ERT207
Analytical Chemistry
Gravimetric Analysis and
Precipitation Equilibria
Pn Syazni Zainul Kamal
CO2 : Ability to classify and use separation
techniques and gravimetric methods for
mass determination
Introduction
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.
Analyte = constituents to be determined
There are two main types of gravimetric analyses
A) 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.
B) 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.
Example for Precipitation:Calcium can be determined gravimetrically by
precipitation of calcium oxalate and ignition of
the oxalate ion to calcium oxide.
Ca2+ + C2O42- →CaC2O4
CaC2O4 → CaO + CO2 + CO
The precipitate thus obtained are weighed and
the mass of calcium oxide is determined.
Example for Volatilisation:The analyte or its decomposition products are volatilised at
a suitable temperature. The volatile product is then
collected and weighed, i.e. the mass of the product is
indirectly determined from the loss in mass of the sample.
Example
Water can be separated from most inorganic compounds by
ignition, the evolved water can then be absorbed on any
one of several solid desiccants. The weight of water evolved
may be calculated from the gain in weight of the absorbent.
Not all insoluble precipitates are well suited for gravimetric
analysis.
It is important to consider what properties are required in order
that a precipitate be applicable for a quantitative precipitation
method:-
Solubility
Filterability
Chemical Composition
Other Desirable Properties
Solubility
The product must be sufficiently insoluble to prevent the loss of
weight.
Filterability
Precipitate formed should be adoptable to simple and rapid
filtration methods.
Chemical Composition
The product must be of known chemical composition.
Other Desirable Properties
Other factors effecting the stability and purity of the precipitate.
For a successful determination in gravimetric analysis the
following criteria should be met :(1)The desired substance must be completely precipitated. In
most determination the precipitate is of such low solubility
that losses from dissolution are negligible. An additional
factor is the common ion effect, this further decrease the
solubility of the precipitate.
E.g. When Ag+ is precipitated out by addition of ClAg+ + Cl- = AgCl
The low solubility of AgCl is reduced further by the excess
of Cl- which is added force to the reaction to proceed
towards right side.
For a successful determination in gravimetric analysis the
following criteria should be met :-
(2)The weighed form of the product should be of
known composition.
(3)The product should be pure and easily filtered. It is
usually difficult to obtain a product which is pure or
which is free from impurities. This could be reduced
by careful precipitation and sufficient washing.
Gravimetric Analysis
Gravimetric analysis is potentially more accurate
and more precise than volumetric analysis.
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Gravimetric analysis avoids problems with
temperature fluctuations, calibration errors,
and other problems associated with
volumetric analysis.
But there are potential problems with
gravimetric analysis that must be avoided to
get good results.
Proper lab technique is critical
Steps in a Gravimetric Analysis
1. Preparation of the solution
2. Precipitation
3. Digestion
4. Filtration
5. Washing
6. Drying or ignition
7. Weighing
8. Calculation
1. Preparation of analyte solution
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Gravimetric analysis usually involves
precipitation of analyte from solution.
1st step – prepare the analyte solution
May need preliminary separation to separate
potential interferences before precipitating
analyte
May need adjustment of solution condition to
maintain low solubility of precipitate
May require pH/temp/volume (of sol.) and/or
other adjustments to maximize precipitate
formation. Eg Calcium oxalate insoluble in basic
medium
2. Precipitation
 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.
 The precipitate should
 Be sufficiently insoluble
 Have large crystals (Easier to filter large
crystals)
 Be free of contaminants
Precipitation process :
When solution of precipitating agent (AgNO3)
added into testing solution (chloride solution) to
form AgCl precipitate,
Supersaturation : the solution phase contains
more dissolved salt than at equilibrium. The
driving force will be for the system to approach
equilibrium (saturation).
- Nucleation : initial phase of precipitation. A min
number of particle precipitate will gather
together to form a nucleus of particle precipitate
(solid phase). Higher degree of supersaturation,
the greater rate of nucleation
Ag+Cl- Ag+Cl-Ag+ClAg+Cl-Ag+Cl-Ag+ClAg+Cl-Ag+Cl-Ag+Cl-
nucleation involves the formation of ion pairs and
finally a group of ions formed.
- Particle growth : particle enlargement process.
Nucleus will grow and forming a crystal of a
specific geometric shape.
Von weimarn discover – the particle size of
precipitates is inversely proportional to the
relative supersaturation of the sol. during the
precipitation process.
