Gravimetric Analysis
CONTENTS
• Gravimetry
• Principle involved in gravimetry
• Types of gravimetric analysis
Precipitation gravimetry
Colloidal precipitates
Crystalline precipitate
• Co-precipitation
• Precipitation from homogenous solution
• Drying and Ignition of precipitate
• Applications of gravimetry
OBJECTIVES
By the end of this
lecture, students will be able to:
• Explain Gravimetric analysis
• Explain the principle involved in gravimetric analysis
• Discuss various co-precipitation techniques in Gravimetric
analysis
• Explain drying and ignition of precipitate in gravimetry
• Brief the applications of gravimetric analysis
Gravimetry
• Quantitative method
that is based on determining the mass of a pure compound to which the analyte
is chemically related.
• Gravimetric methods
are of two types: Precipitation gravimetry Volatilization gravimetry
• Precipitation
gravimetry - Analyte is separated from a solution of the sample as a
precipitate and is converted to a compound of known composition that can be
weighed
• Volatilization
gravimetry - Analyte is separated from other constituents of a sample by
converting it to a gas of known chemical composition Mass of the gas then
serves as a measure of the analyte concentration
Precipitation
Gravimetry
• Analyte is converted to a sparingly soluble precipitate
• Precipitate is then filtered, washed free of impurities
• Converted to a product of known composition by suitable
heat treatment and weighed
• For example, determination of calcium in water is one of
official methods of Association of Official Analytical Chemists
• Excess of oxalic acid is added to an aqueous solution of
the sample
• Ammonia is added to neutralizes the acid
• Also aid in the precipitation of calcium oxalate
• 2NH3 +
H2C2O4 ---------à 2NH4+ +
C2O4-
• Ca+2 + C2O4- ---------à
CaC2O4
• Calcium oxalate precipitate is filtered using a weighed
filtering crucible
• Precipitate is then dried and ignited
• Ignition
converts the precipitate to calcium oxide
• CaC2O4 ---------à CaO + CO + CO2
• After cooling, the crucible and precipitate are weighed
• Mass of calcium oxide is determined by subtracting the
known mass of crucible
• Calcium content of sample can be determined by gravimetric
analysis
Properties of
precipitates and precipitating reagents
• Ideally gravimetrically precipitating reagent should react
specifically or at least selectively with the analyte
• Specific reagents are rare, react only with a single
chemical species
• Selective reagents which are more common react with limited
number of species
Ideal precipitating
reagent would react with the analyte to give a product that is
1) Easily filtered and washed free of contamination
2) Sufficiently low solubility that no significant loss of
analyte occurs during filtration and washing
3) Unreactive with the constituents of the atmosphere
4) Known composition after it is dried or if necessary
ignited
Few reagents produce precipitates that have all these
desirable properties
Particle size and
filterability of precipitates
• Precipitates consisting of large particles are generally
desirable for gravimetric work
• Because these particles are easy to filter and wash free
of impurities
• Precipitates of large particles are usually purer than
precipitates made up of fine particles
Factors that
determine the particle size of precipitates
• Particle size of solids formed by precipitation varies
enormously
• One extreme are colloidal suspensions
• Tiny particles and are invisible to naked eye (10-7 to
10-14 cm in diameter)
• Colloidal particles show no tendency to settle from
solution and are difficult to filtrate
• Other extreme are
particles with dimensions
on the order
of tenths of a
millimeter or greater
• Temporary dispersion of such particles in the liquid phase
is called a crystalline suspension
• Tend to settle spontaneously and are easily filtered
• Mechanism of precipitate formation is not fully understood
• Particle size of precipitate is influenced by
• Precipitate solubility,
• Temperature,
• Reactant concentrations and
• Rate at which reactants are mixed
• By this, particle size is related to single property called
relative supersaturation
• Relative supersaturation = Q-S/S
• Where Q is concentration of solute at any instant
• S is its equilibrium solubility
• Precipitation reactions are slow
• Even precipitating reagent is added drop wise to solution
of an analyte, some supersaturation is likely
• Experimental evidence indicates that particle size of a
precipitate is inversely proportional to relative supersaturation
• Q-S/S is large, precipitate tends to be colloidal
• Q-S/S is small, crystalline solid is more likely
• A supersaturated solution is an unstable solution that
contains higher solute concentration
• Excess solute precipitates out with time, supersaturation
decreases to zero
• Precipitates form by nucleation and particle growth
• If nucleation predominates large number of very fine
particles is produced
