Gravimetric Analysis

2
Points to be covered
• Principle and steps involved in gravimetric
analysis.
• Purity of the precipitate: co-precipitation and
post precipitation,
• Estimation of barium sulphate

3
Introduction
• Gravimetric Analysis is a group of analytical methods in which
the amount of analyte is determined by the measurement of the
mass of a pure substance containing the analyte.
Types of Gravimetric Analyses:
• There are two main types of gravimetric analyses:
A) Precipitation
– analyte must first be converted to a solid (precipitate) by
precipitation with an appropriate reagent.
– The precipitates from solution is filtered, washed, purified
(if necessary) and weighed.
B) Volatilization
– In this method the analyte or its decomposition products
are volatilised (dried) and then collected and weighed, or
alternatively, the mass of the volatilised product is
determined indirectly by the loss of mass of the sample.

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Criteria For a successful determination
• For a successful determination in gravimetric analysis the
following criteria should be met
1. The desired substance must be completely precipitated. In
most determination the precipitate is of such low
solubility that losses from dissolution are negligible. An
additional factor is the common ion effect, this further
decrease the solubility of the precipitate.
– E.g. When Ag+ is precipitated out by addition of Cl-
– Ag+ + Cl- = AgCl
• The low solubility of AgCl is reduced further by the excess
of Cl- which is added force to the reaction to proceed
towards right side.

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Criteria For a successful determination
2. The weighed form of the product should be of
known composition.
3. The product should be pure and easily filtered.
4. Easy in handling i.e. ppt filtering, washing drying
and weighing.
• It is usually difficult to obtain a product which is
pure or which is free from impurities.
• This could be reduced by careful precipitation
and sufficient washing.

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Advantages of Gravimetric Analysis
• Accurate and precise: Gravimetric analysis is
potentially more accurate and more precise than
volumetric analysis
• Possible sources of errors can be checked:
Gravimetric analysis avoids problems with
temperature fluctuations, calibration errors, and
other problems associated with volumetric
analysis.
• It is an ABSOLUTE method.
• Relatively inexpensive

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Disadvantages
• But there are potential problems with
gravimetric analysis that must be avoided to
get good results.
• Proper lab technique is critical
• Careful and time consuming.
• Scrupulously clean glassware.
• Very accurate weighing.
• Coprecipitation.

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Properties of precipitate
• The ppt should be so insoluble that no significant
loss occurs during filtration and washing
• Physical nature of ppt should be such that it cab
be easily separated by filtration
• The PPT should be stable to atmospheric condn.
• The ppt must be convertible to pure compound of
definite composition, either by ignition or by
simple chemical operations such as evaporations.
• Have large crystals (Easier to filter large crystals)
• Be free of contaminants

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Particle Size and Filterability of
Precipitates
• Precipitates made up of large particles are
generally desirable in gravimetric work
because large particles are easy to filter and
wash free of impurities. In addition, such
precipitates are usually purer than are
precipitates made up of fine particles.
• Three types of ppt are produced
– Crystalline, Curdy and gelatinous etc.

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Process of precipitation
• It is a most imp step in gravimetric analysis
• Involves both physical and chemical process
• The physical process consists of three steps
1) Super saturation: the solution phase contains more dissolved salt than at
equilibrium. The driving force will be for the system to approach
equilibrium (saturation).
2) Nucleation : initial phase of precipitation. A min number of particle will
gather together to form a nucleus of particle or precipitate (solid phase).
Higher degree of super saturation, the greater rate of nucleation
• nucleation involves the formation of ion pairs and finally a group of ions
formed.
• it is of two types 1. Spontaneous and 2. Induced
3) Crystal growth : particle enlargement process. Nucleus will grow by
deposition of particles precipitate onto the nucleus and forming a crystal
of a specific geometric shape. Involving two steps diffusion of ion to
surface of nucleus and Deposition on surface.

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Precipitation process (Von weimarn eq)
• Von weimarn discover – the particle size of precipitates is
inversely proportional to the relative supersaturation of the sol.
during the precipitation process.
– The von Weimarn Ratio (The lower the better)
– von Weimarn ratio = (Q – S)/S
• A measure of relative supersaturation or supersaturation ratio
• If high, get excessive nucleation, lots of small crystals, large
surface area
• If low, get larger, fewer crystals, small surface area
• S = solubility of precipitate at equilibrium, ( Keep it high with
high temperatures, adjusting pH)
• Q = concentration of reagents before precipitation (Keep it low
by using dilute solutions, stir mixture well, add reactants slowly)
• Can lower S later by cooling mixture after crystals have formed

