General gravimetric calculations and the Gravimetric Factor

1
Gravimetry

• General gravimetric calculations and the
  Gravimetric Factor
• Example Impure FeCO3 is converted to Fe2O3
  weighing 1.000 g. Whats the weight of FeCO3 in
  the original sample?
• FeCO3 2H Fe2 CO2 H2O
• Fe2 ?Br2 Fe3 Br-
• Fe3 3OH- Fe(OH)3
• Fe(OH)3 Fe2O3 3H2O
• 2FeCO3 Fe2O3 2CO2

?
O
?

• When the principal element or ion appears both in
  the numerator and denominator, determining the
  gravimetric factor is straight forward.

2
Gravimetry

• General gravimetric calculations and the
  Gravimetric Factor
• Sometimes things are somewhat more complicated!
• Example What mass of CuI must have produced
  1.000 g Cu?
• CuI 1e- Cu I-
• CuI Cu2 I- 1e-
• 2CuI Cu Cu2

3
Gravimetry

• General gravimetric calculations and the
  Gravimetric Factor
• Sometimes things are somewhat more complicated!
• Example Br2 is determined by reducing the Br2 to
  Br-, precipitating the Br- with Ag, and
  converting the AgBr to AgCl with Cl2. What mass
  of Br2 is equivalent to 1.000 g AgCl?
• Br2 2Br-
• Br- Ag AgBr
• 2AgBr Cl2 2AgCl Br2
• In this case a common element does not appear in
  the numerator and denominator

4
Gravimetry

• General gravimetric calculations and the
  Gravimetric Factor
• Determining wt of analyte from gravimetric data
• Example If 2.000g impure NaCl is dissolved in
  H2O and excess AgNO3 yields 4.7280 g AgCl, what
  is the chloride in the sample?

5
Gravimetry

• General gravimetric calculations and the
  Gravimetric Factor
• Determining wt of analyte from gravimetric data
• Example A 0.5000 g sample of impure magnetite
  (Fe3O4) is converted to Fe2O3 weighing 0.4110 g.
  What is the Fe3O4 in the original sample?

• Some problems involve analysis for more than one
  analyte in a sample
• See Example 5-7 and problems 5-37 - 5-40 in FAC7
• For each analyte, an independent equation must be
  established.

6
Gravimetry

• Precipitating Agents are
• Specific if they react to form a precipitate with
  only one chemical species
• Selective if they react to form a precipitate
  with a limited number of chemical species
• Desirable properties of precipitates
• Easily filtered, easily washed free of
  contaminants
• Low solubility so that little is lost during
  handling
• Non-reactive toward the atmosphere - H2O, CO2, O2
• Of known composition after drying or ignition
• The mechanism of precipitation is important in
  determining particle size
• Von Weimarn definition of relative
  supersaturation helps explain

Q actual concentration of slightly soluble
compound S equilibrium solubility of slightly
soluble compound
7
Gravimetry

• The mechanism of precipitation is important in
  determining particle size
• During the initial stages of precipitate
  formation, when a solution of precipitating
  agent is added to a solution of analyte
• A localized volume of high relative
  supersaturation occurs until solution is mixed
• QgtgtS and relative supersaturation is high
• Colloidal particles form
• Diameters 10-6 - 10-4 mm
• Larger than typical small molecules
• Not large enough to overcome Brownian motion and
  settle from solution
• Too small to be retained by most filtering media
• In order to produce a filterable precipitate the
  particles must be much larger than colloidal
  dimensions

8
Gravimetry

• There are two competing processes that occur when
  a solution of precipitating agent is added to a
  solution of analyte
• Nucleation is the formation of colloidal sites
  upon which additional precipitate forms
• Particle growth is the increase in particle size
  as additional precipitate builds up on
  nucleation sites

9
Gravimetry
The mechanism of precipitation

• At high relative supersaturation formation of
  colloidal nucleation sites is preferred
• At low relative supersaturation particle growth
  is preferred and crystalline precipitates form

10
Gravimetry

• The mechanism of precipitation
• It is sometimes impossible to avoid formation of
  colloids
• Substances that have very low solubilities are
  formed under conditions of high relative
  supersaturation
• Sulfides, hydroxides, many precipitates of Ag
• Stability of Colloidal precipitates
• Results from adsorption of ions to the colloidal
  particles
• The primary adsorption layer is the layer of
  adsorbed ions directly attached to the colloidal
  particle
• Fajans rule the ion in the primary adsorption
  layer on an ionic colloidal particle will be the
  common ion of the substance in higher
  concentration in the mother liquor from which
  the colloid is formed.
• Example Formation of AgCl by adding AgNO3 to a
  solution of NaCl
• The AgCl formed initially will be in contact with
  a solution having excess Cl-
• Cl- will be the primary adsorbed ion
• Na will be in the secondary adsorption layer
• In excess Ag, Ag will be adsorbed and NO3- is
  secondary ion
• This electrical double layer produces a particle
  that is charged
• Electrostatic repulsion between charged colloidal
  particles prevents agglomeration and formation
  of larger particles that can precipitate

11
Gravimetry

• The mechanism of precipitation
• Coagulating colloidal particles to form
  filterable precipitates
• Heating with stirring
• Reduces adsorption thus the charge of colloidal
  particles
• Gives colloidal particles more kinetic energy -
  collisions between particles will be more
  effective in overcoming repulsions
• Add inert electrolyte such as HNO3
• Causes shrinkage of counter ion layer by
  increasing the concentration of ions of opposite
  charge to the primary adsorbed ions near the
  colloidal particle
• Produces an insulating effect reducing the
  electrostatic repulsion between colloidal
  particles
• Peptization is the resuspension of a colloidal
  precipitate by washing

