it deals with precipitation post and co precipitation organic precipitants

1. Gravimetric Analysis
By

2. Definition
• Gravimetry encompasses all techniques
in which we measure mass or a change
in mass. Measuring mass is the most
fundamental of all analytical
measurements, and gravimetry is
unquestionably the oldest analytical
technique.
• Quantitative estimation by weight

3. • It is the process of isolating and weighing
an element or a definite compound of
the element in as pure form as possible.
• It is concerned with the transformation
of the element or radical into a stable.
• Pure compound can be readily converted
into a suitable form for weighing.
• It is time consuming but are reliable

4. Types of Gravimetric Methods
• Precipitation Gravimetry
A gravimetric method in which the signal is the mass of a
precipitate.
Eg
• The indirect determination of PO3
• 3– by precipitating
• Hg2Cl2 is a representative example, as is the direct
determination of Cl– by
• precipitating AgCl.

5. Electrogravimetry
A gravimetric method in which the signal is
the mass of an electro deposit on the cathode or
anode in an electrochemical cell.
• The oxidation of Pb2+, and its deposition as PbO2
on a Pt anode is one example of
electrogravimetry.
• Reduction also may be used in electrogravimetry.
The electrodeposition of Cu on a Pt cathode, for
example, provides a direct analysis for Cu2+.

6. Volatilization Gravimetry
A gravimetric method in which the
loss of a volatile species gives rise to
the signal.
• In determining the moisture content of food,
thermal energy vaporizes the H2O.
• The amount of carbon in an organic
compound may be determined by using the
chemical energy of combustion to convert C to
CO2.

7. Particulate Gravimetry
A gravimetric method in which the
mass of a particulate analyte is
determined following its separation
from its matrix.
• The determination of suspended solids is one
example of particulate gravimetry.

8. • The solubility of a substance at a given
temperature in a given solvent. – saturation
• If the substance dissolved in excess, it results
in a Supersaturated solution.
• Super saturation is an unstable state which is
brought to a state of stable equilibrium by the
addition of crystal of solute usually termed as
seeding

10. Precipitation Gravimetry
• Precipitation gravimetry is based on the
formation of an insoluble compound following
the addition of a precipitating reagent, or
precipitant [precipitant - A reagent that
causes the precipitation of a soluble species.]
,to a solution of the analyte.
• Any reaction generating a precipitate can
potentially serve as a gravimetric method.

11. Theory and Practice
• The precipitate must be of low solubility, high
purity, and of known composition.
• The precipitate must be in a form that is easy
to separate from the reaction mixture.
• Another important parameter that may affect
a precipitate’s solubility is the pH of the
solution in which the precipitate forms
• Solubility can often be decreased by using a
nonaqueous solvent.

12. • A precipitate’s solubility is generally greater in
aqueous solutions because of the ability of water
molecules to stabilize ions through solvation.
• The poorer solvating ability of nonaqueous
solvents, even those that are polar, leads to a
smaller solubility product.
• For example
• PbSO4 has a Ksp of 1.6 x 10–8 in H2O,
whereas in a
50:50 mixture of H2O/ethanol the Ksp at 2.6 x
10–12 is four orders of magnitude smaller.

13. Avoid impurities
• Precipitation gravimetry is based on a known
stoichiometry between the analyte’s mass and
the mass of a precipitate. It follows, therefore,
that the precipitate must be free from impurities.
• Precipitation typically occurs in a solution rich in
dissolved solids, the initial precipitate is often
impure.
• Any impurities present in the precipitate’s matrix
must be removed before obtaining its weight.

14. • The greatest source of impurities results from
chemical and physical interactions occurring at
the precipitate’s surface.
• Precipitate particles grow in size because of the
electrostatic attraction between charged ions on
the surface of the precipitate and oppositely
charged ions in solution.
• Ions common to the precipitate are chemically
adsorbed, extending the crystal lattice. Other ions
may be physically adsorbed and, unless displaced,
are incorporated into the crystal lattice as a
coprecipitated impurity. Physically adsorbed ions
are less strongly attracted to the surface and can
be displaced by chemically adsorbed ions.

15. Types of impurity
• Inclusion -A coprecipitated impurity in which the
interfering ion occupies a lattice site in the precipitate.
• The probability of forming an inclusion is greatest
when the interfering ion is present at substantially
higher concentrations than the dissolved lattice ion.
• Inclusions are difficult to remove since the included
material is chemically part of the crystal lattice.
• The only way to remove included material is through
reprecipitation [ mass of impure analyte is less].
• This process of reprecipitation is repeated as needed to
completely remove the inclusion.