What Factors Determine Particle Size?
The particle size of solids formed by precipitation
varies enormously. At one extreme are colloidal
suspension, whose tiny particles are invisible to
the naked eye (10-7 to 10-4 cm in diameter).
Colloidal particles show no tendency to settle
from solution, nor are they easily filtered. At the
other extreme are particles with dimensions on
the order of tenths of millimeter or greater. The
temporary dispersion of such particles in the
liquid phase is called a crystalline suspension. The
particles of a crystalline suspension tend to settle
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The particle size of a precipitate is influenced by
experimental variables as precipitate solubility,
temperature, reactant concentrations, and the
rate at which reactants are mixed. The particle size
is related to a single property of the system called
its relative supersaturation, where
relative supersaturation = (Q – S) / S
In this equation, Q is the concentration of the
solute at any instant and S is its equilibrium
solubility.
When (Q – S)/ S is large, the precipitate tends to
be colloidal.
when (Q – S) / S is small, a crystalline solid is more
likely.

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• How do Precipitates Form? Precipitates form in two
ways, by nucleation and by particle growth. The
particle size of a freshly formed precipitate is
determined by which way is faster.
In nucleation, a few ions, atoms, or molecules
(perhaps as few as four or five) come together to form
a stable solid. Often, these nuclei form on the surface
of suspended solid contaminants, such as dust
particles. Further precipitation then involves a
competition between additional nucleation and
growth on existing nuclei (particle growth). If
nucleation predominates, a precipitate containing a
large number of small particles results; if growth
predominates, a smaller number of larger particles is
produced.

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Controlling Particle Size
variables
Experimental
supersaturation and thus
that minimize
lead to crystalline
precipitates include elevated
increase the solubility of the precipitate (S
temperatures to
in
Equation), dilute solutions (to minimize Q), and
slow addition of the precipitating agent with good
stirring. The last two measures also minimize the
concentration of the solute (Q) at any given instant.
Larger particles can also be obtained by pH control,
provided the solubility of the precipitate depends
on pH.

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Colloidal Precipitates
Coagulation
hastened by heating, stirring, and adding
of Colloids: Coagulation can be
an
electrolyte to the medium.
Colloidal suspensions are stable because all the
particles present are either positively or negatively
charged. This charge results from cations or anions
that are bound to the surface of the particles. The
process by which ions are retained on the surface
of a solid is known as adsorption. We can readily
demonstrate that colloidal particles are charged
by observing their migration when placed in an
electrical field.

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Peptization of Colloids
Peptization refers to the process by which a
coagulated colloid reverts to its original dispersed
state. When a coagulated colloid is washed, some
of the electrolyte responsible for its coagulation is
leached from the internal liquid in contact with
the solid particles. Removal of this electrolyte has
the effect of increasing the volume of the counter-
ion layer. The repulsive forces responsible for the
original colloidal state are then reestablished, and
particles detach themselves from the coagulated
mass. The washings become cloudy as the freshly
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Crystalline Precipitates
Crystalline precipitates are generally more easily filtered
and purified than coagulated colloids. In addition, the size
of individual crystalline particles, and thus their
filterability, can be controlled to a degree.
The particle size of crystalline solids can often be
improved significantly by minimizing Q, maximizing S, or
both in Equation. Minimization of Q is generally
accomplished by using dilute solution and adding the
precipitating from hot solution or by adjusting the pH of
the precipitation medium.
Digestion of crystalline precipitates (without stirring) for
some time after formation frequently yields a purer, more
filterable product. The improvement in filterability results
from the dissolution and recrystallization.

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Purity of precipitate
• When the ppt is separated out from solution it is
always not preferably pure and may be
contaminated even after washing
• The amount of impurities depends on nature of
PPt and condition of pptn
• It may be due to
– Co-precipitation
– Post precipitation, Surface adsorption
– Mixed crystal formation
– Occlusion and Mechanical Entrapment

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Coprecipitation
Coprecipitation is the phenomenon in which
soluble compounds are removed from solution
during precipitate formation.
There are four types of coprecipitation:
i) surface adsorption, ii) mixed-crystal formation,
iii) occlusion, iv) mechanical entrapment
Surface adsorption and mixed crystal formation
are equilibrium processes, whereas occlusion
and mechanical entrapment arise from the
kinetics of crystal growth.

