Gravimetric analysis

GRAVIMETRIC
ANALYSIS
By Dr Mark Selby
(from lecture slides developed by
D. Sharma, Department of Chemistry, Simon Fraser
University, British Columbia, Canada)
2/20/2015
PQB313 Analytical Chemistry for Industry
1

GRAVIMETRIC
ANALYSIS
Chapter 10 in Christian (7th Ed.) pages 342f.
2/20/2015 PQB313 Analytical Chemistry for Industry 2

Gravimetric Analysis
Gravimetric analysis is the quantitative determination of
analyte concentration through a process of precipitation
of the analyte, isolation of the precipitate, and weighing
the isolated product.
CVB212 Industrial Analytical Chemistry 3
Uses of gravimetric analysis…
– Chemical analysis of ores and
industrial materials
– Calibration of instrumentation
– Elemental analysis of
inorganic compounds

1. A weighed sample is dissolved
2. An excess of a precipitating agent is added to
this solution
3. The resulting precipitate is filtered, dried (or
ignited) and weighed
4. From the mass and known composition of the
precipitate, the amount of the original ion can
be determined
5. Stoichiometry is important (write down the
chemical equation!)
4CVB212 Industrial Analytical Chemistry

Criteria for Gravimetric Analysis
1. The desired substance must completely precipitate
from solution
• In most determinations the precipitate is of such low
solubility that dissolution of the analyte is negligible
• An additional factor is the "common ion" effect, further
reducing the solubility of the precipitate

When Ag+ is precipitated from solution through
the addition of Cl-
the (low) solubility of AgCl is further reduced
by the excess of Cl- that is added, pushing
the equilibrium to the right (Le Chatelier’s
Principle).
)(sAgClClAg  

2. The weighed form of the product should be
of known composition.
3. The product should be "pure" and easily
filtered.
• It is usually difficult to obtain a product that is
"pure“ (i.e., one that is free from impurities)
• Careful precipitation and sufficient washing may
reduce the level of impurities

Some Organic Precipitants
8
Practical 1:
Determination of
Nickel in Steel
Christian 7th Ed., Table 20.2, pg 354.
CVB212 Industrial Analytical Chemistry

Example: Ni in Steel
• To measure Ni in steel, the alloy is dissolved in 12 M HCl and
neutralised in the presence of citrate ion, which maintains iron in
solution.
• The slightly basic solution is warmed and dimethylglyoxime (DMG)
is added to precipitate the red DMG-nickel complex quantitatively.
• The product is filtered, washed with cold water, and dried at 110 °C.
9
Harris 8th ed., pg 681.

Mechanism of Precipitation
• Induction period
• The time before nucleation occurs after the addition
of the precipitating agent to the solution
• May range from milliseconds to several minutes
• Nucleation
• Formation of small, stable aggregates or nuclei of
precipitate
• Nuclei have sizes down to ~1 nm, composed of a
few atoms, and there may be up to 1010 nuclei per
mole of analyte
• Excess ions from solution collect around the nuclei

11
Silver nitrate is added very slowly to an acidic solution containing chloride. Silver
chloride nuclei form with a surface layer of ions. The “charged” AgCl particles
(or colloidal particles) repel each other.
Harris 8th Ed., Figure 26-2, pg 678.
Nucleus of
AgCl(s) colloid
Primary
adsorbed Ag+
Loosely
associated
counter ion
Illustration of an Electrical
Double Layer
Homogeneous
solution (charges
balanced)

• In addition to the primary adsorbed silver ions, some nitrate
ions form an electrostatic layer around the nucleus.
• These counter ions tend to aggregate around the
[AgCl:Ag]+ center because these centers have a net positive
charge (excess Ag+) and additional negative charge is
required to maintain electrical neutrality.
• Counter ions are less tightly held than the primary adsorbed
ions and the counter ion layer is somewhat diffuse and
contains ions other than those of the counter ions.
• These layers of charged ions associated with the surface of
the nuclei are known as the electric double layer.

