Introduction to Gravimetry

1. TITLE
Theory: atleast 4-5 lines
Apparatus:
Chemicals:
Chemical reaction:
End point:
indicator:
Procedure: atleast 4-5
lines
Precautions: atleast 3
Guide lines for Practical Notebook
Observations and
calculations:
Table
Calculations
Result
(use pencil only)

3. Titrations

4. Titrant
Titrand
End point
Equivalence point
Blank titration
Parallax error (the apparent change in the position f
object when it is viewed from different positions)
Why is the lower meniscus read when a burette
is filled with HCl while the upper meniscus is
read when the it is filled with potassium
permanganate?

6. Standard Solution
The solution that is having exact and known concentration is called standard solution.
Standard solutions can be prepared with the help of primary standards.
Primary Standards
Primary standards are those chemical reagents having high percent purity, stability toward air,
having high molecular mass, readily solubility in the solvent, having medium cost and readily
availability. A few primary standards are as follow,
 Potassium hydrogen phthalate (KHP) is used as a primary standard for standardization of
NaOH
 Na2CO3 is used as a primary standard for standardization of HCl
 Na2C2O4 is used as a primary standard for standardization of KMnO4
 Zn pellets or Mg ribbons are used as a primary standard for standardization of EDTA

7. Secondary Standard Solution
Secondary standard solution is that solution used for further
standardization of solution having not exact known concentration. The
preparation of secondary standard solution is done with the help of
primary standard solution. After standardization with primary standard
then we call it secondary standard solution and can be used for further
standardization. For example the NaOH solution after standardization
against KHP is now a secondary standard solution and this solution can be
used for standardization of HCl solution.

8. Gravimetry is
among the most
accurate
analytical
techniques (but it
is tedious).

9. WHAT STEPS ARE NEEDED?
The steps required in a gravimetric analysis, 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

10. Gravimetric analysis is one of the most accurate and precise methods of
macroquantitative analysis. In this process the analyte is selectively converted to an
insoluble form. The separated precipitate is dried or ignited, possibly to another form,
and is accurately weighed. From the weight of the precipitate and a knowledge of its
chemical composition, we can calculate the weight of analyte in the desired form.
Precipitation
Determination of lead (Pb+2) in water
Reaction of potassium iodide solution
and lead (II) nitrate solution.

11. Titration Types
1. acid-base
HCl + NaOH → H2O + NaCl
2. redox
5NaC2O4 + 2 KMnO4 + 6H+ →
10CO2 + 2Mn2+ +8H2O
3. precipitation
Ag+ + X- → AgX(s)
4. complex formation (EDTA)
ethylenediaminetetraacetic acid

12. Percent Purity Calculations
Weigh accurately a portion of the impure sample and dissolve
it in an arbitrary amount of solvent.
X100
sample
mass
analyte
mass
purity
% 

13. Gravimetric Analysis: is based upon the measurement of
mass
Gravimetric Analysis generalized into two types:
precipitation and volatilization
(i ) A technique in which the amount of an analyte in a sample is
determined by converting the analyte to some product
 Mass of product can be easily measured
(ii) Analyte: the compound or species to be analyzed in a sample
Advantages -
 requires minimal equipment
Disadvantage –
 requires skilled operator,
 slow.
13

14. Gravimetric analysis, or quantitative estimation by weight, is the process of isolating and
weighting an element or a compound of the element in as pure form as possible. The
main object in gravimetric analysis is the transformation of the element or radical into a
stable, pure compound which can be readily converted into a form suitable for weighting.
The weight of the element is calculated from the formula of the compound and atomic
weights of the elements that are constituents of the compound. The separation of the
element or its compound may be accomplished by precipitation methods, volatilization or
electroanalytical methods.
Volumetric (titrimetric) analysis, is the analysis in which we measure the volume of a
reagent reacting stoichiometrically with the analyte. It first appeared as an analytical
method in the early eighteenth century and initially did not receive wide acceptance. The
growth and acceptance of volumetric methods required a deeper understanding of
stoichiometry, thermodynamics and chemical equilibria. By the early 20th century the
accuracy and precision of volumetric methods were comparable to that of gravimetric
methods, establishing an accepted analytical technique. Titrimetric methods are
classified into four categories based on the type of reaction involved: Acid-base,
complexometric, redox and precipitation titrations.

