3. Overall Learning Outcomes?
• Define gravimetric analysis, know its advantages and
disadvantages.
• Understand the mechanism of precipitate formation.
• Explain the precipitation process and state factors which
cause impurities in the precipitate.
• Calculate results from gravimetric data
3
4. • Gravimetric methods are quantitative methods that
are based on determining the mass of a pure
compound to which the analyte is chemically
related.
Gravimetry
5. • Mass is the most fundamental of all analytical
measurements, and gravimetry is unquestionably
the oldest quantitative analytical technique.
• Although gravimetry no longer is the most
important analytical method, it continues to find
use in specialized applications.
Cont.
6. • Precipitation gravimetry, the analyte is separated from
a solution of the sample as a precipitate and is
converted to a compound of known composition that
can be weighed.
• Volatilization gravimetry, the analyte is separated from
other constituents of a sample by conversion to a gas
of known chemical composition.
• Electrogravimetry, the analyte is separated by
deposition on an electrode by an electrical current.
Classifications of Gravimetric methods
7. Precipitation Gravimetry
• The analyte is converted to a sparingly soluble precipitate.
• Precipitate is then filtered, washed free of impurities,
converted to a product of known composition by suitable
heat treatment, and weighed.
• For example; a precipitation method for determining
calcium in natural waters is recommended by the
Association of official Analytical Chemists.
8. Procedure for gravimetric analysis
(Precipitation gravimetry)
• Steps required after the sample has been dissolved,
can be summarized as follows:
1. Preparation of the solution
2. Precipitation
3. Digestion
4. Filtration
5. Washing
6. Drying or igniting
7. Weighing
8. Calculation
9. Weighing the sample Forming a precipitate Filtering the solution Weighing the dry precipitate
10. • Firstly, prepare the analyte solution.
• This may involve several steps including adjustment
of the pH of the solution in order for the precipitate to
occur quantitatively.
• A precipitate of desired properties may be achieved by
removing interferences before precipitating analyte.
• Adjusting the volume of the sample to suit the amount
of precipitating agent to be added.
Preparation of the Solution
11. • 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.
Precipitation
12. • The precipitate is left hot (below boiling) for 30 min
to 1 hour in order for the particles to be digested.
• Digestion involves dissolution of small particles and
reprecipitation on larger ones resulting in particle
growth and better precipitate characteristics.
Digestion of the precipitate
13. Filtration
• Sintered glass crucibles are used to filter the
precipitates.
• The crucibles must first be cleaned thoroughly and then
be 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 weighings differ by
0.3 mg or less.
16. • The precipitate must be washed very well to remove
all coprecipitated impurities which will add to weight
of precipitate.
• Be careful not to use too much water since part of
the precipitate may be lost.
• In the case of colloidal precipitates water should be
avoided as a washing solution since peptization
would occur.
• In such situations dilute nitric acid, ammonium
nitrate, or dilute acetic acid may be used.
Washing the precipitate
17. • After filtration, a gravimetric precipitate is heated
until its mass becomes constant.
• Heating removes the solvent and any volatile
species carried down with the precipitate.
• Some precipitates are also ignited to decompose the
solid and form a known composition.
• This new compound is often called the weighing
form.
• The temperature required to produce a suitable
weighing form varies from precipitate to precipitate.
Drying and Ignition
18. Weighing
• The precipitate is weighed (in the crucible) after it is allowed to cool
(preferably in a desiccator to keep it away from absorbing moisture).
• Properly calibrated analytical balance.
• Good weighing technique.
19. • Ideally, a gravimetric precipitating agent should react
specifically or at least selectively with the analyte.
• Specific reagents, which are rare, react only with a
single chemical species.
• Selective reagents, which are more common, react
with a limited number of species.
• AgNO3 is a selective reagent that precipitates
common ions such as Cl-, Br-, I- and SCN- from acidic
solutions.
Properties precipitating reagents
20. • The ideal precipitating reagent would react with the
analyte to give product that is,
Easily filtered and washed free of contaminants.
Of sufficiently low solubility that no significant loss of
the analyte occurs during filtration and washing.