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Surface Adsorption
Adsorption is a common source of coprecipitation that is
likely to cause significant contamination of precipitates with
large specific surface areas, that is coagulated colloids.
Coagulation of a colloid does not significantly decrease the
amount of adsorption because the coagulated solid still
contains large internal surface areas that remain exposed
to the solvent. The coprecipitated contaminant on the
coagulated colloid consists of the lattice ion originally
adsorbed on the surface before coagulation and the
counter ion of opposite charge held in the film of solution
immediately adjacent to the particle. The net effect of
surface adsorption is therefore the carrying down of an
otherwise soluble compound as a surface contaminant.

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Minimizing Adsorbed Impurities on Colloids
The purity of many coagulated colloids is improved by
digestion. During this process, water is expelled from the
solid to give a denser mass that has a smaller specific
surface area for adsorption.
Washing a coagulate colloid with a solution containing a
volatile electrolyte may also be helpful because any
nonvolatile electrolyte added earlier to cause coagulation
is displace by the volatile species. Washing generally does
not remove much of the primarily adsorbed ions because
the attraction between these ions and the surface of the
solid is too strong. Exchange occurs, however between
existing counter ions and ions in the wash liquid.

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Reprecipitation
A drastic but effective way to minimize the effects
of adsorption is reprecipitation, or double
precipitation. Here, the filtered solid is redissolved
and reprecipitated. The first precipitate ordinarily
carries down only a fraction of the contaminant
present in the original solvent. Thus, the solution
containing the redissolved precipitate has a
significantly lower contaminant concentration
than the original, and even less adsorption occurs
during the second precipitation. Reprecipitation
adds substantially to the time required for an
analysis.

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Mixed-Crystal Formation
In mixed-crystal formation, one of the ions in the crystal
lattice of a solid is replaced by an ion of another element.
For this exchange to occur, it is necessary that the two
ions have the same charge and that their sizes differ by no
more than about 5%. Furthermore, the two salts must
belong to the same crystal class. For example, MgKPO4, in
MgNH4PO4, SrSO4 in BaSO4, and MnS in CdS.
The extent of mixed-crystal contamination increases as
increases. Mixed-crystal formation is
the ratio of contaminant to analyte concentration
troublesome
because little can be done about it. Separation of the
interfering ion may have to be carried out before the final
precipitation step. Alternatively, a different precipitating
reagent may be used.

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Occlusion and Mechanical Entrapment
When a crystal is growing rapidly during precipitate
formation, foreign ions in the counter-ion layer may become
trapped, or occluded, within the growing crystal.
Mechanical entrapment occurs when crystals lie close
together during growth. Here, several crystals grow together
and in so doing trap a portion of the solution in a tiny pocket.
Both occlusion and mechanical entrapment are at a minimum
when the rate of precipitate formation is low, that is, under
conditions of low supersaturation. Digestion is often
remarkably helpful in reducing these types of copreipitation.
The rapid solution and reprecipitation that goes on at the
elevated temperature of digestion opens up the pockets and
allows the impurities to escape into the solution.

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Precipitation from Homogeneous Solution
technique in which a precipitating agent
Precipitation from homogeneous solution is a
is
generated in a solution of the analyte by a slow
chemical reaction. Local reagent excesses do not
occur because the precipitating agent appears
gradually and homogeneously throughout the
solution and reacts immediately with the analyte.
As a result, the relative supersaturation is kept low
homogeneously formed precipitates,
during the entire precipitation. In general,
both
colloidal and crystalline, are better suited for
analysis than a solid formed by direct addition of a
precipitating reagent.

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Steps in a gravimetric analysis
1. Preparation of the solution
2. Precipitation
3. Digestion
4. Filtration
5. Washing
6. Drying or ignition
7. Weighing
8. Calculation

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1.Preparation of analyte solution
1st
• Gravimetric analysis usually involves precipitation of
analyte from solution.
step – Sampling; Representative of bulk
2nd step - prepare the analyte solution (Dissolution)
• May need :
– preliminary separation to separate potential interferences
before precipitating analyte
– adjustment of solution condition (pH/temp/vol/conc of test
substance) to maintain low solubility of precipitate & max
precipitate formation. Eg Calcium oxalate insoluble in basic
medium
– Most of the substances are readily solute in water and can
be used as such. Some required special treatment as
treatment with HCl, HNO3, Aquaregia or fusing with basic
flux

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2.Precipitation process :
• The precipitating reagent is added at a
concentration that favors the formation of a "good"
precipitate.
• This may require low concentration, extensive
heating (often described as "digestion"), or careful
control of the pH.
• A large excess of pptant should be avoided because
this increases chances of adsorption on ppt.
• Test for completeness of pptation
• No new ppt should be formed after addition of
drop of ppting agent.

