Fundamentals and principles of gravimetry

2. Gravimetric Analysis
Gravi – Metric
(Weighing - Measure)
 To measure the purity.
 Most accurate analytical technique.
 It is an ABSOLUTE method.
 Precise methods of macro quantitative analysis.
 Possible sources of errors can be checked.

3. Analyte (or) Sample
↓ (selectively convert)
Precipitate form ( Insoluble form )
↓
Filtration (Separate the precipitate)
↓
Drying
↓
Igniting
↓
Weighing
↓
Calculations

5. PRINCIPLE & STEPS INVOLVED IN
GRAVIMETRIC ANALYSIS
 Preparation of the sample solution
 Precipitation process
 Digestion (or) Ostwald ripening
 Filtration
 Washing
 Drying
 Igniting
 Weighing
 Calculation

6. PREPARATION OF THE
SAMPLE SOLUTION
 Factors to be considered during preparation of
sample solution
Volume of solution during precipitation
 PH
Temperature.

7. Volume of solution during precipitation
• Solution condition
(volume) must be
adjusted to maintain
low solubility of
precipitate
• Excess volume of
solution during
precipitation
• Leads to precipitate will
be coagulate

8. PH
Influence the solubility of the analytical precipitate
Possibility of interference from other substance
Example 01 : During Calcium oxalate precipitation
REASON

9. Example 02 : 8-Hydroxy quinoline –precipitating reagent
Example : In PH
– 4 condition
Aluminium ions - can be easily precipitated - at PH
4.
.
Magnesium ion
Concentration of anion form of oxine is too low at PH
4 to precipitate magnesium ion.
Higher PH
is required to shift the ionisation step and precipitate the magnesium.
If the PH
is too high magnesium hydroxide will precipitate causing interference

10. TEMPERATURE
Increase the temperature
Increase solubility of precipitate
Precipitate will dissolved or coagulated & Difficult to filter

11. PRECIPITATION PROCESS
Actually precipitate occurs in series of steps.
Solution of precipitating reagent added to
Sample solution
Precipitate formed
Eg: AgNO3 + NaCl →AgCl + NaNO3
Precipitation process
involves heterogeneous
equilibrium as of steps
Solubility product

12. SUPERSATURATION
Super saturation is one of the important parameter in precipitation process
It ‘s play vital role in gravimetry
Increase super saturation - Increase nucleation
Should avoid Super saturation & nucleation

13. Nucleation- Individual ions/atoms/molecules coalescence to
form “nuclei (small particles come together to form colloid
particles)

14. NUCLEATION
Effects of Increase nucleation
Initial nucleus will grow by the deposition of their precipitate
particles
To form a crystal of certain geometric shape.
Again the greater super saturation
More rapid crystal growth rate & colloidal precipitate formed
Increase the growth rate increased chance of imperfection in the
crystal & surface area of precipitate increase this leads to easy
trapping of impurities.
Trapping of impurities

15. Surface area of the precipitate Increase –
Increase the impurity adsorption.

16. Relationship between
supersaturation & Nucleation
High relative supersaturation Low relative supersaturation

17. Von weimarn – discovered
Particle size of precipitate is inversely proportional to the relative
supersaturation of the solution during the precipitation process
Relative supersaturation = Q-S
S
Q- concentration of mixed reagents before precipitation occurs(Degree of supersaturation)
S- solubility of precipitate at equilibrium
This ratio is also called the Von weimarn ratio.
Relationship between particle sizes & supersaturation – In
precipitation process

18. So we want keep
Q value is low & S value is high - during precipitation
 Precipitate from dilute solution. This keeps Q low.
 Add dilute precipitating reagents slowly, with effective stirring. This also keeps Q low.
Local excess of the reagent are prevented by stirring.
 Precipitate from hot solution. This increases S. The solubility should not be too great
or the precipitation will not be quantitative (with less than one precipitate remaining).
The bulk of the precipitation may be performed in the hot solution, and then this
solution may be cooled to make the precipitation quantitative.
 Precipitate at as low a pH as is possible to maintain quantitative precipitation. Many
precipitates are more soluble in acid medium and this slows the rate of precipitation.
They are more soluble because the anion of the precipitate combines with protons in
the solution.

