Sodium carbonate
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encyclopedia
Sodium carbonate
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Sodium carbonate
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Soda ash, Washing soda, Soda crystals
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Identifiers
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VZ4050000
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Jmol-3D images
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Properties
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Na2CO3
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105.9888 g/mol
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Appearance
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Odorless
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Decahydrate:
7 g/100 mL (0 °C) 16.4 g/100 mL (15 °C) 34.07 g/100 mL (27.8 °C) Heptahydrate: 48.69 g/100 mL (34.8 °C) Monohydrate: 50.31 g/100 mL (29.9 °C) 48.1 g/100 mL (41.9 °C) 45.62 g/100 mL (60 °C) 43.6 g/100 mL (100 °C)[3] |
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3.67
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Structure
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α = 90°,
β = 101.35(8)°, γ = 90°
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Octahedral (Na+, anhydrous)
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Thermochemistry
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Hazards
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Warning
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EU Index
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011-005-00-2
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Related compounds
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Related compounds
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Except where noted
otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
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Sodium carbonate (also known as washing soda, soda ash and soda crystals), Na2CO3, is a sodium salt of carbonic acid(soluble in water).
It most commonly occurs as a crystalline heptahydrate, which
readily effloresces to form a white
powder, the monohydrate. Pure sodium carbonate is a white, odourless powder
that is hygroscopic (absorbs moisture from
the air), has an alkaline taste, and forms a strongly alkaline water solution.
Sodium carbonate is well known domestically for its everyday use as a water softener. It can be extracted from the ashes of many
plants growing in sodium-rich soils, such as vegetation from the Middle East,
kelp from Scotland and seaweed from Spain. Because the ashes of these
sodium-rich plants were noticeably different from ashes of timber (used to
create potash), they became known
as "soda ash".[12] It is synthetically produced in large
quantities from salt (sodium chloride) andlimestone by a method known as the Solvay process.
Contents
The manufacture of glass is one of the most important uses of sodium carbonate. Sodium
carbonate acts as a flux for silica, lowering the melting point of the mixture to something
achievable without special materials. This "soda glass" is mildly
water soluble, so somecalcium carbonate is added to the pre-melt mixture to make the
glass produced insoluble. This type of glass is known as soda lime glass: "soda" for the sodium carbonate
and "lime" for the calcium carbonate. Soda lime glass has been the
most common form of glass for centuries.
Sodium carbonate is also used as a relatively strong base in various settings. For example, sodium
carbonate is used as a pHregulator to maintain stable alkaline
conditions necessary for the action of the majority of photographic film developing agents.
It is a common additive in municipal pools used to neutralize
the corrosive effects of chlorine and raise the pH.
In cooking, it is sometimes used in place of sodium hydroxide for lyeing, especially
with German pretzels and lye rolls. These dishes are treated with a
solution of an alkaline substance to change the pH of the surface of the food
and improve browning.
In taxidermy,
sodium carbonate added to boiling water will remove flesh from the skull or
bones of trophies to create the "European skull mount" or for
educational display in biological and historical studies.
In chemistry, it is often used as an electrolyte. This is because electrolytes are usually
salt-based, and sodium carbonate acts as a very good conductor in the process
of electrolysis. In addition, unlike chloride ions, which form chlorine gas,
carbonate ions are not corrosive to the anodes. It is also used as a primary
standard for acid-base titrations because it is solid
and air-stable, making it easy to weigh accurately.
In domestic use, it is used as a water softener in laundering.
It competes with the magnesium and calcium ions in hard water and prevents them
from bonding with the detergent being used. Sodium carbonate can be used to
remove grease, oil and wine stains. Sodium carbonate is also used as a descaling agent in boilers such as those found in coffee
pots and espresso machines.
In dyeing with fiber-reactive
dyes, sodium carbonate (often under a name such as soda ash fixative or soda
ash activator) is used to ensure proper chemical bonding of the dye with
cellulose (plant) fibers, typically before dyeing (for tie dyes), mixed with
the dye (for dye painting), or after dyeing (for immersion dyeing).