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3.Digestion of the Precipitate
• Let precipitate stand in contact with mother
liquor (the solution from which it was
precipitated), usually at high temp
• This process is called digestion, or
Ostwaldripening. The small particles tend to
dissolve and precipitate on the surfaces of the
larger crystals
• Digestion make larger crystals, reduce surface
contamination, reduce crystals imperfection

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• Ppt is separated from mother liquor
• Choice depends on nature of ppt , cost of media
and heating temp required for drying.
• Filtration medium used are
– Filter papers
– Filter pulps
– Filter mats
– Crucible fitted with porous plate (Sintered glass filters)
– Crucible to be used at high temperature
4. Filtration

Filtration media
• Filter papers: eg ash less filter papers
– No 41 (Fast), 40 (Medium), 42(Slow)
– 541, 540, 542 Grater mechanical strength
• Filter pulps
• Filter mats (Gooch crucible)
• Sintered glass crucibles
– Types G1 (100-120 ), G2 (40-50 ), G3 (20-30 ),
G4 (5-10 )
– They are resistant to chemical and easy to clean
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• Sintered glass crucibles are used
to filter the precipitates.
• The crucibles first cleaned
thoroughly and then subjected to
the same regimen of heating and
cooling as that required for the
precipitate.
• This process is repeated until
constant mass has been
achieved, that is, until
consecutive weighing differ by
0.3 mg or less.
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5. Washing
• Co precipitated impurities esp those on
surface, removed by washing the precipitate
• Wet precipitate with mother liquor and which
will also be remove by washing
• Need to add electrolyte to the wash liquid
bcoz some precipitate cannot be wash with
pure water, peptization occur.
• Eg HNO3 for AgCl precipitate

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6) Drying or ignition
• To remove solvent and wash electrolytes
• Done by heating at 110 to 120°C for 1 to 2 hrs.
• Converts hygroscopic compound to non-
hygroscopic compound
• May used high temp if precipitate must be
converted to a more suitable form before
weighing
• Eg MgNH4PO4 convert to pyrophosphate
Mg2P2O7 by heating at 900°C.

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7) Weighing
• After the precipitate is allowed to cool
(preferably in a desiccator to keep it from
absorbing moisture), it is weighed (in the
crucible).
• Properly calibrated analytical balance
• Good weighing technique

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Organic Precipitates
• Organic precipitating agents have the
advantages of giving precipitates with very
solubility in water and a favorable gravimetric
factor.
• Most of them are chelating agents that forms
slightly insoluble, uncharged chelates with the
metal ions.

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Gravimetric Analysis: Weight
Relationship
• in gravimetric method – the analyte (solute) is converted to
precipitate which is then weight
• From the weight of the precipitate formed in a gravimetric
analysis, we can calculate the weight of the analyte
• Gravimetric factor (GF) = weight of analyte per unit weight of
precipitate.
• Obtain from ratio of F Wt of the analyte per F Wt precipitate,
multiplied by moles of analyte per mole of precipitate
obtained from each mole of analyte
• GF = f wt analyte (g/mol) x a (mol analyte/mol precipitate)
• f wt precipitate (g/mol) b
• = g analyte / g precipitate

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Example 1 of gravimetric factor
• If Cl2 in a sample is converted to chloride and precipitated as AgCl, the
weight of Cl2 that gives
• 1g of AgCl is?
• F wt Cl = 35.453, F wt Ag = 107.868
• GF = f wt analyte (g/mol) x a (mol analyte/mol precipitate)
• f wt precipitate (g/mol) b
• = g analyte / g precipitate
• GF = f wt analyte (g/mol) x a (mol analyte/mol precipitate)
• f wt precipitate (g/mol) b
• = g analyte / g precipitate
• g Cl2 = g AgCl x f wt analyte (g/mol) x a
• f wt precipitate (g/mol) b
• = 1 AgCl x 70.906 g/mol x 1 mol
• 143.321 g/mol 2 mol
• = 0.2474 g

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EXAMPLE 2
• A 0.3516g sample of commercial phosphate
detergent was ignited at a red heat to destroy the
organic matter. The residue was then taken up in
hot HCl which converted P to H3PO4. The
phosphate was precipitated with Mg2+ followed
by aqueous NH3 to form MgNH4PO4.6H2O. After
being filtered and washed, the precipitate was
converted to Mg2P2O7 (FW=222.57) by ignition
at 100ºC. This residue weighed 0.2161g. Calculate
the percent P (FW = 30.974) in the sample.

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