• If particle growth predominates, a small number of larger
particles is obtained
• To increase the particle size of precipitate, minimize the
relative supersaturation during precipitate formation
• Experimentally methods to minimize supersaturation and produce
crystalline precipitates include:
• Elevated temperatures to increase the solubility of the
precipitate
• Dilute solutions
• Slow addition of precipitating agent with good stirring
Colloidal
Precipitates
• It is very difficult to filter the particles of a
colloidal suspension
• To trap these particles, the pore size of the filtering
medium must be so small that filtration takes a very long time
• With suitable treatment, individual colloidal particles
can be made stick together or coagulate to produce larger particles that are
easy to filter
Coagulation of
colloids
• Colloidal suspensions are stable
• Because all of the particles are either positive or
negative charge and repel
• Can be observed by placing electrodes
• Process by which ions are retained on the surface of a solid
is known as adsorption
• Can be hastened by heating, stirring and by adding an
electrolyte to the medium
• Adsorption of ions on ionic solid originates from the
normal bonding forces
• For example, a silver ion at the surface of a silver chloride
particle has partially unsatisfied bonding capacity for anions
• Chloride ions at the surface of solid exert an analogous
attraction for cations dissolved in the solvent
Peptization of
colloids
• Peptization is a process by which a coagulated colloid
returns to its dispersed state
• Coagulated
colloid on washing-
some of the
electrolyte responsible for
its coagulation is leached
• Repulsive forces will be reestablished and particles
detach themselves
• Practical treatment of colloidal precipitates
• Best precipitated from hot, stirred solutions containing
sufficient electrolyte to ensure coagulation
• Filterability of coagulated colloid often improves if
allowed to stand for an hour or more in contact with hot solution
• Process is known as digestion
• Result is denser mass that is easy to filter
Crystalline
precipitates
• More easily filtered and purified than coagulated colloids
• Particle size of crystalline solid can be improved by
• Using dilute solutions
• Adding precipitating reagent slowly with good mixing
• Adjusting pH of the solution
Digestion of crystalline precipitates often yields a purer
and more filterable product
Coprecipitation
Process in which otherwise soluble compounds are carried out
of solution by a precipitate
Four types of coprecipitation:
i) Surface adsorption
ii) Mixed crystal formation
iii) Occlusion
iv) Mechanical entrapment
(i) And (ii) are equilibrium processes
(iii) and (iv) arise from kinetics of crystal growth
Surface adsorption
• Common source of coprecipitation
• Major source of contamination in coagulated colloids with
large specific surface areas
• Effects on purity and are usually undetectable
• In adsorption, a normally soluble compound is carried out
of the solution on the surface of a coagulated colloid
• Net effect of surface adsorption is carrying down of an otherwise
soluble compound as a surface contaminant
• For example, coagulated silver chloride formed in
gravimetric determination of chloride ion is contaminated with
• Primarily adsorbed silver ions and nitrate or other ions
in counter-ion layer
• Result is silver nitrate, a normally soluble compound is
coprecipitated with silver chloride
Minimizing adsorbed
impurities on colloids
• Purity of coagulated colloids is improved by digestion
• In digestion, water is expelled from the solid to give a
denser mass that has a smaller specific surface area for adsorption
• Washing a coagulated colloid with electrolyte solution
improves purity
• For example, determination
of silver by
precipitation with chloride
ion- the primarily adsorbed
species is chloride
• Washing with
an acidic solution
converts the counter-ion
layer largely to hydrogen ions
• So that both chloride and hydrogen ions are retained by
the solid
• Volatile HCl is then given off when the precipitate is
dried
Reprecipitation
• Effective way to minimize the effects of adsorption
• Filtered solid is redissolved and reprecipitated
• First precipitate usually carries a fraction of
contaminant
Mixed-crystal
formation
• Is a type of co precipitation in which contaminant ion replaces
an ion in the lattice of a crystal
• For this
exchange to occur,
both the ions should have
same charge and difference in size should be not more
than 5%
• Both the salts should belong to same crystal class
• For example, barium sulfate formed by adding barium
chloride to a solution containing sulfate, lead and acetate ions
• Can be contaminated by lead sulfate
• This contamination occurs even though acetate ions prevent
precipitation of lead sulfate by complexing with lead
• In this case, lead ions replace some of the barium ions in
the barium sulfate crystals
• It is a troublesome type of coprecipitation
