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March 6,2013 INORGANIC ASSIGNMENT
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Reaction Mechanism
The calcium is precipitated as calcium oxalate CaC2O4.H2O by
treating a hothydrochloric acid solution with ammonium oxalate and
slowly neutralizing withaqueous ammonia solution
Ca+2 + C2O4-2 + H2O CaC2O4.H2O
Calcium oxalate monohydrate has the solubility of 0.0067g/dm3
and0.0140g/dm3 at 25oC and 950C respectively. The solubility is
less in neutralsolutions containing moderate concentrations of
ammonium oxalate owing toCommon Ion Effect, Since a dilute solution
of ammonium oxalate is employedas Wash Liquid in gravimetric
determination.Calcium oxalate being the salt of a weak acid, its
solubility increases withincreasing hydrogen ion concentration of
the solution because of the removal ofthe oxalate ions to form
hydrogen oxalate ions and oxalic acid.
CaC2O4 Ca+2 + C2O4-2
C2O4+2 + H+ HC2O4-
HC2O4- + H+ H2C2O4
This Common Ion Effect gives precipitates of Calcium Oxalate
which dissolved indilute H2SO4 and the liberated oxalic acid its
titrated with standard KMnO4solution.
H2C2O4 + KMnO4 + H2SO4MnSO4 + K2SO4 + CO2 + H2O
Effect of Heating
Anhydrous calcium oxalate appears to be stable between 225oC and
398oC.However by water free calcium oxalate along with H2SO4 will
give satisfactoryresult at about 70-80oC. At this temperature
coarse crystals are obtained.Precipitation from cold neutral or
ammoniacal solutions yields a very finelydivided precipitate, which
is difficult to filter. Good results are obtained byadding ammonium
oxalate to a hot acid solution of calcium salt (more or lesscalcium
oxalate may precipitate, depending upon the pH of the solution)
andfinally neutralizing with aqueous ammonia solution. The
precipitation afterdigesting for about half an hour consists of
relatively coarse crystals which arereadily filtered.
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March 6,2013 INORGANIC ASSIGNMENT
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INDUSTRIAL USES OF CALCIUM
Metallurgical IndustriesMetallurgical industries use large
amount of calcium or calcium containing alloysfor a variety of
purposes. In Ferrous Metallurgy, calcium and certain of its
alloysare used extensively as addition agents to deoxidize, degas
steel, desulfurize andcast iron to control the type and
distribution of non metallic inclusions in steeland to promote a
uniform microstructure in gray iron. Because of addition ofpure
calcium to molten steel, it is difficult due to its reactivity.
Using alloys suchas calcium-silicon may overcome this problem but
introduces unwanted alloyelements into the iron or steel. In a new
and promising technique a steel-cladcalcium wire is quickly fed
into molten steel.
Lead Industry
Calcium is used in the lead industry as Refining Agent and as an
alloyingingredient. When added to molten lead during refining,
calcium metal removesbismuth impurities through formation of the
insoluble intermetallic compoundBi2Ca3
As a Reducing Agent
Calcium metal is an excellent Reducing Agent for production of
the less commonmetals because of the large free energy of its
oxides and halides
For same processes, calcium metal first reacts with hydrogen to
form calcium
hydride which is then used as the actual reducing agent.
Additional amounts of calcium metal are converted into calcium
hydride for useas a portable source of hydrogen gas.
In Thermal BatteriesCalcium metal is also used in strip form as
the anode material in ThermalBatteries, which are used as the Power
Source in artillery fuses.
In Automobile IndustryThe mechanical and electrical properties
of aluminum alloys are improved
through addition of small amounts of calcium. Such calcium
containing alloys areused for die casting of automobile trim.
REFERENCES
Vogels (Quantitative Analysis)Encyclopedia
Ferman Inorganic Analysis
