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CAUSTIC SODA

Sodium hydroxide (NaOH), also known as lye, caustic
soda and (incorrectly, according to IUPAC
nomenclature)[1] as sodium hydrate, is a caustic
metallic base. Sodium hydroxide forms a strong
alkaline solution when dissolved in a solvent such as
water. It is used in many industries, mostly as a
strong chemical base in the manufacture of pulp and
paper, textiles, drinking water, soaps and detergents
and as a drain cleaner. Worldwide production in 1998
was around 45 million tonnes. Sodium hydroxide is the
most used base in chemical laboratories.

Pure sodium hydroxide is a white solid; available in
pellets, flakes, granules and as a 50% saturated
solution. It is deliquescent and readily absorbs
carbon dioxide from the air, so it should be stored in
an airtight container. It is very soluble in water
with liberation of heat. It also dissolves in ethanol
and methanol, though it exhibits lower solubility in
these solvents than potassium hydroxide. It is
insoluble in ether and other non-polar solvents. A
sodium hydroxide solution will leave a yellow stain on
fabric and paper.

Physical properties

Δ H° dissolution for diluted aqueous -44.45 kJ /
mol;

From aqueous solutions at 12.3-61.8°C, it
crystallizes in monohydrate, with a melting point of
65.1 ° C and density of 1.829 g/cm 3;

Δ H° form -734.96 kJ / mol;

Monohydrate from -28 to -24°C;

Heptahydrate from -24 to -17.7°C;

Pentahydrate from -17.7 to -5.4°C;

Tetrahydrate (α- changed), at -5 , 4 – 12.3°C Also
know metastable β- NaOH 4* H2O. Which above 61.8°C
are crystallized.

Chemical properties

Sodium hydroxide is completely ionic, containing
sodium ions and hydroxide ions. The hydroxide ion
makes sodium hydroxide a strong base which reacts with
acids to form water and the corresponding salts, e.g.,
with hydrochloric acid, sodium chloride is formed:
NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)

In general such neutralization reactions are
represented by one simple net ionic equation:
OH−(aq) + H3O+(aq) → 2H2O

This type of reaction with a strong acid, releases
heat, and hence is referred to as exothermic. Such
acid-base reactions can also be used for titrations,
which is a common method to determine the
concentration of acids. Another type of reaction that
sodium hydroxide is involved in is with acidic oxides.
The reaction of carbon dioxide has already been
mentioned, but other acidic oxides such as sulfur
dioxide (SO2) also react completely. Such reactions
are often used to "scrub" harmful acidic gases (like
SO2 and H2S) and prevent their release into the
atmosphere.
2NaOH + CO2 → Na2CO3 + H2O

Sodium hydroxide slowly reacts with glass to form
sodium silicate, so glass joints and stopcocks exposed
to NaOH have a tendency to "freeze".[citation needed]
Flasks and glass-lined chemical reactors are damaged
by long exposure to hot sodium hydroxide, and the
glass becomes frosted. Sodium hydroxide does not
attack iron since Iron does not have amphoteric
properties. A few trasition metals, however, may react
with Sodium Hydroxide in a vigorous way. In 1986 an
aluminium road tanker in the UK was mistakenly used to
transport 25% sodium hydroxide solution, causing
pressurisation of the contents and damage to the
tanker.

Unlike NaOH, the hydroxides of most metals are
insoluble, and therefore sodium hydroxide can be used
to precipitate metal hydroxides. One such hydroxide is
aluminium hydroxide, used as a gelatinous floc to
filter out particulate matter in water treatment.
Aluminium hydroxide is prepared at the treatment plant
from aluminium sulfate by reacting with NaOH. This
reaction is highly profitable, and is hence an
important synthesis reaction.

Sodium hydroxide reacts readily with carboxylic acids
to form their salts and is even a strong enough base
to form salts with phenols. NaOH can be used for the
base-driven hydrolysis of esters (as in
saponification), amides and alkyl halides. However,
the limited solubility of NaOH in organic solvents
means that the more soluble KOH is often preferred.

Manufacture

In 1998, total world production was around 45 million
tonnes. North America and Asia collectively
contributed around 14 million tonnes, while Europe
produced around 10 million tonnes.

Methods of production

Sodium hydroxide is produced (along with chlorine and
hydrogen) via the chloralkali process. This involves
the electrolysis of an aqueous solution of sodium
chloride. The sodium hydroxide builds up at the
cathode, where water is reduced to hydrogen gas and
hydroxide ion:
2Na+ + 2H2O + 2e− → H2 + 2NaOH

To produce NaOH it is necessary to prevent reaction of
the NaOH with the chlorine. This is typically done in
one of three ways, of which the membrane cell process
is economically the most viable.
Mercury cell process (also called the Castner-Kellner
process) – Sodium ions are reduced to sodium metal,
which forms an amalgam with a mercury cathode; this
sodium is then reacted with water to produce NaOH.
There have been concerns about mercury releases,
although modern plants claim to be safe in this
regard.[2]
Diaphragm cell process – uses a steel cathode, and
the reaction of NaOH with Cl2 is prevented using a
porous diaphragm, often made of asbestos fibers. In
the diaphragm cell process the anode area is separated
from the cathode area by a permeable diaphragm. The
brine is introduced into the anode compartment and
flows through the diaphragm into the cathode
compartment. A diluted caustic brine leaves the cell.
The sodium hydroxide must usually be concentrated to
50% and the salt removed. This is done using an
evaporative process with about three tonnes of steam
per tonne of sodium hydroxide. The salt separated from
the caustic brine can be used to saturate diluted
brine. The chlorine contains oxygen and is purified by
liquefaction and evaporation.[3][4]
Membrane cell process – similar to the diaphragm
cell process, with a Nafion membrane to separate the
cathode and anode reactions. Only sodium ions and a
little water pass through the membrane. It produces a
higher quality of NaOH. Of the three processes, the
membrane cell process requires the lowest consumption
of electric energy and the amount of steam needed for
concentration of the caustic is relatively small (less
than one tonne per tonne of sodium hydroxide).[5][6]

An older method for sodium hydroxide production was
the Leblanc process, which produced sodium carbonate,
followed by roasting to create carbon dioxide and
sodium oxide. This method is still occasionally used.
It helped establish sodium hydroxide as an important
commodity chemical.

The LeBlanc process was superseded by the Solvay
process in the late 19th century.

Major producers
In the United States, the major producer of sodium
hydroxide is the Dow Chemical Company, which has
annual production around 3.7 million tonnes from sites
at Freeport, Texas, and Plaquemine, Louisiana. Other
major US producers include Oxychem, PPG, Olin, Pioneer
Companies, Inc. (PIONA), and Formosa. All of these
companies use the chloralkali process

Uses
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

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). The 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. This can be useful in
etching through a resist or in 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

Illegal drugs

Sodium hydroxide is a key reagent in the process of
making methamphetamine 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 also the most common ingredient
in oven cleaners.

Safety

Solid sodium hydroxide or solutions containing high
concentrations of sodium hydroxide may cause chemical
burns, permanent injury or scarring, and blindness.

Dissolution of sodium hydroxide is highly exothermic,
and the resulting heat may cause heat burns or ignite
flammables.

Sodium Hydroxide is extremely caustic, and can react
with fats and oils on skin, in a reaction which
creates salts. For this reason, sodium hydroxide is
very dangerous, and skin should be washed thoroughly
with water following contact with this substance.

________________________________________________________
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April 11, 2008 - Posted by | Uncategorized

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