15. • The quantitative determination of a substance by the
precipitation method of gravimetric analysis involves
isolation of an ion in solution by:
1.precipitation reaction,
2.filtering,
3.washing the precipitate free of contaminants, conversion
of the precipitate to a product of known composition,
4.drying
5.weighing the precipitate and determining its mass by
difference.
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16. Determination of lead (Pb+2) in water
Pb+ + 2Cl-  PbCl2(s)
 By adding excess Cl- to the sample,
essentially all of the Pb+2 will precipitate as
PbCl2.
 Mass of PbCl2 is then determined.
used to calculate the amount of Pb+2 in
original solution
Reagent
Analyte Solid Product
16

17. Mechanism of precipitation
1. Induction period time between mixing and visual
appearance of a precipitate called the induction period
2. Nucleation is the formation, in a super saturation
solution, of the smallest aggregate of molecules capable
of growing into a large precipitate particle.
17

18. 3. Crystal growth Once 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.
4. Aggregate growth Natural cohesive forces exist
between particles having the same composition and,
as a result, most precipitate to consist of a relatively
few large aggregate of crystals.
Crystal Growth
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19. 1. Precipitation Methods
Ag+(aq) + Cl-(aq) AgCl(s)
2. Volatilization Methods
CuSO4
. 5H20 (s) CuSO4 (s) + 5H2O(g)

20. Some Common
Gravimetric Analysis

21. Gravimetric Procedures
• Sample Preparation
• Preparation of Solution
• Precipitation
• Digestion
• Filtration & washing of precipitate
• Drying &/or igniting of precipitate
• Calculations

22. I. Sample Preparation
• Collect sample
• Dry sample
• Determine mass of sample
• Dissolve Sample

23. II. Preparation of Solution
• Volume of solution
• Good concentration range
• Concentrations of interferences
• Temperature
• pH

24. III. Precipitation
• The precipitating agent should be
specific (at least selective) in its
reaction with the analyte

25. Precipitating Agents
• Inorganic precipitating Reagents
• Homogeneous generation of
precipitating agents
• Organic precipitating agents
• Reducing agents used in
gravimetric analysis
• Organic functional groups

26. 1. Inorganic precipitating Reagents

27. Ideal properties of a precipitate
• Easily filtered & washed free of
contaminants
• Low solubility to reduce loss of mass
during filtration and washing
• Un-reactive with environment
• Known composition after drying or
ignition

28. The Ideal Precipitate
Large Particles
Should be easy to filter &
wash free of impurities

29. Particle Size
Collodial systems vs Crystalline suspensions
• Precipitate solubility
• temperature
• reactant concentration
• rate of mixing

30. Precipitate Formation
nucleation & particle growth
• Nucleation
– supersaturated solutions
– initiated by suspended solid contaminants
• Particle growth
– low relative supersaturation
• hot solutions
• slow addition of precipitating agent w/ stirring

31. IV. Digestion
• Colloidal Particles
– Primary Adsorption layer
– Counter-ion layer
• Ostwald Ripening
• Flocculation
• Purification
– impurities

32. Ostwald Ripening
The precipitate (small crystals) is
allowed to stand in the presence
of the mother liquor ( solution
from which it was precipitated)
LARGE CRYSTALS grow at the
expense of the small crystals

33. V. Filtration and Washing
of precipitate
• Wash with electrolyte
• Avoids peptization
– (reverse of coagulation)

34. VI. Drying &/or igniting of
precipitate
• Heat to constant mass
– removal of solvent
• Ignition
– conversion to another substance
– MgNH4PO4  MgP2O7 (900oC)

35. Solubility:
The solubility of a precipitate can be
decreased by:
 Decreasing temperature of solution
 Using a different solvent
- usually a less polar or organic solvent
(likes dissolves likes)
Solubility vs. pH Solubility vs. Temperature
Solubility vs. Common Ion Effect
35

36. Filterability:
product be large enough to collect on filter:
• Doesn’t clog filter
• Doesn’t pass through filter
Best Case: Pure Crystals
Worst Case: Colloidal suspension
Difficult to filter due to small size
Tend to stay in solution indefinitely  suspended by
Brownian motion
usually 1-100 nm in size
36

37. Conditions for analytical precipitation
An analytical precipitate for gravimetric analysis should
consist of perfect crystals large enough to be easily washed
and filtered. The perfect crystal would be large and free
from impurities. The precipitate should also be "insoluble".
Colloidal suspension
Crystal formation
Want to
Convert to
37

38. Methods used to improve particle size and filterability
1. Precipitation from hot solution The solubility S of
precipitates increases with temperature and so an increase
in S decreases the supersaturation.
2. Precipitation from dilute solution This keeps Q low. Slow
addition of precipitating reagent with effective stirring.
This also keeps Q low; stirring prevents local high
concentrations of the precipitating agent.
3. Precipitation at a pH near the acidic end of the pH range
Many precipitates are more soluble at the lower (more
acidic) pH values and so the rate of precipitation is slower.
4. Digestion of the precipitate. Heating the precipitate in the
precipitating solution, a process called digestion, results in
larger and purer particles by giving the crystal a chance to
dissolve and reprecipitate.
38

39. Impurities in Precipitates
Impurities can be incorporated into a precipitate during its
formation, called co-precipitation, or after its formation while
still in contact with the precipitating solution, called
postpricipitation
Co-precipitation
a) Surface adsorption
39