The sodium carbonate test (not to be confused with sodium
carbonate extract test) is used to distinguish between some common metal ions,
which are precipitated as their respective carbonates. The test can distinguish
between Cu, Fe and Ca/Zn/Pb. Sodium carbonate solution is added to the salt of
the metal. A blue precipitate indicates Cu2+ ion. A dirty green
precipitate indicates Fe2+ ion. A yellow-brown precipitate indicates Fe3+ ion. A white precipitate indicates Ca2+, Zn2+ or Pb2+ ion. The compounds
formed are, respectively, copper(II) carbonate, iron(II) carbonate, iron(III) oxide, calcium carbonate, zinc carbonate and lead(II) carbonate. This test is used to precipitate the ion present as almost all
carbonates are insoluble. While this test is useful for telling these cations
apart, it fails if other ions are present, because most metal carbonates are
insoluble and will precipitate. In addition, calcium, zinc and lead ions all
produce white precipitates with carbonate, making it difficult to distinguish
between them. Instead of sodium carbonate,sodium hydroxide may be added, this gives nearly the same
colours, except that lead and zinc hydroxides are soluble in excess alkali, and
can hence be distinguished from calcium. For the complete sequence of tests
used for qualitative cation analysis, see qualitative inorganic analysis.
Sodium carbonate is a food additive (E500) used as an acidity
regulator, anti-caking agent, raising agent, and stabilizer. It is one of the
components of kansui (かん水?),
a solution of alkaline salts used to give ramen noodles their characteristic flavor and texture. It is also used
in the production of snus (Swedish-style snuff) to stabilize the pH of the final product. In
Sweden, snus is regulated as a food product because it is
put into the mouth, requires pasteurization, and contains only ingredients that
are approved as food additives.
Sodium carbonate is also used in the production of sherbet powder. The cooling and fizzing sensation
results from the endothermic reaction between sodium carbonate and a weak acid,
commonly citric acid,
releasing carbon dioxide gas, which occurs when the sherbet is moistened by
saliva.
In China, it is used to replace lye-water in the crust of
traditional Cantonese moon cakes,
and in many other Chinese steamed buns and noodles.
Sodium carbonate is used by the brick industry as a wetting
agent to reduce the amount of water needed to extrude the clay.
In casting, it is referred to as "bonding agent" and
is used to allow wet alginate to adhere to gelled
alginate.
Sodium carbonate is used in toothpastes, where it acts as a
foaming agent and an abrasive, and to temporarily increase mouth pH.
Sodium carbonate is used by the cotton industry to neutralize
the sulfuric acid need for acid de-linting of fuzzy cottonseed.
Sodium carbonate, in a solution with common salt, may be used
for cleaning silver. In a non-reactive container (glass, plastic or ceramic)
aluminium foil and the silver object are immersed in the hot salt solution. The
elevated pH dissolves the aluminium oxide layer on the foil and enables an
electrolytic cell to be established. Hydrogen ions produced by this reaction
reduce the sulfide ions on the silver restoring silver metal. The sulfide can
be released as small amounts of hydrogen sulfide. Rinsing and gently polishing
the silver restores a highly polished condition.[13]
The integral enthalpy of solution of sodium carbonate is
−28.1 kJ/mol for a 10% w/w aqueous solution.[14] Mohs hardness of sodium carbonate monohydrate is 1.3.[6]
1.
sodium carbonate
decahydrate (natron)
2.
sodium carbonate
heptahydrate (not known in mineral form)
3.
sodium carbonate
monohydrate (thermonatrite).
Structure of monohydrate at 346 K.
Sodium carbonate is soluble in water, and can occur naturally in
arid regions, especially in mineral deposits (evaporites) formed when
seasonal lakes evaporate. Deposits of the mineral natron have been mined from dry lake bottoms in Egypt
since ancient times, when natron was used in the preparation of mummies and in the early manufacture of glass.
The anhydrous mineral form of sodium carbonate is quite rare and
called natrite. Sodium carbonate also erupts from Ol
Doinyo Lengai, Tanzania's unique volcano, and it is presumed to have
erupted from other volcanoes in the past but, due to these minerals'
instability at the earth's surface, are likely to be eroded. All three
mineralogical forms of sodium carbonate, as well as trona,
trisodium hydrogendicarbonate dihydrate, are also known from ultra-alkaline pegmatitic rocks,
that occur for example in the Kola Peninsula in Russia.