16. CACACACACACACACACACA
ACACACACACACACACACAC
CACACACAMACACACACACA
ACACACACACACAMACACAC
CACACACACACACACACACA
AMACACACACACACACACAC
CACACACACACACACACACA
• Occlusion- A coprecipitated impurity trapped
within a precipitate as it forms.
• Occlusions form in two ways.
• The most common mechanism occurs when
physically adsorbed ions are surrounded by
additional precipitate before they can be
desorbed or displaced

17. CACACACACACACACACACA
ACACACACACACACACACAC
CACACACACACACACACACA
ACACACACACACACACACAC
CACACACACACACACACACA
ACACACACACACACACACAC
CACACACACACACACACACA
• Occlusions also form when rapid precipitation
traps a pocket of solution within the growing
precipitate

18. • Occlusions are minimized by maintaining the
precipitate in equilibrium with its supernatant
solution for an extended time. This process is
called digestion.
• After precipitation is complete the surface
continues to attract ions from solution These
surface adsorbates, which may be chemically or
physically adsorbed, constitute a third type of
coprecipitated impurity.
• Adsorption is maximum in gelatinous precipitate
and is least for micro crystalline precipitates.
• Surface adsorbates also may be removed by
washing the precipitate. Potential solubility
losses, however, cannot be ignored.

19. • Inclusions, occlusions, and surface adsorbates
are called coprecipitates
• Precipitation consists of two distinct events:
nucleation,or the initial formation of smaller
stable particles of precipitate.
homogeneous precipitation
• A precipitation in which the precipitant is
generated in situ by a chemical reaction.
• Coagulation- The process of smaller particles
of precipitate clumping together to form
larger particles.

20. • The amount of electrolyte needed to cause
spontaneous coagulation is called the critical
coagulation concentration.
• Precipitation should be carried out in dilute
solutions as this minimises errors due to
coprecipitation.
• Reagent should be mixed slowly with constant
stirring, as it helps the growth of large regular
crystals.
• Precipitate should be washed with dilute
solutions as pure water tends to cause
peptisation [ hydrolysis ]

21. Schematic model of the solid–solution
interface at a particle of AgCl in a solution
containing excess AgNO3.

22. Post precipitation-Precipitation occur on the
surface of the first precipitate after its formation
Post ppt example

23. • Peptization- The reverse of coagulation in
which a coagulated precipitate reverts to
smaller particles.
• Rinsing the precipitate to remove this residual
material must be done carefully to avoid
significant losses of the precipitate.
• Filtering removes most of the supernatant
solution.
• Usually the rinsing medium is selected to
ensure that solubility losses are negligible.

24. • In many cases this simply involves the use of cold
solvents or rinse solutions containing organic
solvents such as ethanol.
• Precipitates containing acidic or basic ions may
experience solubility losses if the rinse solution’s
pH is not appropriately adjusted.
• When coagulation plays an important role in de-
termining particle size, a volatile inert electrolyte
is often added to the rinse water to prevent the
precipitate from reverting into smaller particles
that may not be retained by the filtering device.
This process of reverting to smaller particles is
called peptization.

25. • The volatile electrolyte is removed when drying
the precipitate.
• In general, solubility losses are minimized by
using several small portions of the rinse solution
instead of a single large volume.
• Testing the used rinse solution for the presence
of impurities is another way to ensure that the
precipitate is not over rinsed.
• This can be done by testing for the presence of a
targeted solution ion and rinsing until the ion is
no longer detected in a freshly collected sample
of the rinse solution.

26. • Finally, after separating the precipitate from its
supernatant solution the precipitate is dried to
remove any residual traces of rinse solution and
any volatile impurities.
• A temperature of 110 °C is usually sufficient when
removing water and other easily volatilized
impurities.
• A conventional laboratory oven is sufficient for
this purpose.
• Higher temperatures require the use of a muffle
furnace, or a Bunsen or Meker burner, and are
necessary when the precipitate must be
thermally decomposed before weighing or when
using filter paper.

