Analyze of metal

1. GRAVIMETRIC ANALYSIS
The term gravimetric pertains to a Weight Measurement.
Gravimetric method is one in which the analysis is completed
by a weighing operation.
Gravimetric Analysis is a group of analytical methods in which
the amount of analyte is determined by the measurement of the
mass of a pure substance containing the analyte.
Gravimetric Methods can also be defined as quantitative
methods based on the determining the mass of a pure
compound to which the analyte is chemically related.

2. Types of Gravimetric Analyses:
1- Precipitation:
Analyte must first be converted to a solid (precipitate) by
precipitation with an appropriate reagent. The precipitates from
solution is filtered, washed, purified (if necessary) and weighed.
2- Volatilization:
In this method the analyte or its decomposition products are
volatilised (dried) and then collected and weighed, or alternatively,
the mass of the volatilised product is determined indirectly by the
loss of mass of the sample.

3. Types of Gravimetric Analyses:
3- Electrogravimetry:
Electrolysis used to deposit a metal on a solid electrode so that its
mass can be determined.
4-Thermogravimetry:
The measurement of changes in weight as a function of changes in
temperature used as a technique of chemically analyzing substances.

4. How to Perform a Successful Gravimetric Analysis
(Precipitation)
• 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 sample solution
2. Precipitation
3. Digestion
4. Filtration
5. Washing
6. Drying or igniting
7. Weighing
8. Calculation

5. Requirements of the gravimetric analysis:
• I- Requirements of the precipitated form:
• 1-The ppt. must be so insoluble that no appreciable loss occur
when it is collected by filtration. )Ksp ≤ 10-8 (
• 2-The ppt. must be readily separated from the solution by
filtration, and can be washed free of soluble impurities
• 3-The ppt. must be convertible into a pure substance of definite
chemical composition, this may be effected either by ignition or
by a simple chemical operation
II- Requirements of the weighted form:
1-The weighted form must be only one compound with constant
chemical formula for simple calculation.
2-The weighted form must be stable without any change ( no
oxidation or reduction) during ignition.
3-the amount of element or ion in the weighted form must be small
as much as possible

6. • III- Requirements of the precipitating agent
• 1-These agents must have volatilization properties in order to
remove the excess amount during ignition or heating.
• 2-The precipitating agent must be specific or selective
• Selective
• Ag+ + Halides (X-)  AgX(s)
• Ag+ + CNS-  AgCNS(s)
• Specific
• Dimethylglyoxime (DMG)
• 2 DMG + Ni2+  Ni(DMG)2(s) + 2 H+
Requirements of the gravimetric analysis:

7. Advantages of Gravimetric Analysis
• Accurate and precise: Gravimetric analysis is potentially more
accurate and more precise than volumetric analysis
• Possible sources of errors can be checked
• Gravimetric analysis avoids problems with temperature
fluctuations, calibration errors, and other problems associated
with volumetric analysis.
• Relatively inexpensive
Disadvantages
• Proper lab technique is critical.
• Careful and time consuming.
• Carefully clean glassware.
• Very accurate weighing.
• Coprecipitation.

8. Steps of Gravimetric Analysis
I-Preparation of the solution
1- Preliminary separation may be necessary to eliminate interfering
materials
2- Adjust the solution conditions to maintain low solubility of the
ppt. and to obtain it in a form suitable for filtration.
The conditions adjusted are:
• Masking of the potential interferences
• Volume of the solution during precipitation
• Concentration range of the test substance
• Temperature
• PH

9. Steps of Gravimetric Analysis
• II- Precipitation step:
• Adding the precipitating reagent to the test sample to form a ppt.
• It is the most important step in gravimetric analysis.
• Precipitation occurs in a series of three steps
• 1) Super saturation: the solution phase contains more dissolved salt
than at equilibrium. The driving force will be for the system to
approach equilibrium (saturation).

10. • 2) Nucleation : initial phase of
precipitation. A min number of particle
will gather together to form a nucleus of
particle of precipitate (solid phase).
• it is of two types: Spontaneous and
Induced.
3) Crystal growth :
particle enlargement process.
Nucleus will grow by deposition
of particles precipitate onto the
nucleus and forming a crystal of a
specific geometric shape.

11. • 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.
Ideal Analytical Precipitation
• Colloidal suspensions
• (large number of small particles)
– 10-7 to 10-4 cm diameter
– Normally remain suspended
– Very difficult to filter
Filterability of Precipitates:
Crystalline suspensions
(small number of large particles).
 tenths of mm diameter
 Normally settle out spontaneously
 Readily filterable

12. Ideal 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.
High Relative supersaturation → many small crystals (high surface area)
Low relative super saturation→ fewer, large crystals (low surface area)

13. Conditions for Analytical Precipitation
• 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.)
• Precipitate at a low pH “acidic “:
• 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 crystallinenuclei, such as
expulsion of foreign atoms (or other impurities).

