Gravimetry
Gravimetry or gravimetric analysis is a quantitative analytical
method that is based upon the measurement of mass of a pure
compound to which the analyte is chemically related.
1.1 Types of Gravimetric Methods
1. Precipitation method
analyte is converted to an insoluble product, filtered,
washed and heated. The mass of the resulting residue
is determined.
The “Ideal” Product:
- Insoluble
- Very pure
- Easily filterable
- Possess a known composition
2. Volatilization method
analyte is heated and the analyte or its decomposition product
is collected. Either the change of sample weight is determined
(thermogravimetry), or the combustion gases are trapped and
weighed (combustion analysis).
NaHCO3(aq) + H2SO4(aq)  CO2(g) + H2O(l) + NaHSO4(aq)
CO2 (g) + 2 NaOH(s)  Na2CO3(s) + H2O(l)
1.2 Mechanism of Precipitation
Stages of Precipitate Formation
1. Nucleation
Nucleation is the formation, in a supersaturated solution, of the
smallest aggregate of molecules capable of growing into a large
precipitate.
The number of molecules comprising the nucleation aggregate
depends both on the substance itself and the conditions of the
precipitation.
The time between mixing and the visual appearance of the
precipitate is called the induction period.
The induction period varies with
(a) the nature of the substance being precipitated,
(b) the concentration of the reagents being mixed, and
(c) the order of addition of the reagents.
* Concentrated solutions = short induction period, and precipitation
appears to occur instantly upon mixing.
* Dilute soultions = appearance of precipitates or crystals may take
minutes or even days.
2. Crystal Growth
* After a nucleation aggregate has formed, it begins to grow as
ions or molecules from the solution deposit on the surface in a
regular, geometric pattern.
- can be in the form of single crystals or aggregates of
crystals
* Aggregation process is important because it determines the final
size of the precipitate particles and therefore the ease with which
they can be separated from the solution.
Which is better:
faster OR slower nucleation
compared to crystal growth?
Goal : Large crystals suitable for filtration
Factors affecting particle size
It has been observed that the particle size of a precipitate is
influenced partly by variables such as
- precipitate solubility
- reactant concentration
- rate of addition and mixing of reactants
- temperature.
To account for the effect of these variables on particle size, it is
theorized that particle size is related to a single solution property
called the supersaturation (von Weimarn) ratio, or relative
supersaturation, given by
supersaturation ratio = Q – S
S
where Q = concentration of the solute at any given instant
S = equilibrium solubility.
The numerator, Q – S, is a measure of the degree of supersaturation.
It is believed that the rate of
nucleation increases exponentially
with the supersaturation ratio, while Rate
the rate of growth increases
linearly
Nucleation
Growth
Q-S
S
The most favorable relationship between growth and nucleation
occurs when the supersaturation ratio is at its smallest finite value.
How to keep supersaturation ratio small?
1) Keeping Q small by having a dilute solution, slow addition of
precipitating agent and rapid mixing
2) Increasing S by increasing temperature.
* In a highly supersaturated solution = nucleation proceeds faster
than particle growth, resulting in a suspension of tiny particles or,
worse, a colloid.
* In a less supersaturated solution, nucleation is slower, and the
nuclei have a chance to grow into a larger, more tractable
particles.
Techniques that promote particle growth:
1. Raise the temperature = increases solubility and thereby
decrease supersaturation.
2. Add precipitant slowly with vigorous mixing = avoids a local,
highly superpsaturated condition where the stream of
precipitant first enters the analyte.
3. Keep volume of solution large = the concentrations of analyte
and precipitant are low.
3. Aging
The natural cohesive forces existing between particles having the
same composition will cause to form larger aggregate of crystals.
However, small particles can have other properties that tend to
counteract the natural forces of aggregation and lead to the
formation of colloidal suspension.
The formation of individual particles in such a suspension must be
prevented in a gravimetric determination.
Silver chloride precipitates, for example, are well known for their
tendency to form colloidal suspensions
Case 1 : Colloidal AgCl in a solution with excess AgNO3.
Case 2 : Colloidal AgCl in an electrolyte solution.
Case 1:
In a solution of AgNO3 where
there is excess Ag+ ions, the
primary adsorption layer
contain an excess of Ag+ ions
and is positively charged.
