Everything about Sodium Hydroxide totally explained
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Sodium hydroxide (
NaOH), also known as
lye,
caustic soda and (incorrectly, according to
IUPAC nomenclature) 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 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* H
2O. 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, for example, 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) → 2H
2O
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 (SO
2) also react completely. Such reactions are often used to "scrub" harmful acidic gases (like SO
2 and H
2S) and prevent their release into the atmosphere.
» 2NaOH +
CO2 →
Na2CO3 + H
2O
Sodium hydroxide slowly reacts with glass to form
sodium silicate, so glass joints and
stopcocks exposed to NaOH have a tendency to "freeze".
Flasks and glass-lined
chemical reactors are damaged by long exposure to hot sodium hydroxide, and the glass becomes frosted. Sodium hydroxide doesn't attack
iron since Iron doesn't have amphoteric properties. A few transition 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 pressurization of the contents and damage to the tanker. The pressurization was due to the hydrogen gas which is produced in the reaction between sodium hydroxide and aluminium:
2Al
(s) + 6NaOH
(aq) → 3H
2(g) + 2Na
3AlO
3(aq)
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
+ + 2H
2O + 2e
− → H
2 + 2NaOH
To produce NaOH it's 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.
- 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.
- 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).
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
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's 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.
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 isn't 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's 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.
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.
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 isn't 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's 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.
A solution of 0.5M or more of sodium hydroxide should be labelled corrosive, while a solution between 0.5-0.05M should be labelled irritant.
Further Information
Get more info on 'Sodium Hydroxide'.
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