by the
Montreal Protocol. Water and sodium hydroxide detergent-based parts washers are considered to be an environmental improvement over the solvent-based cleaning methods. s grade sodium hydroxide to be used as a type of
drain cleaner. Sodium hydroxide is used in the home as a type of
drain openers to unblock clogged drains, usually in the form of a dry crystal or as a thick liquid gel. The alkali reacts with
greases to produce
water soluble soap and
glycerol. It also
hydrolyzes proteins, such as those found in
hair, which may block waste water pipes. Dissolving sodium hydroxide in water is an
exothermic reaction producing considerable quantities of heat which assists in speeding up the reactions with grease and other organic matter. 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 incidence and intensity of chemical burns, manufacturers of chemical relaxers use other alkaline chemicals in preparations available to consumers. Sodium hydroxide relaxers are still available, but they are used mostly by professionals.
Paint stripper A solution of sodium hydroxide in water was traditionally used as the most common paint stripper on wooden objects. Its use has become less common, because it can damage the wood surface, raising the grain and staining the colour.
Water treatment Sodium hydroxide is sometimes used during
water purification to raise the pH of water supplies. Increased pH makes the water less corrosive to plumbing and reduces the amount of lead, copper and other toxic metals that can dissolve into drinking water.
Historical uses Sodium hydroxide has been used for detection of
carbon monoxide poisoning, with blood samples of such patients turning to a
vermilion color upon the addition of a few drops of sodium hydroxide. 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 segregation of sands and cement, decreases the amount of water required in a mix and increases workability of the cement product, be it mortar, render or concrete. ==Safety==