A positively-charged primary
adsorption layer attracts an
excess of negative ions into
an adjacent secondary or
counter-ion
layer
(ionic
atmosphere).
The two charged layers of the
particle constitute an electrical
double layer.
Colloidal silver chloride particles in a solution of AgNO3
The EDL exerts a repulsive force toward
other similarly charged particles,
preventing them from getting close
enough to coalesce or coagulate into
larger aggregates.
• Colloidal particles must collide with one another to coalesce.
• However, the negatively charged ionic atmosphere of the particles
repel one another.
• The particles must have enough kinetic energy to overcome
electrostatic repulsion before they can coalesce.
•How to overcome this?
• avoid excess precipitating
reagents (minimize excess
Ag+)
• heating = increases KE
• increasing electrolyte
concentration = decreases
volume of ionic atmosphere
* HNO3 can be volatilized
upon heating
Case 2: Colloidal silver chloride particles in a solution of HNO3
Purity
It is a basic requirement in gravimetry that the precipitate formed be
of known composition, which means that the isolated precipitate
must be pure.
Impurities can be incorporated into a precipitate during its
formation, called coprecipitation, or after its formation while still in
solution, called postprecipitation.
Coprecipitation = impurity is precipitated along with the
desired product
Adsorbed : bound to the surface of a crystal
Absorbed : impurity within the crystal (inclusion or occlusion)
Inclusions = impurity ions randomly occupy sites in the
crystal lattice normally occupied by ions that belong in the
crystal. More likely to occur when impurity ion has a size and
charge similar to the ion on the product.
Occlusions = impurity that are trapped inside the crystal
lattice
Postprecipitation : formation of a second insoluble substance on
an existing precipitate.
* It is always the result of a difference in the rates of
precipitation of the analyte and the contaminant.
* For example, calcium oxalate can be contaminated with
magnesium oxalate upon standing in precipitating solution
containing magnesium ions.
Reducing impurities
1) Wash away the mother liquor (liquid from which a
susbstance
precipitates),
redissolve
precipitate,
reprecipitate.
2) Heating these precipitates in the mother liquor, a process
called digestion, usually results in larger and purer particles
by giving the crystals a chance to dissolve and reprecipitate
under equilibrium conditions.
During digestion at elevated temperature:
Small particles tend to dissolve and reprecipitate on larger ones.
Individual particles agglomerate.
Adsorbed impurities tend to go into solution.
©Gary Christian,
Analytical Chemistry,
6th Ed. (Wiley)
Fig. 10.1. Ostwald ripening.
Fundamental Steps in Gravimetry
1. Forming the Precipitate
The principal goals of every gravimetric procedure are to produce
a precipitate that is pure and that can be filtered easily.
Thus, crystal growth should be favored over nucleation, colloids
should coagulate, and impurities should be minimized if not
totally prevented.
2. Separating and Rinsing Precipitates
A precipitate may be separated by filtering it through paper, sintered
glass, or sintered porcelain. Once the precipitate has been collected
on the filter, traces of the precipitating solution with its dissolved
substances must be rinsed away.
a) Wash with water = generally sufficient with crystalline precipitates
b) Wash with an electrolyte = to prevent peptization
* Peptization : when water washes the ions that neutralizes
the charges of ionic precipitates, causing the particles to repel and
disintigrate, and pass through the filter
3. Drying and Igniting Precipitates
After a precipitate has been filtered it is heated over a burner or
in an oven until it reaches a constant weight.
During the heating process, moisture and volatile electrolytes are
removed and, in some cases, the chemical form of the precipitate
is changed.
Weight
BaSO4
Weight
CaC2O4.H2O
CaC2O4
Al2O3.xH2O
CaCO3
Al2O3
200
600
1000
Temperature (◦C)
CaO
200
600
1000
Temperature (◦C)
4. Calculating the Results
The result of a gravimetric determination is usually reported as a
percentage of analyte:
% analyte = weight of analyte x 100
weight of sample
The precipitate is seldom the analyte itself, but a definite
relationship between the two exists.
This relationship is called gravimetric factor and is used to
convert the known weight of the precipitate to the corresponding
weight of the analyte.