14. • Why much excess precipitating agent should be avoided?
• A slight excess of precipitating reagent is added to decrease the
solubility by mass action (common ion effect) and to insure
complete precipitation.
• A large excess of precipitating agent should be avoided because
this increases chances of adsorption on the surface of the ppt. If
the approximate amount of analyte is known, a 10 % excess of
reagent is generally added.
• How completeness of precipitation done?
• Completeness of precipitation is checked by waiting until the
precipitate has settled and then adding a few drops of
precipitating reagent to the clear solution above it. If no new
precipitate forms, precipitation is complete.

15. 1- Crystalline precipitate (small number of large particles).
2- Colloidal precipitate (large number of small particles).
Colloidal particles are very small and have a very large surface to- mass ratio,
which promotes surface adsorption.
Types of precipitates
Coagulation: when counter-layer neutralizes
the primary layer and is close to it, so the
particles will collect together to form larger
size particles.
Peptization:
When coagulated particles are filtered,
they retain the adsorbed primary and
secondary ion layers a long with solvent.
If washed with water, the extent of layer to
be loosely bound, and the particles revert
to the colloidal state.

16. • The precipitate is allowed to stand in the presence of the mother liquor (the solution
from which it was precipitated).
• The large crystal grow at the expense of the small ones.
• Digestion is usually done at elevated temperature to speed the process, although in
some cases it is done at room temperature.
• Function of the digestion
• Improves the purity and crystallinity of the ppt.
• 1-Imperfection of the crystals tend to disappear.
• 2-Adsorbed or trapped impurities tend to go into solution,
• 3-Improves the filterability of the precipitate.
During digestion at elevated temperature:
• Small particles tend to dissolve and reprecipitate
on larger ones.
* Individual particles agglomerate.
* Adsorbed impurities tend to go into solution.
III- Digestion of the precipitate :

17. • The function of the filtration step, is the separate the precipitate from the
mother liquor.
IV- Filtration of the precipitate :
The method of the filtration
depending on the:
• nature of the precipitate
• the temperature of the drying
before weighing.
Filtration can be done by two ways:
a- Filtering Crucibles
If the precipitate need not to be heated above 500 0 C, a glass sintered crucible
may be used. They are generally available in different kinds.
b- Filter paper and Funnels
If the precipitate must be heated over 500 oC to be converted to a suitable
weighing form.. It can be filtered by an ashless filter paper, then transferred to a
clean porcelain crucible.

18. • The main function of this step is the removing of :
• Excess of precipitating agent.
• The mother liquor
• The impurities adsorbed on the surface of the
precipitate.
V- Washing of the precipitate :
Properties of the washing solution:
* should be an electrolyte
* should contain common ion to decreases the errors due to the dissolving
* should be volatile at drying or ignition step
* should not formed volatile compound with the precipitate
* should not formed easy soluble compound with the precipitate
Test for Completeness of washing:
This usually done by testing the filtrate for the presence of the ion of the
precipitating reagent. After several washing with small volumes of the wash
solution, a few drops of the filtrate are collected in a test tube for testing.

19. • Drying:
• If the collected precipitate is in a form suitable for weighing, it must be
heated to remove water and the adsorbed electrolyte from the wash
solution.
• This drying can usually be done by heating at 110 to 120◦C for 1 to 2 h. in
an oven
VI- Drying or Ignition the ppt.:
The ppt. is ignited at a much higher
temperature is usually required if it must
be converted to a more suitable form for
weighing. For example Hydrous ferric
oxide, Fe2O3 ·xH2O, is ignited to the
anhydrous ferric oxide.
Ignition:
Steps of ignition
drying of the filter paper
charring of the filter paper
ignition of the precipitate

20. VII- Weighing the ppt.:
After the precipitate is allowed to cool
(preferably in a desiccator to keep it from
absorbing moisture), it is weighed (in the
crucible).
• Properly calibrated analytical balance
VIII- Calculation:
In general the precipitate we weigh is usually in a different form than the analyte
whose weight we wish to report.
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:
The gravimetric factor (GF),

21. • GF = (fwt analyte (g/mol) / fwt precipitate(g/mol)) x (a/b)
• weight analyte (g) = (wt ppt (g) x GF)
• % analyte = (weight analyte (g)/ weight sample (g)) x 100%
Substance weighed Substace sought G F
Mg2P2O7 Mg2+
K2PtCl6 K2O
Ag2CrO4 Cr2O3

