Ammonium bicarbonate is an inorganic compound with formulaHCO3, simplified to
NH5CO3. The compound has many names, reflecting its long history. Chemically
speaking, it is the bicarbonate salt of the ammonium ion. It is a colourless
solid that degrades readily to carbon dioxide, water and ammonia.
Production Ammonium bicarbonate is produced by
combining carbon dioxide and ammonia: CO2 + NH3 + H2O →HCO3
Since ammonium bicarbonate is thermally unstable, the reaction solution is kept
cold, which allows the precipitation of the product as white solid. About
100,000 tons were produced in this way in 1997.
Ammonia gas passed into a strong aqueous solution of the
sesquicarbonateHCO3,2CO3, and H2O) converts it into normal ammonium
carbonate2CO3), which can be obtained in the crystalline condition from a
solution prepared at about 30 °C. This compound on exposure to air gives off
ammonia and reverts to ammonium bicarbonate.
= Salt of Hartshorn= Compositions containing ammonium
carbonate have long been known. They were once produced commercially,
formerly known as sal volatile or salt of hartshorn. It was obtained by the dry
distillation of nitrogenous organic matter such as hair, horn, leather. In
addition to ammonium bicarbonate, this material contains ammonium carbamate,
and ammonium carbonate2CO3). It is sometimes called ammonium
sesquicarbonate. It possesses a strong ammoniacal smell, and on digestion with
alcohol, the carbamate is dissolved leaving a residue of ammonium
bicarbonate. A similar decomposition takes place when
the sesquicarbonate is exposed to air. Uses
Ammonium bicarbonate is used in the food industry as a raising agent for flat
baked goods, such as cookies and crackers, and in China in steamed buns
and Chinese almond cookies. It was commonly used in the home before modern
day baking powder was made available. In China it is called edible or food-grade
"smelly powder". Many baking cookbooks may still refer to it as hartshorn or
hornsalt In many cases it may be substituted with baking soda or baking
powder or a combination of both, depending on the recipe composition and
leavening requirements. Compared to baking soda or potash, hartshorn has the
advantage of producing more gas for the same amount of agent, and of not leaving
any salty or soapy taste in the finished product, as it completely decomposes
into water and gaseous products that evaporate during baking. It cannot be
used for moist, bulky baked goods however, such as normal bread or cakes,
since some ammonia will be trapped inside and will cause an unpleasant
taste. It is commonly used as an inexpensive
nitrogen fertilizer in China, but is now being phased out in favor of urea for
quality and stability. This compound is used as a component in the production of
fire-extinguishing compounds, pharmaceuticals, dyes, pigments, and it
is also a basic fertilizer being a source of ammonia. Ammonium bicarbonate
is still widely used in the plastic and rubber industry, in the manufacture of
ceramics, in chrome leather tanning, and for the synthesis of catalysts.
It is also used for buffering solutions to slightly alkaline pH during chemical
purification, such as High Performance Liquid Chromatography. Because it
entirely decomposes to volatile compounds this allows rapid recovery of
the compound of interest by freeze-drying.
Ammonium bicarbonate is also a key component of the expectorant cough syrup
"Senega and Ammonia". Reactions
It dissolves in water to give a mildly alkaline solution. It is insoluble in
acetone and alcohols. Ammonium bicarbonate decomposes above
about 36 °C into ammonia, carbon dioxide, and water in an endothermic
process and so causes a drop in the temperature of the water:
NH4HCO3 → NH3 + H2O + CO2. When treated with acids, carbon dioxide
is also produced: NH4HCO3 + HCl → NH4Cl + CO2 + H2O.
Reaction with base produces ammonia. It reacts with sulfates of
alkaline-earth metals to precipitating their carbonates:
CaSO4 + 2 NH4HCO3 → CaCO3 +2SO4 + CO2 + H2O.
It also reacts with alkali metal halides, giving alkali metal bicarbonate
and ammonium halide: NH4HCO3 + NaCl → NH4Cl + NaHCO3;
NH4HCO3 + KI → NH4I + KHCO3; NH4HCO3 + NaBr → NH4Br + NaHCO3.
Safety Ammonium Bicarbonate is an irritant to
the skin, eyes and respiratory system. Short-term health effects may occur
immediately or shortly after exposure to Ammonium Bicarbonate. Breathing Ammonium
Bicarbonate can irritate the nose, throat and lungs causing coughing,
wheezing and/or shortness of breath. Repeated exposure may cause bronchitis
to develop with cough, and/or shortness of breath. Health effects can occur some
time after exposure to Ammonium Bicarbonate and can last for months or
years. No exposure limits have been established
for Ammonium Bicarbonate. This however, does not mean that this substance is not
harmful. Safe work practices are recommended and should be followed at
all times. Where possible, operations should be
enclosed and the use of local exhaust ventilation at the site of chemical
release is recommended. If local exhaust ventilation or enclosure is not used,
respirators are necessary. Wear protective work clothing. Change clothes
and wash thoroughly immediately after exposure to Ammonium Bicarbonate.
Ammonium Bicarbonate from China used to make "cookies" was found to be
contaminated with melamine, and imports were banned in Malaysia following the
2008 Chinese milk scandal. References