Trona, trisodium hydrogendicarbonate dihydrate (Na3HCO3CO3·2H2O), is mined in
several areas of the US and provides nearly all the domestic consumption of
sodium carbonate. Large natural deposits found in 1938, such as the one near Green River, Wyoming, have made mining more economical than industrial production in
North America. There are important reserves of Trona in Turkey; two million
tons of soda ash have been extracted from the reserves near Ankara. It is also
mined from some alkaline lakes such asLake Magadi in Kenya by dredging. Hot saline springs
continuously replenish salt in the lake so that, provided the rate of dredging
is no greater than the replenishment rate, the source is fully sustainable.
Several "halophyte" (salt-tolerant) plant species and
seaweed species can be processed to yield an impure form of sodium carbonate,
and these sources predominated in Europe and elsewhere until the early 19th
century. The land plants (typicallyglassworts or saltworts) or the
seaweed (typically Fucus species) were harvested, dried, and burned. The ashes were then
"lixiviated" (washed with water) to form an alkali solution. This
solution was boiled dry to create the final product, which was termed
"soda ash"; this very old name refers to the archetypal plant source
for soda ash, which was the small annual shrub Salsola
soda ("barilla
plant").
The sodium carbonate concentration in soda ash varied very
widely, from 2–3 percent for the seaweed-derived form ("kelp"), to 30 percent for the best barilla produced from saltwort plants in Spain. Plant and seaweed sources for
soda ash, and also for the relatedalkali "potash", became
increasingly inadequate by the end of the 18th century, and the search for
commercially viable routes to synthesizing soda ash from salt and other
chemicals intensified.[15]
In 1791, the French chemist Nicolas Leblanc patented a process for producing sodium
carbonate from salt, sulfuric acid, limestone,
and coal. First, sea salt (sodium chloride) was boiled in sulfuric acid to yield sodium sulfate and hydrogen chloride gas, according to the chemical equation
Next, the sodium sulfate was blended with crushed limestone (calcium carbonate) and coal, and the mixture was burnt,
producing calcium sulfide.
The sodium carbonate was extracted from the ashes with
water, and then collected by allowing the water to evaporate.
The hydrochloric acid produced by the Leblanc process was a major source of air pollution, and the calcium sulfide byproduct also presented waste disposal
issues. However, it remained the major production method for sodium carbonate
until the late 1880s.[15][16]
In 1861, the Belgian industrial chemist Ernest Solvay developed a method to convert sodium chloride
to sodium carbonate using ammonia.
The Solvay process centered around a large hollow tower. At the
bottom, calcium carbonate (limestone) was heated to release carbon dioxide:
At the top, a concentrated solution of sodium chloride and
ammonia entered the tower. As the carbon dioxide bubbled up through it, sodium
bicarbonate precipitated:
The sodium bicarbonate was then converted to sodium carbonate by
heating it, releasing water and carbon dioxide:
Meanwhile, the ammonia was regenerated from the ammonium chloride byproduct by treating it with the lime (calcium hydroxide) left over from carbon dioxide generation:
Because the Solvay process recycles its
ammonia, it consumes only brine and limestone, and has calcium chloride as its only waste product. This made it
substantially more economical than the Leblanc process, and it soon came to
dominate world sodium carbonate production. By 1900, 90% of sodium carbonate
was produced by the Solvay process, and the last Leblanc process plant closed
in the early 1920s.
Developed by Chinese chemist Hou Debang in the 1930s. The earlier steam reforming byproduct carbon dioxide was pumped through a saturated
solution of sodium
chloride and ammonia to produce
sodium bicarbonate via the following reactions:
The sodium bicarbonate was collected as a precipitate due to its
low solubility and then heated to yield pure sodium carbonate similar to last
step of the Solvay process. More sodium chloride is added to the remaining
solution of ammonium and sodium chlorides; also more ammonia is pumped at
30-40 °C to this solution. The solution temperature is then lowered to
below 10 °C. Solubility of ammonium chloride is higher than that of sodium
chloride at 30 °C and lower at 10 °C. Due to this temperature
dependent solubility difference and the common-ion effect, ammonium chloride is precipitated in a
sodium chloride solution.
The Chinese name of Hou's process (联合制碱法) means "Coupled
Manufacturing Alkali Method": Hou's process is coupled to the Haber process and offers better atom economyby eliminating the production of calcium
chloride since ammonia no longer needs to be regenerated. The byproduct
ammonium chloride can be sold as a fertilizer.
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