22. Alternative Gravimetry Technique
• Homogeneous Precipitation
• What?
• Precipitating agent generated slowly by chemical reaction in analyte
solution
• Why?
– Precipitant appears gradually throughout
– Keeps relative supersaturation low
– Larger, less-contaminated particles
• How?
– (OH-) by urea decomposition
– (NH2)2CO  2 OH- + CO2 + 2 NH4
+
– (S=) by thioacetamide decomposition
– CH3CSNH2 H2S + CH3CONH2
– (DMG) from biacetyl + hydroxylamine
– CH3C(=0)-C(=0)CH3 + 2 H2NOH DMG + 2 H2O

23. Purity of precipitate “ppt”
• When the ppt is separated out from solution it is always not preferably
pure and may be contaminated even after washing.
• The amount of impurities depends on nature of ppt and condition of
precipitation.
• It may be due to:
– Co-precipitation.
– Post precipitation.
Coprecipitation:
• A process in which normally soluble compounds are carried down with
insoluble precipitate. It may resulted in impurities within the desired
precipitates.
• Coprecipitated impurities may cause either negative or positive errors.
• There are four types of coprecipitation:
• Surface adsorption, Mixed-crystal formation, Occlusion, and
mechanical entrapment (Inclusion).

24. Surface Adsorption:
The impurity is chemically or physically adsorbed onto the surface of precipitates
• Adsorption is the major source of contamination in coagulated colloids but of
no significance in crystalline precipitates.
• Minimizing Adsorbed Impurities on Colloids:
• Washing a coagulated colloid with a solution containing a volatile electrolyte.
• Digestion: during this process, water is expelled from the solid to give a denser
mass that has a smaller specific surface area for adsorption.
• Reprecipitation: In this process, the filtered solid is redissolved and
reprecipitated. The solution containing the redissolved precipitate has a
significantly lower contaminant concentration than the original, and even less
adsorption occurs during the second precipitation

25. • A type of coprecipitation in which a contaminant ion replaces an ion in the
lattice of a crystal.
• Ex:
• SrSO4 in BaSO4
• MgKPO4 in MgNH4PO4
• MnS in CdS
• Mixed-crystal formation may occur in both colloidal and crystalline
precipitates
• Problem solving:
•Change to another more selective precipitating agent
•Reprecipitation
Mixed-Crystal Formation:

26. Occlusion: ( ‫االيونات‬ ‫انحباس‬ )
* A type of co-precipitation in which a compound (foreign
ions in the counter-ion layer ) is physically trapped within a
precipitate during rapid precipitate formation.
* Problem solving: Digestion
Mechanical Entrapment: ( (
‫المذيب‬ ‫انحصار‬
A type of co-precipitation in which coprecipitated physically
trap a pocket of solution within a precipitate during rapid
precipitate formation.
Problem solving: Digestion
Mixed-crystal formation may occur in both colloidal and crystalline
precipitates, but occlusion and mechanical entrapment are confined to
crystalline precipitates.

27. Postprecipitation
Sometimes, when the precipitate is allowed to stand in contact with
the mother liquor, a second substance will slowly form a precipitate
with the precipitating reagent.
This is called post precipitation.
For example, when calcium oxalate is precipitated in the presence
of magnesium ions, magnesium oxalate does not immediately
precipitate because it tends to form supersaturated solutions.
But it will precipitate if the solution is allowed to stand too long
before being filtered.
Postprecipitation is a slow equilibrium process.

29. Q. 1- A 0.5000 g of a mixture containing " Na2CO3 and
NaCl "was dissolved , treated and ppted. as silver
chloride AgCl ( 143.5 g/mol) , the wt. of the ppt was
0.5035 g. What is the % of Na2CO3

30. Q. 2- A gold alloy containing Gold Au , Silver Ag , and
Copper Cu weights 1.0000 g. The Ag was ppted. and
weighed as 0.2000 g of AgCl. Cu was ppted. and weighed
as 0.4000 g of CuO. Calculate the % of Gold in the alloy?
(( Ag = 108 , CuO = 79.5 , AgCl = 143.5 ,, Au = 197 , Cu = 63.5 ))

31. Precipitation Equilibria: The Solubility Product
• Solubility of Slightly Soluble Salts:
• AgCl(s)(AgCl)(aq) Ag+ + Cl-
• Solubility Product KSP :
When substances have limited solubility and their solubility is
exceeded, the ions of the dissolved portion exist in equilibrium with the
solid material.
So-called insoluble compounds generally exhibit this property.
When a compound is referred to as insoluble, it is actually not
completely insoluble but is slightly soluble. For example, if solid
AgCl is added to water, a small portion of it will dissolve:

32. • AgCl(s)  Ag+ + Cl-
• KSP = [Ag+][Cl-]
BaSO4(s) ----> Ba2+(aq) + SO4
2-(aq)
Ksp = [Ba2+][SO4
2-]
PbCl2(s) -----> Pb2+(aq) + 2 Cl-(aq)
Ksp = [Pb2+][Cl-]2
Ag2CrO4(s)  2 Ag+ + CrO4
2-
KSP = [Ag+]2[CrO4
2-]
The solubility product constant, Ksp, is the product of the concentrations
of the ions involved in solubility equilibrium, each raised to a power equal
to the stoichiometric coefficient of that ion in the chemical equation for
the equilibrium.

33. The molar solubility depends on the stoichiometry of the salt.
A 1:1 salt is less soluble than a nonsymmetric salt with the same Ksp.

34. Examples
Sol. S mol/L
Ksp
Sol. Pr.
General
Formula
AgCl / PbSO4
( Ksp )1/2
S2
[M] [ A]
MA
PbI2 / Ag2S
( Ksp / 4 )1/3
4S3
[M] [ A]2
MA2
Fe(OH)3 / La(IO3)3
( Ksp / 27 )1/4
27S4
[M] [ A]3
MA3
Bi2S3
( Ksp / 108 )1/5
108S5
[M]2 [ A]3
M2A3
The relation between Ksp and solubility:

37. The molar solubility depends on the stoichiometry of the salt.
A 1:1 salt is less soluble than a nonsymmetric salt with the same Ksp.

38. Precipitation Equilibria: The Common and diverse Ion Effect
• The Common Ion Effect will decrease the solubility of a slightly
soluble salt.
• For this reason a slight excess of precipitating reagent (10%) is
added to the sample to decrease the solubility.
Ex: Calculate S for AgCl in : a- water. b – 0.1 M AgNO3 .
The Common Ion Effect

39. Diverse Ion Effect on Solubility:
• Presence of diverse ions will increase the solubility of
precipitates due to shielding of dissociated ion species.
The important factors that affect solubility of crystalline solids
are:
1- Temperature
2- Nature of the solvent
3- Common ion
4- Diverse ion
5- pH
Factors affecting solubility

40. • The solubility of most compounds increases with increasing
temperature.
• Some steps of gravimetric analysis are carried out by using
hot solutions in order to:
• Improve the ppt. properties in terms of particle size .
• Eliminate impurities by increasing their solubility.
• Increase the size of ppt. particles, which reduces the possibility
of contamination by decreasing the surface area.
• Precipitates with high solubility should be precipitated from a
cold solution
1- Temperature

41. 2- Nature of the solvent
• Most inorganic compounds more soluble in water than in
organic solvents.
• The high polarity of water enables it to attract and surround the
positive and negative ions (hydrated ions), which results in the
dissolution of the compound.
• The energy released as a result of this process is used to
overcome the attraction forces between ions within the crystal
structure .
• This attraction does not occur between the ions and the organic
solvents and thus the solubility does not occur

42. 3- The Common ion:
• The common ion is one of the ions that make up the ppt.
• The effect of the common ion is the effect of the presence of one
of the ions of the precipitant in the solution.
• The source of which is a dissolved compound in the solution and
not as a result of the dissolved part of the ppt.
• A slight increase in the common ion reduces the solubility of the
precipitate. See the previous example.
• For this reason, a slight excess (10%) of the precipitating reagent
is added.
• Washing the ppt. by a washing liquid containing a common ion.
• The common ion should not be the ion to be estimated ”Analyte”

43. To maintain favorable conditions for
precipitations, precipitate from hot and dilute
solution at low pH.
4- Diverse Ion Effect on Solubility:
• Presence of diverse ions will increase the solubility of
precipitates due to shielding of dissociated ion species.
5- Effect of pH:
*Generally solubility increases as pH of the solution decreased.

44. Diverse Ion Effect on Solubility:
• Presence of diverse ions will increase the solubility of
precipitates due to shielding of dissociated ion species.
• KSP
o and Activity Coefficients
• AgCl(s)(AgCl)(aq) Ag+ + Cl-
• Thermodynamic solubility product KSP
o
• KSP
o = aAg+
. aCl- = [Ag+]ƒAg+
. [Cl-]ƒCl-
• KSP
o = KSP ƒAg+
. ƒCl-
• KSP = KSP
o/(ƒAg+
. ƒCl)