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Sodium hydroxide

2007 Schools Wikipedia Selection. Related subjects: Chemical compounds

                     Sodium hydroxide
            Sodium hydroxide Sodium hydroxide
                          General
   Systematic name     Sodium hydroxide
   Other names         Lye, Caustic Soda
   Molecular formula   NaOH
   Molar mass          39.9971 g/mol
   Appearance          White flakes
   CAS number          [1310-73-2]
                        Properties
   Density and phase   2.1 g/cm³, solid
   Solubility in water 111 g/100 ml (20°C)
   Melting point       318°C (591 K)
   Boiling point       1390°C (1663 K)
   Basicity (pK[b])    0.2
                          Hazards
   MSDS                External MSDS
   EU classification   Corrosive (C)
   R-phrases           R35
   S-phrases           S1/2, S26, S37/39, S45
   NFPA 704

                       0
                       3
                       1

   Flash point         Non-flammable.
                  Supplementary data page
   Structure and
   properties          n, ε[r], etc.
   Thermodynamic
   data                Phase behaviour
                       Solid, liquid, gas
   Spectral data       UV, IR, NMR, MS
                     Related compounds
   Other anions        Sodium chloride
                       Sodium sulfate.
   Other cations       Potassium hydroxide
                       Calcium hydroxide
   Related bases       Ammonia, lime.
   Related compounds   Chlorine
     Except where noted otherwise, data are given for
   materials in their standard state (at 25 °C, 100 kPa)
   Infobox disclaimer and references

   Sodium hydroxide (Na OH), also known as lye or caustic soda, is a
   caustic metallic base. An alkali, caustic soda is widely used in many
   industries, mostly as a strong chemical base in the manufacture of pulp
   and paper, textiles, drinking water, and detergents. Worldwide
   production in 1998 was around 45 million tonnes. Sodium hydroxide is
   also the most common base used in chemical laboratories, being able to
   test for quite a number of cations (this is called Qualitative
   Inorganic Analysis), as well as to provide alkaline mediums for some
   reactions that need it, such as the Biuret test.

General properties

   Pure sodium hydroxide is a white solid, available in pellets, flakes,
   granules, and also 50% saturated solution. It is deliquescent and also
   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 does 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.

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) + H[2]O( l)

   In general such neutralization reactions are represented by one simple
   net ionic equation:

          OH^−(aq) + H^+(aq) → H[2]O

   This type of reaction releases heat when a strong acid is used. Such
   acid-base reactions can also be used for titrations, and indeed this is
   a common way for measuring the concentration of acids. Related to this
   is the reaction of sodium hydroxide 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[2]S) and
   prevent their release into the atmosphere.

          2NaOH + CO[2] → Na[2]CO[3] + H[2]O

   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 does not attack iron or copper, but many other metals such as
   aluminium, zinc and titanium are attacked rapidly. 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. For this same reason aluminium pans should never be
   cleaned with lye.

          2Al( s) + 6NaOH(aq) → 3H[2](g) + 2Na[3]AlO[3](aq)

   Many non-metals also react with sodium hydroxide, giving salts. For
   example phosphorus forms sodium hypophosphite, while silicon gives
   sodium silicate.

   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 reaction with
   NaOH:

          6NaOH(aq) + Al[2](SO[4])[3](aq) → 2 Al(OH)[3](s) +
          3Na[2]SO[4](aq)

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

Manufacture

   In 1998, total world production was around 45 million tonnes. Of this,
   both North America and Asia contributed around 14 million tonnes, and
   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[2]O + 2e^− → H[2] + 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 – sodium metal forms as an amalgam at 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 reaction of NaOH
       with Cl[2] is prevented using a porous diaphragm. 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 caustic
       soda 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 caustic soda. The salt separated from the
       caustic brine can be used to saturate diluted brine. The chlorine
       contains oxygen and must often be 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
       caustic soda).

   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 to establish sodium hydroxide as an
   important commodity chemical.