Unreactive with constituents of the atmosphere.
Of known chemical composition after it is dried or, if
necessary, ignited.
Cont.
21. • Precipitates consisting of large particles are generally
desirable for gravimetric work because these particles are
easy to filter and wash free of impurities.
• In addition, precipitates of this type are usually purer than
precipitates made up of fine particles.
Particle size and filterability of
precipitates
22. Particle size of solids formed by precipitation varies enormously.
• Colloidal suspensions
Whose tiny particles are invisible to the naked eye (10-7 -
10-4 cm in diameter).
Colloidal particles show no tendency to settle from
solution.
Not easily filtered.
• Crystalline suspension
Particles with dimensions on the order of tenths of a
millimeter or greater.
The particles of a crystalline suspension tend to settle
spontaneously and are easily filtered.
Factors that determine the particle size
of precipitates
23. According to von Weimarn ratio which state that the particle
size of precipitates is inversely proportional to the relative
supersaturation of the solution during precipitation process
Q is the concentration of the solute at any instant
S is its equilibrium solubility
• Experimental evidence indicates that when (Q-S)/S is
large then the precipitate tends to be colloidal.
• When (Q-S)/S is small, a crystalline solid forms.
S
S
Q
24. Particle size of a precipitate is influenced by;
precipitate solubility,
temperature,
reactant concentrations, and
rate at which the reactants are mixed.
25. Mechanism of Precipitates formation
• The precipitates forms in two ways,
by nucleation and
by particle growth.
Nucleation is a process in which a minimum number of
atoms, ions, or molecules join together to produce a stable
solid.
• If nucleation predominates, a precipitate containing a
large number of small particles results.
• If particle growth predominates, a smaller number of
large particles is produced.
26. • Individual colloidal particles are so small that they
are not retained by ordinary filters.
• Fortunately, however, we can coagulate, or
agglomerate, the individual particles of most
colloids to give a filterable, amorphous mass that
will settle out of solution.
Colloidal Precipitates
27. Coagulation of colloids
• Can be accelerated by heating, stirring, adding
electrolyte.
Why colloidal suspensions are stable and do not
coagulate spontaneously?
Colloidal suspensions are stable because all colloidal
particles are either positively or negatively charged.
Colloidal particles are very small and have a very
large surface-to-mass ratio, which promotes surface
adsorption.
The process by which ions are retained on the
surface of a solid is known as adsorption.
28. • The extent of adsorption and the charge on a given
particle increase rapidly as the common ion
concentration becomes greater.
• For example, colloidal silver chloride particle in a solution
that contains an excess of silver nitrate.
The primary adsorption layer - attach directly to the solid
surface, for e.g. consists mainly of adsorbed silver ions.
The counter–ion layer - surrounding the charged particle is a
layer of solution, for e.g. which contains sufficient excess of
negative ion (principally nitrate) to balance the charge on the
surface of the particle.
Electrical double layer – constitute the primarily adsorbed and
the counter-ion layer. This double layer exerts an electrostatic
repulsive force that prevents particles from colliding and
adhering.
29.
30. Nucleation
– Individual ions/atoms/molecules coalesce to form “nuclei”
• Particle Growth
– Condensation of ions/atoms/molecules with existing
“nuclei” forming larger particles which settle out
• Colloidal Suspension
– Colloidal particles remain suspended due to adsorbed ions
around a small particle yielding a net + or - charge
• Coagulation, agglomeration
– Suspended particles coalesce (lump together) to form
larger filterable particles
• Peptization
– Re-dissolution and dispersion of coagulated colloids by
washing and removing inert electrolyte (not good)
Terms Used In Gravimetry
31. Method of improving particle size and
Filterability
• Minimization of Q
Using dilute solution and adding the precipitating
reagent slowly and with good mixing.
