CAS Number 497-19-8
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Linear Formula Na2CO3
·
Molecular Weight 105.99
·
Beilstein Registry
Number 4154566
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EC Number 207-838-8
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PubChem Substance ID 24899762
Detailed d
It most commonly occurs as a crystalline decahydrate,
which readily effloresces to form a white
powder, the monohydrate. Pure sodium carbonate is a white, odorless powder that
is hygroscopic (absorbs moisture
from the air). It has a strongly alkaline taste, and forms a moderately basic
solution in water. Sodium carbonate is well known domestically for its everyday
use as a water softener. Historically it
was extracted from the ashes of 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) and limestone by a method known as
the Solvay process.
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 some calcium carbonate is added to the 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, it is used as a pH regulator to
maintain stable alkaline conditions necessary for the action of the majority of
photographic film developing agents. It acts as
an alkali because when dissolved in water, it dissociates into the weak acid:
carbonic acid and the strong alkali: sodium hydroxide. This gives sodium
carbonate in solution the ability to attack metals such as aluminium with the
release of hydrogen gas.[13]
It is a common additive in swimming pools used to raise the ph
which can be lowered by chlorine tablets and other
additives which contains acids.
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. 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.
Domestic use
Soda ash 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, but doesn't prevent scaling.[14] Sodium carbonate can
be used to remove grease, oil, and wine stains.
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).
Sodium carbonate test
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 copper (Cu), iron (Fe), and calcium (Ca), zinc (Zn) or lead
(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.
Main applications
By far the largest consumption of sodium carbonate,
is in the manufacture of glass, paper, rayon, soaps, and detergents. It is also
used as a water softener, since carbonate can precipitate the calcium and
magnesium ions present in "hard" water.
Sodium carbonate is a food additive (E500) used as
an acidity regulator, anticaking 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.
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 needed for acid delinting of fuzzy cottonseed.
Sodium carbonate, in a solution with common salt,
may be used for cleaning silver. In a nonreactive 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.
Sodium carbonate is used in some aquarium water pH buffers to
maintain a desired pH and carbonate hardness (KH).
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