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
   for chemical reactions and also for the neutralization of acidic
   materials. It can be used also as a neutralizing agent in petroleum
   refining

Experiment

   Sodium hydroxide has also been used in conjunction with zinc for
   creation of the famous "Gold pennies" experiment. A penny is boiled in
   a solution of NaOH together with some granular zinc metal, the colour
   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.
   The reason this happens is that granular zinc dissolves in NaOH to form
   Zn(OH)[4]^2-. 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.

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.

Soap making

   Soap making via saponification is the most traditional chemical process
   using sodium hydroxide. The Arabs began producing soap in this way in
   the 7th century, and the same basic process is still used today.

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 water and lye
   would turn the fat into soap which would be tainted with methanol.

   It is used more often than potassium hydroxide because it costs less,
   and a smaller quantity is needed for the same results. Another
   alternative is sodium silicate.

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 allodizing the surface
   may become corroded, either under normal use or in severe atmospheric
   conditions.

Food preparation

   Food uses of lye include washing or chemical peeling of fruits and
   vegetables, chocolate and cocoa processing, caramel colour production,
   poultry scalding, soft drink processing, and thickening ice cream.
   Olives are often soaked in lye to soften them, while pretzels and
   German lye rolls are glazed with a lye solution before baking to make
   them crisp.

   Specific foods processed with lye 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 cooking in hot oil and salting to form corn nuts.
       Nixtamal is similar, but uses calcium hydroxide instead of sodium
       hydroxide.
     * Hominy is also known in some areas of the Southeastern United
       States, as the breakfast food grits, dried and ground into a coarse
       powder. They are prepared by boiling in water, with the addition of
       butter and other ingredient to suit the tastes of the preparer.
     * 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 hydroxide solution
       before baking, which contributes to their unique crust.

Domestic uses

   Sodium hydroxide is used in the home as an agent for unblocking drains,
   provided as a dry crystal (e.g. " Drāno") or as a thick liquid gel. The
   chemical mechanism employed is the conversion of grease to a form of
   soap, and so forming a water soluble form to be dissolved by flushing;
   also decomposing complex molecules such as the protein of hair. Such
   drain cleaners (and their acidic versions) are highly caustic and
   should be handled with care.

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 lye 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.

   In this framework, sodium hydroxide has also been used by criminals and
   serial killers to dispose of their victim's bodies.

Illegal drugs

   Because it is a key ingredient in the process of making
   Methamphetamine, it is now impossible to purchase pure Sodium hydroxide
   as a consumer product in the United States. Products containing pure
   Sodium hydroxide, such as Red Devil, are no longer available for sale.
   As a result, amateur soapmakers must now purchase Sodium hydroxide in
   bulk.

Safety

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

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

   The combination of aluminium and sodium hydroxide results in a large
   production of hydrogen gas:
   2Al( s) + 6NaOH(aq) → 3H[2](g) + 2Na[3]AlO[3](aq).
   Mixing these two in a closed container is therefore dangerous.

   For more information, consulting an MSDS is suggested.

Trivia

     * This danger was shown in a scene of the 1999 movie Fight Club,
       where the character Tyler Durden puts it on the protagonist's
       freshly kissed hand to create a lip-shaped scar, symbolizing their
       commitment to the plan that makes up the movie's plot. This is the
       only scene in the movie which Brad Pitt's parents have seen - he
       showed it to them before its release to convince them not to watch
       the movie.
     * Lye is used as an assault weapon in an episode of US crime drama
       CSI: New York, in which the victim has the chemical thrown over his
       face, causing a chemical burn, and his eventual death.
     * Mythbusters episode 20 tested the theory that jawbreakers mixed
       with sodium hydroxide would explode under heat. They heated solid,
       dry sodium hydroxide with jawbreakers in a microwave, finding that
       the mixture didn't explode before the sodium hydroxide itself
       exploded. Unofficial Mythbusters Guide: Episode 20 It's important
       to note that solid sodium hydroxide is extremely caustic, therefore
       it is unlikely to be found in food.

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