27. • To ensure that drying is complete the precipitate is
repeatedly dried and weighed until a constant weight is
obtained.
• Precipitates containing volatile ions or substantial
amounts of hydrated water are usually dried at a
temperature that is sufficient to completely remove
the volatile species.
• An additional problem is encountered when the
isolated solid is nonstoichiometric.
• For example, precipitating Mn2+ as Mn(OH)2, followed
by heating
• to produce the oxide, frequently produces a solid with
a stoichiometry of MnOx, [ formation of a mixture of
several oxides that differ in the oxidation state
• of manganese ]

28. VOLATILIZATION GRAVIMETRY

29. • Thermogravimetry - A form of volatilization
gravimetry in which the change in a sample’s
mass is monitored while it is heated.
• Theory and Practice
• The analysis is direct or indirect, volatilization
gravimetry requires that the products of the
decomposition reaction be known.
• For inorganic compounds, however, the
identity of the volatilization products may
depend on the temperature at which the
decomposition is conducted.

30. • Thermogravimetry The products of a thermal
decomposition can be deduced by monitoring
the sample’s mass as a function of applied
temperature.
• The loss of a volatile gas on thermal
decomposition is indicated by a step in the
Thermogram - A graph showing change in mass
as a function of applied temperature.
• Thermal decomposition or combustion is
accomplished using a Bunsen or Meker burner, a
laboratory oven or a muffle furnace, with the
volatile products vented to the atmosphere.

31. • The weight of the sample and the solid
residue are determined using an analytical
balance.

32. PARTICULATE GRAVIMETRY

33. • Two approaches have been used to separate the
analyte from its matrix in particulate gravimetry.
• The most common approach is filtration, in which
solid particulates are separated from their gas,
liquid, or solid matrix.
• A second approach uses a liquid-phase or solid-
phase extraction.
• Liquid samples are filtered by pulling the liquid
through an appropriate filtering medium, either
by gravity or by applying suction from a vacuum
pump or aspirator.

34. Role of organic precipitants in
gravimetric analysis
• The organic reagents which are used in
gravimetric analysis are generally termed as
organic precipitants.
• The separation of one or more inorganic ions
from mixtures may be made with the aid of
organic reagents.
• The compounds have high molecular weight
which in turn facilitates determination of a small
amount of ion with a large amount of precipitate.

35. • The ideal organic precipitant should be
specific in character.
• The precipitate so formed with organic
reagents is weighed after drying or it is ignited
and weighed as an oxide or the precipitate is
dissolved in mineral acids and determined
volumetrically.
• Most important organic reagents are chelating
agent.

36. • When a poly functional ligand which can
occupy more than two positions in the co
ordination sphere it combines with a central
metal ion to form a co ordination compound
with a closed ring structure is termed as
chelate.
• A closed member ring incorporating a metal is
thus called a chelate.
• It has special place in inorganic analysis
because the precipitants which they form are
for the most part qualitatively different from
the purely inorganic precipitates like BaSO4

37. • In order to form chelate the ligand must have
a replacable hydrogen atom and a pair of
unshared electrons for co ordination.
• Organic reagents are preferred because they
are selective.
• Steric hinderance often causes the selectivity
of the chelating agent.

38. Organic precipitants

39. • The reagents IV or VI are selective for
copper on account of steric hinderance.
• The reagent V is selective for copper on
account of steric hinderance.
• The selectivity can be further improved
by control of PH.
• By using sequestering agents the reaction
can be selective.

40. Criteria for choice of an organic
reagent
• Organic reagent must be selective for a
particular metal use of dimethyl glyoxime or 1
nitro 2 napthol for precipitation for nickel or
cobalt respectively.
• They do not coprecipitate impurities and
other ionic precipitates.
• N-phenyl N- benzoyl hydroxyl amine –
Niobium and tantalum

42. Application of gravimetric Analysis
• Determination of purity and thermal stability
of both primary and secondary standard.
• Determination of the composition of complex
mixture.
• Studying the sublimation behaviour of various
substances.

43. Limitation of Gravimetric Analysis
• Only few derivatives are available which are
quantitatively insoluble.
• The impurities present in the samples are also
converted into equally insoluble derivatives
under the experimental conditions.
• The insoluble precipitate is washed thorough to
remove reagents and their impurities. This
washing may dissolve some amount of the
precipitate and also increases the volume of the
solvent water.

44. • Precipitate is to be collected quantitatively by
filtration. Some colloidal form of the precipitate
may pass through the filter.
• Final drying is carried out by heating for removing
the moisture. This step may bring about the loss
of some precipitate by spurting or cracking.
• The process become complicated, tedious time
consuming and reduces precision.
• Hence pharmacopoeia usually avoids gravimetric
methods of assay when other equally accurate
methods are available.