12
Gravimetry

• Impurities associated with colloidal precipitates
• Coprecipitation is the contamination by an
  otherwise soluble component during formation of
  a precipitate
• This is not contamination with another
  precipitate that happens to form
• For colloidal precipitates coprecipitation
  involves adsorption of impurity ions
• If AgCl is in contact with a solution containing
  excess Ag, Ag will be adsorbed along with
  counter ions such as NO3-
• The surface area of colloidal precipitates is
  enormous 3 000 ft2/g!
• Washing with water will not remove the adsorbate
  - it will peptize

13
Gravimetry

• Impurities associated with colloidal precipitates
• Producing a pure colloidal precipitate
• Form the precipitate from a hot, stirred solution
  to which an appropriate electrolyte has been
  added to aid agglomeration
• Digest the samples after precipitation - allow
  the precipitate to sit in contact with the
  precipitating agent while hot
• This will remove weakly adsorbed water from the
  precipitate and produce a more dense, easier to
  filter precipitate
• Filter the precipitate from hot solution
• Wash with an electrolyte that produces a volatile
  adsorbate
• Precipitation of Cl- by Ag has Ag as the
  primary adsorption ion
• Washing with dilute HNO3 can exchange Ag with H
  and put NO- in the secondary adsorption layer
• Heating to 110 oC will decompose the HNO3 to
  NO2(g) and NO(g)
• Reprecipitate the compound which will minimize
  adsorption because there will be a lower
  concentration of adsorbing ions
• For AgCl, redisolve using dilute NH3 - Ag(NH3)2
  is formed - and destroy the complex with HNO3
• If possible form the precipitate by using
  homogeneous formation of the precipitating
  agent
• OH- can be formed from hydrolysis of urea

Equations for formation of other precipitating
agents are shown in Table 5-3, p. 92 in FAC7.
14
Gravimetry

• Formation of crystalline precipitates
• Crystalline precipitate formation is preferred at
  low relative supersaturations
• Keep Q small
• Use dilute solution of of precipitating agent
• Use small increments of precipitating solution
• Use slow addition of precipitating solution with
  stirring
• Keep S large
• Keep the solution hot during the precipitation
  process

15
Gravimetry

• Impurities associated with crystalline
  precipitates
• Specific surface area of crystalline precipitates
  is small
• Little interference from adsorption
• Inclusion is the random distribution of impurity
  ions throughout the volume of the crystals
• Isomorphic inclusion involves including ions that
  are the right size to fit in the crystal lattice
  and thus replace ions that should be there
• No strain is imposed on the crystal
• Nonisomorphic inclusion involves including ions
  that do not fit in the lattice as a result of
  the too rapid growth of the crystal
• Occlusion involves trapping droplets of solution
  containing dissolved impurities in pockets of
  the growing crystal
• Occlusion can also result from mechanical
  entrapment where two crystals come together and
  fuse as they grow

16
Gravimetry

• Formation of crystalline precipitates
• Purifying crystalline precipitates
• Washing will not remove included or occluded
  impurities
• Digestion is the best method of improving the
  purity and quality of a crystalline precipitate
• Heat the precipitate in contact with its mother
  liquor
• The crystals undergo dynamic dissolution and
  reformation
• Trapped impurities are released and will be less
  likely to be trapped during the slow reformation
  of the precipitate
• The reformed crystals are usually larger because
  of bridging between multiple crystals and are
  easier to filter

17
Gravimetry

• Direction of Coprecipitation errors
• Positive errors always occur if the contaminant
  is not a compound of the analyte
• Contamination of Al(OH)3 with Fe3 increases the
  mass of precipitate over that expected if the
  precipitate were assumed to be pure Al(OH)3
• Positive or negative errors can occur if the
  contaminant is a compound of the analyte
• Contamination of Fe(OH)3 (molar mass107) by
  FeCl3 (molar mass162) gives a positive error
  since the mass of the precipitate is more than
  would be expected given the mass of Fe3 in the
  sample
• Contamination of Fe(OH)3 (molar mass107) with
  Fe(OH)2 (molar mass89) gives a negative error
  since the mass of the precipitate is less than
  would be expected given the mass of Fe3 present
  in the sample

18
Gravimetry

• Heating and Ignition of precipitates
• Filtered precipitates normally are heated or
  ignited to produce a constant weight of
  precipitate
• Heating precipitates usually removes water and
  other volatile components
• Some precipitates (AgCl) can be brought to
  constant weight at 110 oC
• Others require heating at considerably higher
  temperature - Ignition
• BaSO4 requires heating to above 700 oC
• Al2O3 requires heating above 1000 oC
• CaC2O4 undergoes chemical reaction at various
  temperatures
• 110 oC CaC2O4H2O is formed
• 225 oC CaC2O4 is formed
• 450 oC CaC2O4 CaCO3 CO

19
Gravimetry

• Applications of Gravimetric Methods
• Inorganic preciptating agents see Table 5-4, p.
  94 FAC7.
• Reducing agents see Table 5-5, p 94, FAC7
• Organic preciptating agents see p. 94-96, FAC7
• Organic functional group analysis see Table 5-6,
  p. 96, FAC7
• Volatilization methods see p. 96-96, FAC7