40. • b) Occlusion Impurities absorbed or trapped within pockets in the crystal
• c) Inclusion Impurities placed in the crystal instead of analyte
• Surface adsorption
• Isomorphous replacement
• Post precipitation
40

41. There are several requirements that must be met to make
precipitation reliable:
• The precipitate must have a very low solubility in water; i.e.
its Ksp must be very small number
• It must precipitate in a high state of purity or be capable of
reprecipitation for further purification.
• It must be capable of drying or of ignition.
• It should not be hydroscopic at room temperature.
41

42. Ageing &digestion
• The precipitate should be in contact with the solution
from which the precipitate is formed.
• Warm the solution that contains the precipitate for
some time to obtain complete precipitation in a form
which can be readily filtered.
42

43. During the process of ageing and digestion, two changes occur:
• After precipitation has occurred, the very small particles, which
have a greater solubility than the greater ones, tend to pass into
solution and will redeposit upon the larger particles. Thus co
precipitation on the minute particles is eliminated.
• The rapidly formed crystals are irregular. Thus on ageing they will
become regular and the surface area is reduced, so adsorption will
be reduced. The net result of digestion is usually to reduce the
extent of co precipitation and to increase the size of the particles,
rendering filtration easier.
43

44. Filtration
• A precipitate may be separated by filtering it through
• paper,
• sintered glass,
• or sintered porcelain.
• The choice depends on the nature of the precipitate and
on the temperature to which it will be heated after
filtering.
44

45. Washing
• The precipitate and filter must be washed with suitable
electrolyte to remove dissolved solids that remain in the
precipitate and wetted filter.
• Problems with coprecipitation and surface adsorption may
be reduced by careful washing of the precipitate.
• With many precipitates, peptization occurs during
washing.
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46. Precipitates from ionic compounds
- need electrolyte in wash solution TO keep
precipitate from breaking up and redissolving
(peptization)
Electrolyte should be volatile removed by drying
- HNO3, HCl, NH4, NO3, etc.
Example:
AgCl(s) should not be washed with H2O, instead
wash with dilute HNO3
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47. Drying/Igniting Precipitates
 Precipitates are dried at about 120oC for
accurate, stable mass measurements
47

51. Gravimetric Factor

52. Solubility Equilibria
16.6
AgCl (s) Ag+ (aq) + Cl- (aq)
Ksp = [Ag+][Cl-] Ksp is the solubility product constant
MgF2 (s) Mg2+ (aq) + 2F- (aq) Ksp = [Mg2+][F-]2
Ag2CO3 (s) 2Ag+ (aq) + CO3
2- (aq) Ksp = [Ag+]2[CO3
2-]
Ca3(PO4)2 (s) 3Ca2+ (aq) + 2PO4
3- (aq) Ksp = [Ca2+]3[PO3
3-]2
Dissolution of an ionic solid in aqueous solution:
(Q= reaction quotient)
Q = Ksp Saturated solution
Q < Ksp Unsaturated solution No precipitate
Q > Ksp Supersaturated solution Precipitate will form
Note: We are assuming ideal behavior. In reality, there may be hydrolysis
or ion pairs that decrease solubility.

53. 16.6

54. 16.6

55. The Common Ion Effect and Solubility
The presence of a common ion decreases
the solubility of the salt.
What is the molar solubility of AgBr in (a) pure water
and (b) 0.0010 M NaBr?
AgBr (s) Ag+ (aq) + Br- (aq)
Ksp = 7.7 x 10-13
s2 = Ksp
s = 8.8 x 10-7
NaBr (s) Na+ (aq) + Br- (aq)
[Br-] = 0.0010 M
AgBr (s) Ag+ (aq) + Br- (aq)
[Ag+] = s
[Br-] = 0.0010 + s  0.0010
Ksp = s x 0.0010
s = 7.7 x 10-10
16.8
Ksp = [Ag+] [Br-]
Ksp = [Ag+] [Br-]

56. pH and Solubility
• The presence of a common ion decreases the solubility.
• Insoluble bases dissolve in acidic solutions
• Additional of H+ions uses up OH- ions
• Insoluble acids dissolve in basic solutions
• Additional OH- ions use up H+ ions
Mg(OH)2 (s) Mg2+ (aq) + 2OH- (aq)
Ksp = [Mg2+][OH-]2 = 1.2 x 10-11
Ksp = (s)(2s)2 = 4s3
4s3 = 1.2 x 10-11
s = 1.4 x 10-4 M
[OH-] = 2s = 2.8 x 10-4 M
pOH = 3.55 pH = 10.45
At pH less than 10.45
Lower [OH-]
OH- (aq) + H+ (aq) H2O (l)
remove
Increase solubility of Mg(OH)2
At pH greater than 10.45
Raise [OH-]
add
Decrease solubility of Mg(OH)2
16.9

57. Flame Test for Cations
lithium sodium potassium copper
16.11