• Because little can be done about it when certain
combinations of ions present in a sample matrix
Occlusion and mechanical
entrapment
• Occlusion is a type of coprecipitation in which a compound
is trapped within a pocket formed during rapid crystal growth
• Foreign ions may get trapped or occluded in the rapidly
growing crystal
• Amount of occluded material is greatest in the part of a
crystal that forms first
• Mechanical entrapment occurs when crystal lie close
together during growth
• Crystals grow together and trap a portion of solution in a
tiny pocket
• Both occlusion and mechanical entrapment are minimum at
low supersaturation
Precipitation
from homogenous solution
• Homogenous precipitation is a process in which a
precipitate is formed by slow generation of a precipitating reagent homogenously
throughout a solution
• Solids formed by homogenous precipitation are generally
purer
• More easily filtered than precipitates generated by direct
addition of reagent to the analyte solution
• Because precipitating agent appears gradually and
homogenously throughout the solution and reacts immediately with the analyte
• Relative supersaturation is kept low during entire
precipitation
• Urea is often used for the homogenous generation of
hydroxide ion
• NH2CONH2 + 3H2O ------------à
CO2 + 2NH4+ + 2OH-
• This hydrolysis process proceeds slowly at temperatures
just 100 0C
• 1 to 2 hours needed to complete a typical precipitation
• Urea is particularly valuable for the precipitation of
hydrous oxides or basic salts
•For example, hydrous
oxides of iron
(III) and aluminium
formed by direct addition of base are bulky and
gelatinous masses
• Heavily contaminated and difficult to filter
• In contrast, same products produced by homogenous
generation of hydroxide ion are dense and easily filtered, considerably high
purity
• Results better crystal size as well as improvements in
purity
Drying and
Ignition of Precipitate
• After filtration, a gravimetric precipitate is heated until
its mass becomes constant
• Heating removes the solvent and any volatile species carried
down with the precipitate
• Some precipitates are also ignited to decompose the solid
and form a compound of known composition
• New compound is often called weighing form
• Temperature
required to produce
a suitable weighing
form varies from precipitate to precipitate
• Can be done in automatic thermobalance, an instrument that
records the mass of a substance continuously as its temperature is increased at
a constant rate
• Moisture is completely removed from silver chloride at a
temperature higher than 110 0C
• Dehydration of aluminium oxide is not complete until a
temperature greater than 1000 0C is achieved
• Aluminium oxide formed homogenously with urea can be completely
dehydrated at about 650 0C
• For calcium oxalate, about 1350C unbound water is
eliminated to give monohydrate
• About 4500C signals the decomposition of calcium
carbonate and carbon monoxide
• Final step depicts the conversion of calcium carbonate to
calcium oxide and carbon dioxide after reaching about 1000 0C
Applications
of Gravimetric Methods
• Developed for most inorganic anions and cations
• For neutral species as water, sulfur dioxide, carbon
dioxide and iodine
• Organic substances can also be determined gravimetrically
• Like lactose in milk products, salicylates in drug
preparations, nicotine in pesticides, cholesterol in cereals
• Gravimetric analysis do not require a calibration or
standardization step
• Because results are calculated directly from the
experimental data and atomic masses
• Requires less time and effort
SUMMARY
• Quantitative
methods that are
based on determining
the mass of
a pure compound to which the
analyte is chemically related.
• Gravimetric methods are of two types: Precipitation
gravimetry and volatilization gravimetry
• Analyte is converted to a sparingly soluble precipitate
• Ideally gravimetrically precipitating reagent should react
specifically or at least selectively with the analyte
• Precipitates consisting of large particles are generally
desirable for gravimetric work
• A supersaturated solution is an unstable solution that
contains higher solute concentration
• Major source of contamination in coagulated colloids with
large specific surface areas
• Reprecipitation- Effective way to minimize the effects of
adsorption
• Mixed crystal formation- Is a type of coprecipitation in
which contaminant ion replaces an ion in the lattice of a crystal
• Occlusion is a type of coprecipitation in which a compound
is trapped within a pocket formed during rapid crystal growth
• Homogenous precipitation is a process in which a
precipitate is formed by slow generation of a precipitating reagent homogenously
throughout a solution
• Urea is often used for the homogenous generation of
hydroxide ion
• After filtration, a gravimetric precipitate is heated until
its mass becomes constant
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