MgCl2 + 2AgNO3 ➞ 2AgCl + Mg(NO3)2
m MgCl2 = m AgCl x 1 mol MgCl2 (FW MgCl2)
2 mol AgCl (FW AgCl)
gravimetric factor
Important Note !!!
In many cases it is not necessary to write reactions to
determine the stoichiometry involved, but only to look at what is
desired and what is weighed.
For example, the amount of Na2S2O4 in a sample is being
determined by converting the sulfur to sulfate and precipitating it
as barium sulfate, BaSO4.
What do we need to know?
* 2 moles BaSO4 will be produced for each mole of
Na2S2O4 present in the sample.
2 moles BaSO4  1 mole Na2S2O4
We do not need to know how Na2S2O4 gets converted to BaSO4!

Example
A 0.3516-g 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 the P to H3PO4.
The phosphate was precipitated as MgNH4PO4.6H2O by addition
of Mg+2 followed by aqueous NH3. After being filtered and
washed, the precipitate was converted to Mg2P2O7 (222.57
g/mol) by ignition at 1000°C. This residue weighed 0.2161 g.
Calculate the % P (30.974 g/mol) in the sample.
Solution:
FWofP
2molP
mP  mMg2 P2O7 x
x
FWofMg2 P2O7 1molMg2 P2O7
30.974 2
mP  0.2161gx
x  0.06015gP
222.57 1
Thus,
0.06015g
%P 
x100  17.11%P
0.3516g
Examples of Gravimetric Calculations
1. Relating Mass of product to mass of reactant
2. Calculating how much precipitant to use
3. A problem with two components
4. Combustion analysis
5. A problem with two analytes and two products
1. Relating Mass of Product to Mass of Reactant : The
piperazine (FW 86.136) content of an impure commercial
material can be determined by precipitating and weighing
the diacetate (FW 206.240). In one experiment, 0.3126 g of
the sample was dissolved in 25 mL of acetone, and 1 mL of
acetic acid (FW 60.052) was added. After 5 mins, the
precipitate was filtered, washed with acetone, and dried at
110°C, and found to weigh 0.7121 g. What is the weight
percent of piperazine in the commercial product?
HN
NH +
2CH3CO2H
+H
2N
NH2+ (CH3CO2-)2
2. Calculating How Much Precipitant to Use : To
measure the nickel (FW 58.69) content in steel, the alloy
is dissolved in 12 M HCl and neutralized in the presence
of citrate ion, which maintains iron in the solution. The
slightly basic solution is warmed, and dimethylglyoxime
(DMG, FW 116.12) is added to precipitate the red DMGnickel complex (FW 288.91) quantitatively. The product is
filtered, washed with cold water, and dried at 110°C. If the
nickel content is known to be near 3 wt% and you wish to
analyze 1.0 g of the steel, what volume of 1.0 wt%
alcoholic DMG solution should be used to give a 50%
excess of DMG for the analysis? Assume that the density
of the alcohol solution is 0.79 g/mL.
3. A Problem with Two Components : A mixture of the
8-hydroxyquinoline complexes of Al (AlQ3, FW 459.43)
and Mg (MgQ3, FW 312.61) weighed 1.0843 g. When
ignited in a furnace open to air, the mixture decomposed,
leaving a residue of Al2O3 (FW 101.96) and MgO (FW
40.304) weighing 0.1344 g. Find the weight percent of
AlQ3 [or Al(C9H6NO)3] in the original mixture.
4. Combustion Analysis : A compound weighing 5.714
mg produced 14.414 mg of CO2 and 2.529 mg of H2O
upon combustion. Find the weight percent of C and H in
the sample.
5. A Problem with Two Analytes and Two Products : A
6.881-g sample containing magnesium chloride and
sodium chloride was dissolved in sufficient water to give
500 mL solution. Analysis of the chloride content of 50.0mL aliquot resulted in the formation of 0.5923 g of AgCl.
The magnesium in a second 50.0-mL aliquot was
precipitated as MgNH4PO4; upon ignition 0.1796 g of
Mg2P2O7 was found. Calculate the percentage of
MgCl2.6H2O and of NaCl in the sample.