The document discusses gravimetric analysis, which is a form of quantitative chemical analysis based on measuring the mass of a substance. It specifically focuses on precipitation gravimetry, where an analyte is converted to an insoluble compound through the addition of a precipitating reagent. Key points covered include common forms of precipitation gravimetry, factors that influence precipitation, and sources of error like coprecipitation and postprecipitation of impurities. Methods to minimize impurities, such as washing, masking, digestion, and reprecipitation are also outlined.
1. GRAVIMETry
Dr Bincy Joseph
ASSISTANT PROFESSOR
DEPARTMENT OF CHEMISTRY
ST. MARY’S COLLEGE, THRISSUR
Gravimetry,Dr.Bincy Joseph,St.Mary's college,Thrissur.
2. Gravimetric Analysis
► It is the form of quantitative analysis which is based upon
the measurement of mass.
► Mass or change in mass and involves weighing.
Gravimetry,Dr.Bincy Joseph,St.Mary's college,Thrissur.
3. Various Forms of Gravimetric Analysis
► Precipitation gravimetry
► Volatilization gravimetry
► Electrogravimetry
► Thermogravimetry …
Gravimetry,Dr.Bincy Joseph,St.Mary's college,Thrissur.
4. Precipitation gravimetry
► It is based on the conversion of the analyte in solution into an
insoluble compound by the addition of a precipitating reagent or
precipitant.
► The precipitate thus obtained is filtered and washed free of
impurities.
► The estimation of the analyte is done by determining the weight of
the dry ppt.
► In some other cases ,the precipitated substance is converted into a
product of known composition by suitable heat treatment.
► He analyte is then estimated from the mass of the new pdt through
stoichiometric calculations.
Gravimetry,Dr.Bincy Joseph,St.Mary's college,Thrissur.
6. Precipitation gravimetry…
► Among steps involved in precipitation gravimetry, most important is
the precipitation.
► The choice of right precipitant
► The quantity of precipitant added
► The conditions of precipitation
► Nature of ppt formed etc are noted.
Gravimetry,Dr.Bincy Joseph,St.Mary's college,Thrissur.
7. Impurities in precipitates.
► The gretest source of error in gravimetry is the presence
of impurities within the lattices or on the surfaces of the
precipitate particles.
They occur mainly by
► Coprecipitation and
► Postprecipitation
Gravimetry,Dr.Bincy Joseph,St.Mary's college,Thrissur.
8. Coprecipitation
► The process in which substances that are normally soluble become
incorporated into a precipitate during its formation is known as
coprecipitation.
For an example,
► The precipitation of the contaminant barium nitrate with the desired
precipitate of Barium sulphate on adding sulfuric acid to a solution of
Barium Chloride containing a small amount of nitrate ion is a typical
example of coprecipitation.
Gravimetry,Dr.Bincy Joseph,St.Mary's college,Thrissur.
9. Types of Coprecipitation
1.Coprecipitation through surface adsorption
► Consider precipitate of Barium sulphate by adding sulphuric acid to
Barium chloride solution. primary particles of the ppt Barium
sulphate adsorb the common lattice ions of Barium ion and thus
become positively charged .The charge attracts negative ions ,
chloride, nitrate from the solution .In such cases ,the ion forming
least soluble salt is held by the ppt .Thus here, it is the impurity ion
nitrate, preferentially adsorbed ,not chloride ,because barium nitrare
is less soluble than barium chloride.Thus,barium nirate get
coprecipitate with barium sulphate due to surface adsorption.
Gravimetry,Dr.Bincy Joseph,St.Mary's college,Thrissur.
10. Types of Coprecipitation….
2.Coprecipitation through mixed crystal formation ( isomorphous
coprecipitation or inclusion)
► Coprecipitation through mixed crystal formation occurs when a
potential contaminant replaces a lattice ion of the solid.
► For example ,when barium sulphate is precipitated from a solution
containing traces of chromate ions , the latter get incorporated into
the lattice with sulphate ions forming mixed crystals and thus
barium chromate gets precipitated with barium sulphate.
Gravimetry,Dr.Bincy Joseph,St.Mary's college,Thrissur.
11. Types of Coprecipitation….
3. Coprecipitation through occlusion: Internal adsorption and mechanical
entrapment:
Occlusion is a type of coprecipitation in which an interfering compound is
entrapped within the pockets of the growing precipitate during rapid
crystal growth.
For example,barium sulphate is precipitated by adding sulphuric acid to
barium chloride solution may contain barim chloride through internal
adsorption .if nitrate ions is present, it will be Barium nitrate ,that is
preferentially trapped within the ppt because of its lower solubility.
Gravimetry,Dr.Bincy Joseph,St.Mary's college,Thrissur.
12. Methods of minimizing coprecipitation
► 1.Washing
► 2.separation or masking of impurity
► 3.Choice of precipitant
► 4.Digestion
► 5.Reprecipitation
► 6.Precipitation from homogeneous solution
Gravimetry,Dr.Bincy Joseph,St.Mary's college,Thrissur.
13. Post precipitation
► The process by which an impurity gets deposited on the surface of
the desired ppt after its formation is called post precipitation.
► For example, Ca2+ as precipitated as oxalate in the presence of Mg2+ ,
magnesium oxalate slowly postprecipitates on calcium oxalate.
► Postprecipitation causes considerable errors in gravimetric analysis.
► substances that normally get post precipitated are sparingly soluble
ones capable of forming relatively stable supersaturated solutions.
► They generally have an ion common with the primary precipitate.
Gravimetry,Dr.Bincy Joseph,St.Mary's college,Thrissur.
14. Methods of minimizing Post precipitation
► Regulation of acidity.
► Masking the species likely to get post precipitated
through complexation.
► Filter off the primary precipitate as early as possible.
Gravimetry,Dr.Bincy Joseph,St.Mary's college,Thrissur.