19. Maintain crystalline precipitate
& Avoid colloidal precipitate
Small or less no of large particles
Crystalline precipitate
Why ? should avoid the colloidal precipitate
 More or large no of Small particles(1-100 m)ʍ
 Difficult to wash & filter
 Because colloidal precipitate may contain
Hydrophilic particle (Water loving) - It forms viscous layer
Hydrophobic particle (Water hating) – It forms gelatinous precipitate

20. DIGESTION OF PRECIPITATE
 Digestion is a process keeping 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
Ostwald ripening
Ostwald ripening improves the
purity and crystallinity of the
precipitate

21. POSIBLE IMPURITIES DURING PRECIPITATION
PROCESS
 OCCLUSION & INCLUSION
 SURFACE ADSORPTION
 POST PRECIPITATION
 ISOMORPHISM

22. OCCLUSION & INCLUSION
OCCLUSION
Trapping of impurities
with in the crystal.
Example :
Water may trapped in
pockets when silver
nitrate crystals are
formed
INCLUSION
Trapping of impurities
with in the crystal
lattice.
Example :
K+
in NH4MgPO4
Impurities
Occlusion &
Inclusion
impurities are
difficult to
remove.
Digestion may
help some but
this is not
completely
effective.
Purification by
dissolving &
reprecipitating
may be helpful

23. SURFACE ADSORPTION
Its reduced by
proper precipitation
technique, digestion
and washing
High relative supersaturation
Increase nucleation
Many small crystals formed
More surface area
More surface adorption
More impurities formed

24. Imagine this is a large crystal
It have less surface area
Supersaturation increase nucleation occur,precipitate will
become small crystal & convert into colloidal form &
increase the surface area
It have more surface area – chance to bind more
impurities
Surface area – 8 nos
Take one chalk piece & imagine that is large crystal. It have 4 surface area
Imagine - break that chalk piece ,now the large crystal are become small
crystals. It have 8 surface area.
Story
imagination
to
understand
the concept

25. POST PRECIPITATION
When the precipitate is allowed to stand in contact with mother liquor, a second
substance will slowly form a precipitate with precipitating reagent
 EXAMPLE : 01
When calcium oxalate is precipitated in the presence of magnesium ions,
magnesium oxalate does not form immediately precipitate because it tends
to form supersaturated solution.
 EXAMPLE : 02
Copper sulfide will precipitate in acid solution in the presence of zinc ion but
eventually zinc sulfide will precipitate

26. ISOMORPHOUS REPLACEMENT
Two compounds are isomorphs – it means same type of formula and crystallize in
similar geometric forms & same lattice dimensions.
So one ion can replace another in crystal easily.
 EXAMPLE : 01
In the precipitation of Mg2+
as magnesium ammonium phosphate, K+
has nearly same
ionic size as NH4+
and can replace it to form magnesium potassium phosphate.

27. WASHING & FILTERING
THE PRECIPITATE
 Co-precipitate impurities, especially these on the substance can be removed by
washing the precipitate after filtering
 Precipitate will be wet by mother liquid that can also be removed by washing
 Many precipitate cannot be washed with pure water because peptization occurs
(reverse of coagulation)
 Eg:HNO3 and NH4NO3 is used for washing AgCl precipitate

28. DRYING & IGNITING
THE PRECIPITATE
 Precipitate must be in suitable form for weighing.
 Precipitate must be pure, stable, of certain known composition.
 Remove water efficiently; electrolytes in washing solvent.
 Appropriate chemical changes during heating.
EXAMPLE:
AgCl–drying in over 100 -1300
C to remove physically-bound
water.
Higher temp. is necessary if water is trapped in crystals /
chemically-bound water / or ensure appropriate chemical changes
•Mg2P2O7--> MgNH4PO 900o
C
•CaC2O4--> CaO 1100o
C

29. WEIGHING & CALCULATION

30. • Applications
– Extensive numbers of inorganic ions are determined
with excellent precision and accuracy.
– Routine assays of metallurgical samples.
– Relative precision 0.1 to 1%.
– Good accuracy
Disadvantages
Careful and time consuming.
Scrupulously clean glassware.
Very accurate weighing.
Coprecipitation.

33. Desicooler
Crucibles
Crucible tongs
Beaker tongs

37. References
 Analytical chemistry- Gary D.Christian 5th edition. Page no. 145-168.
en.wikipedia.org/wiki/Gravimetry
http://ull.chemistry.uakron.edu/analytical/03_Gravimetry.pdf
http://www.ecs.umass.edu/cee/reckhow/courses/572/572bk15/572BK15.html