More Terminology
• Adsorption is a process in which a substance
(gas, liquid, or solid) condenses onto the surface
of a solid
• The electric double layer of a colloid consists of
a layer of charge associated with the surface of
the particles and a layer with a net opposite
charge in the solution surrounding the particles
• A colloid is a finely divided particle (typically with
diameters from 10 nm to 1 m) that forms a stable
dispersion within a medium (air or liquid)

Digestion
• Heating the precipitate within the mother liquor (or
solution from which it precipitated) for a certain period
of time to encourage densification of nuclei.
• During digestion, small particles dissolve and larger
ones grow (Ostwald ripening). This process helps
produce larger crystals that are more easily filtered
from solution
14
T

Ideal Analytical Precipitation
• In an ideal world, an analytical precipitate for
gravimetric analysis should consist of perfect crystals
large enough to be easily washed and filtered.
• The perfect crystal would be free from impurities and
be large enough so that it presented a minimum
surface area onto which foreign ions could be
adsorbed.
• The precipitate should also be "insoluble" (i.e., low
solubility such that loses from dissolution would be
minimal).

Conditions for Analytical Precipitation
• Von Weimarn showed that particle size of
precipitates is inversely proportional to the relative
supersaturation of the solution during precipitation
• Relative supersaturation = (Q-S)/S
• Where Q is the molar concentration of the mixed
reagents before any precipitation occurs and S is the
molar solubility of the product (precipitate) when the
system has reached equilibrium.
• For the best possible results, conditions need to be
adjusted such that Q will be as low as possible and S will
be relatively large.

• Precipitation from hot solution
• The molar solubility (S) of precipitates increases with
an increase in temperature
• An increase in S decreases the supersaturation and
increases the size of the particle.
• Precipitation from dilute solution
• This keeps the molar concentration of the mixed
reagents low. Slow addition of precipitating reagent
and thorough stirring keeps Q low. (Uniform stirring
prevents high local concentrations of the
precipitating agent.)
17PQB313 Analytical Chemistry for Industry

• Precipitation at a pH near the acidic end of
the pH range in which the precipitate is
quantitative.
• Many precipitates are more soluble at the lower
(more acidic) pH values and so the rate of
precipitation is slower.
• Digestion of the precipitate.
• The digestion period can lead to improvements in
the organization of atoms within the crystalline
nuclei, such as expulsion of foreign atoms (or other
impurities).
18PQB313 Analytical Chemistry for Industry

Impurities in Precipitates
• Coprecipitation…
…is the precipitation of an unwanted species along
with your analyte of interest;
… occurs to some degree in every gravimetric
analysis;
• A major factor for precipitations of barium sulfate and those
involving hydrous oxides
… and cannot be avoided, but can be minimized by
careful precipitation and a thorough washing of the
precipitate.

• Surface adsorption
• Unwanted material is
adsorbed onto the surface
of the precipitate
• Digestion of a precipitate
reduces the relative
surface area and,
therefore, the area
available for adsorption of
impurities
• Washing can remove
impurities bound to the
surface
20
0 2 4 6 8 10 12 14
Particle Surface Area = 4(r
2
)
Particle Volume = 4/3(r
3
)
Particle Surface Area
Particle Volume
Particle Radius (A.U.)
Scaling per Particle

• Occlusion
• A type of coprecipitation
in which impurities are
trapped within the growing
crystal
21
• Post-precipitation
– Sometimes a precipitate in contact with the mother liquor is
contaminated by the precipitation of an impurity

• Inclusion
• A type of coprecipitation in which
the impurities occupy the crystal
lattice sites
• Peptidization
• A procedure where the precipitate is
washed and filtered, but part of the
precipitate reverts to the colloidal
form because supporting electrolyte
is gone.
• Cooling the system with an ice-
water bath minimizes loss of
precipitate due to dissolution
22
AgCl (s) → AgCl (colloid)

Increasing Purity
• Re-precipitation
• a procedure including washing away the mother
liquor, redissolving the precipitate, and precipitating
the product again
• Drying the solid
• Generally the solids are dried at
~120 oC, but conditions for
drying can vary considerably.