• Maximizing of S
S increased by precipitating from hot solution or
adjusting the pH of the precipitation medium.
relative supersaturation
S
S
Q
32. Precipitation titrimetry
• Based on reaction that yield ionic compounds of limited
solubility, i.e. precipitates
• Because of slow formation rate of most precipitates, there are
few reagents that can be used
• By far the most common precipitation reagent is silver nitrate
(AgNO3), which is used for the determination of halides
(practical exp. 2 for Cl- determination), halide-like anions (SCN-,
CN-, CNO-), mercaptans, fatty acids, etc.
Argentometric methods: Titrimetric methods based on the use
of silver nitrate
33. Indicators for Argentometric titrations
3 types of indicators for titrations with silver:
• Chemical
• Chromate ion (The Mohr Method)
• Adsorption indicators (The Fajans Method)
• Iron(III) ion (The Volhard Method)
• Potentiometric
• Amperometric
Not considered
34. Indicators for Argentometric titrations
Chromate ion (The Mohr Method)
• Sodium chromate can serve as an indicator for an argentometric
determination of chloride, bromide and cyanide ions. Ag+ + Cl- ↔
AgCl (s)
• Sodium chromate reacts with silver to form brick-red silver
chromate precipitate at the equivalence-point
2Ag+ + CrO4
2- ↔ Ag2CrO4 (s)
• Must be carried out at pH 7 to 10
• Above, because the chromate ion is the conjugate base of the weak
chromic acid (pKa ~6.5). Consequently in more acidic solutions, the
chromate ion concentration is too low to produce the precipitate
near the equivalence point (next slide)
• Normally pH is maintained by saturation with NaHCO3
• Not used that frequently any more, since Cr(VI) is carcinogenic
37. Indicators for Argentometric titrations
Adsorption Indicators (The Fajans Method)
• An adsorption indicator is an organic compound that tends to be
adsorbed onto the surface of the solid in a precipitation titration
• Ideally the adsorption occurs near the equivalence point and results
not only in a color change, but also in a transfer of color from the
solution to the solid (or the reverse)
• Fluorescien is a typical adsorption indicator, that is used for the
titration of chloride ion with silver nitrate
• In aqueous solution, fluorescein partially dissociates into hydronium
ions and negatively charged fluoresceinate ions that are yellow-
green
• The fluoresceinate ions form an intensely red silver salt after
adsorption
38. Fluorescein color in dissociated form
Adsorption Indicators (The Fajans Method cont.)
Fluorescein colour in dissociated form under UV light. Where else is
fluorescein used?
39. Use of fluorescein
Adsorption Indicators (The Fajans Method cont.)
Fluorescein used in medical applications as a dye
40. Use of fluorescein
Adsorption Indicators (The Fajans
Method cont.)
Fluorescein used in emergency
safety equipment as a dye.
41. Indicators for Argentometric titrations
Iron(III) ion (The Volhard Method)
Silver ions are titrated with a standard solution of thio-cyanate ion:
Ag+ + SCN- ↔ AgSCN (s)
• Iron(III) serves as the indicator
• Solution turns red with the first slight excess of thio-cyanate ion:
Fe3+ + SCN- ↔ FeSCN2+
• Titration must be carried out in acidic solution to prevent precipitation of Fe(III)
as the hydrated oxide
• [Indicator] is not critical and can be between 0.002M – 1.6M
42. Indicators for Argentometric titrations
Ion(III) ion (The Volhard Method) (cont.)
• The most important application of this method is the indirect
determination of halide ions
• In above, a measured excess of standard silver nitrate is added to
the sample, and the excess silver determined by back-titration with
a standard thio-cyanate solution
• The strong acid environment required for the Volhard method
prevents interferences from ions such as carbonate, oxalate and
arsenate
43. The Volhard Method – Exp. 2
Red colour is masked by the white precipitate, therefore appear brick red.
However, once precipitate settle, red colour is very evident.
44. Take home message
Know and apply:
• Precipitation titrimetry
• Argentometric methods
• Indicators for Argentometric titrations
• Chemical indicators in titrations with silver nitrate (Mohr
method, Fajans method and Volhard method)
• https://www.youtube.com/watch?v=yhNTPL3uHxs
• https://www.youtube.com/watch?v=ODnPyAy-Z54