Increasing Purity
• Precipitation in the presence of electrolyte
• Coulombic repulsion is diminished in the presence of
electrolyte because of a compression of the volume
of the ionic atmosphere
• Digestion
• Raising the temperature will increase the collision
energy for colloidal particles and overcome
Coulombic repulsion, leading to formation of larger
particles (coalescence)

• For example: determination of silver or chloride
by the formation of AgCl (s)
• Precipitation occurs when the value of [Ag+][Cl-]
exceeds the solubility product Ksp of AgCl
(1.810-10).
)(sAgClClAg  

26
A 10.0 mL solution containing Cl- was treated with excess
AgNO3 to precipitate 0.4368 g of AgCl. What was the
concentration of Cl- in the unknown? (AgCl = 143.321
g/mol)
Number of moles of Cl- = number of moles of AgCl
mol103.048
ol143.321g/m
0.4368g 3-

Concentration of Cl- M0.3048
L0.01000
mol103.048 3




Harris 8th Ed., pg 674.

Other Analytes
27
Why is the form weighed different from the precipitate?
Christian 7th Ed., Table 10.1, page 353

Gravimetric Factor
In general the precipitate we weigh is usually in a different form
than the analyte whose weight we wish to report.
The gravimetric factor (GF), represents the weight of analyte
per unit weight of precipitate. It is obtained from the ratio of the
formula weight of the analyte to that of the precipitate, multiplied
by the moles of analyte per mole of precipitate obtained from
each mole of analyte, that is:
2/20/2015 Footer Text 28
analyte g/mol
precipitate g/
FW ( )
(mol analyte/mol precipitate)
FW ( m )ol
a
GF
b
 

Gravimetric Factor - Example
Question: Calculate the grams of analyte per gram of
precipitate for the following conversion:
Answer:
2/20/2015 Footer Text 29
Analyte Precipitate
Bi2S3 BaSO4
2 3
2 3 4 2 3 4
4
2 3
2 3 4
4
FW Bi S (g/mol) 1
g Bi S /g BaSO = (mol Bi S / molBaSO )
FW BaSO (g/mol) 3
514.15(g Bi S /mol) 1
GF = 0.73429 g Bi S /g BaSO
233.40(g BaSO /mol) 3

 

Sample Calculation
30
A certain barium halide exists as the hydrated salt BaX2
.2H2O, where X is
the halogen.
The barium content of the salt can be determined by gravimetric methods. A
sample of the halide (0.2650 g) was dissolved in water (200 cm3) and
excess sulphamic acid added. The mixture was then heated and held at
boiling for 45 minutes. The precipitate (barium sulfate) was filtered off,
washed and dried. Mass of precipitate obtained = 0.2533 g.
Determine the identity of X.

Sample Calculation
31
The precipitate is barium sulfate. The first stage is to determine the
number of moles of barium sulfate produced, this will, in turn give us
the number of moles of barium in the original sample.
Relative Molecular Mass (Mr) of barium sulfate
Mr = 137.34 (Ba) + 32.06 (S) + (4 x 16.00) (4 x O) = 233.40 g/mol
Number of moles = mass / Mr = 0.2533 (g) / 233.40 (g/mol)
= 1.09 x 10 –3 (mol)

Sample Calculation
32
This is the number of moles of barium present in the precipitate and,
therefore, the number of moles of barium in the original sample. Given the
formula of the halide, (i.e., it contains one barium per formula unit), this
must also be the number of moles of the halide. From this information we
can deduce the relative molecular mass of the original halide salt:
Mr = mass / number of moles = 0.2650 (g) / 1.09 x 10-3 (mol)
= 244.18 (g/mol)

33
Sample Calculation
The atomic mass (Ar) of 2 X will be given by the Ar of the whole salt – that of the
remaining components:
Ar of 2 X = 244.18 (g/mol) – 137.34 (g/mol Ba) – 2 x 18.02 (g/mol H2O) = 70.81
(g/mol)
2 X = 70.81, so X = 35.41 (g/mol)
The Ar of chlorine is 35.45 (g/mol), which is in good agreement with the result
obtained and hence the halide salt is hydrated barium chloride and
X = Chlorine
Final formula is BaCl2
.2H2O
Compare this example with the worked solution for RaCl2, in Harris 8th Ed., 674.
PQB313 Analytical Chemistry for Industry

Gravimetric analysis

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