The von Weimarn Ratio
von Weimarn ratio = (Q – S)
S
A measure of relative supersaturation or
supersaturation ratio
 The lower the better
If high, get excessive nucleation, lots of
small crystals, large surface area
If low, get larger, fewer crystals, small
surface area
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S = solubility of precipitate at equilibrium
Keep it high with high temperatures,
adjusting pH
Q = concentration of mixed reactants before
precipitation
Keep it low by using dilute solutions, stir
mixture well, add reactants slowly
Can lower S later by cooling mixture after
crystals have formed
3. Digestion of the Precipitate
Let precipitate stand in contact with mother
liquor (the solution from which it was
precipitated), usually at high temp.
This process is called digestion, or Ostwald
ripening. The small particles tend to dissolve
and precipitate on the surfaces of the larger
crystals
Large crystal (small surface area) have lower
free energy than small crystal (large surface
area)
Digestion make larger crystals, reduce surface
contamination, reduce crystals imperfection
4. Filtration
Sintered glass crucibles are used to filter the
precipitates.
The crucibles first cleaned thoroughly and then
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.
(you may see the proper technique of filtration in
chapter 2)
5. Washing
Coprecipitated impurities esp those on
surface can be removed by washing the
precipitate
Wet precipitate with mother liquor and
which will also be remove by washing
Need to add electrolyte to the wash liquid
bcoz some precipitate cannot be wash
with pure water, peptization occur.
Eg HNO3 for AgCl precipitate
6) Drying or ignition
To remove solvent and wash electrolytes
Done by heating at 110 to 120°C for 1 to 2
hrs.
Converts hygroscopic compound to nonhygroscopic compound
May used high temp if precipitate must be
converted to a more suitable form before
weighing
Eg MgNH4PO4 convert to pyrophosphate
Mg2P2O7 by heating at 900°C.
7) Weighing
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
Good weighing technique
Organic Precipitates
Organic precipitating agents have the
advantages of giving precipitates with very
solubility in water and a favorable gravimetric
factor.
Most of them are chelating agents that forms
slightly insoluble, uncharged chelates with the
metal ions.
Organic Precipitates
Gravimetric Analysis: Weight
Relationship
in gravimetric method – the analyte (solute) is
converted to precipitate which is then weight.
From the weight of the precipitate formed in a
gravimetric analysis, we can calculate the weight of
the analyte
Gravimetric factor (GF) = weight of analyte per unit
weight of precipitate. Obtain from ratio of the F Wt of
the analyte per F Wt precipitate, multiplied by moles of
analyte per mole of precipitate obtained from each mole
of analyte
GF = f wt analyte (g/mol)
x a (mol analyte/mol precipitate)
f wt precipitate (g/mol)
b
= g analyte / g precipitate
Example 1
If Cl2 in a sample is converted to chloride and
precipitated as AgCl, the weight of Cl2 that gives 1 g of
AgCl is???
F wt Cl = 35.453
F wt AgCl = 107.868
GF = f wt analyte (g/mol)
x a (mol analyte/mol precipitate)
f wt precipitate (g/mol)
b
= g analyte / g precipitate
GF = f wt analyte (g/mol) x a (mol analyte/moprecipitate)
f wt precipitate (g/mol) b
= g analyte / g precipitate
g Cl2 = g AgCl x f wt analyte (g/mol) x
f wt precipitate (g/mol)
= 1 AgCl x 70.906
x 1
143.321
2
= 0.2474 g
a
b
percent composition by weight of the analyte
in the sample :
% A = gA
gsample
x 100%
gA – grams of analyte (the desired test substance)
gsample – grams of sample taken for analysis
EXAMPLE 2
A 0.3516 gm sample of commercial phosphate detergent
was ignited at a red heat to destroy the organic matter.
The residue was then taken up in hot HCl which
converted P to H3PO4. The phosphate was precipitated
with Mg2+ followed by aqueous NH3 to form
MgNH4PO4.6H2O. After being filtered and washed, the
precipitate was converted to Mg2P2O7 (FW=222.57) by
ignition at 1000ºC. This residue weighed 0.2161 gm.
Calculate the percent P (FW = 30.974) in the sample.
GF = f wt analyte (g/mol)
x a (mol analyte/mol precipitate)
f wt precipitate (g/mol)
b
= g analyte / g precipitate
g P = 0.2161 g x 30.974 x 2
222.57 1
= 0.0601g
% A = gA
x 100%
gsample
= 0.0601 g x 100%
0.3516 g
= 0.1709 %
Orthophosphate (PO43-) is determined by
weighing as ammonium phosphomolybdate
(NH4)PO4.12MoO3. Calculate the percent P
and the percent P2O5 if 2.1771g precipitate
(ppt) were obtained from a 0.3588g sample.
[F wt P = 30.97], [F wt P.molybdate = 1876.5],
[F wt P2O5 = 141.95]
g P = 2.1771g x 30.97 x 1 mol
1876.5
1 mol
= 0.0359g
% P = 0.0359g x 100%
0.3588g
= 10.01 %
g P2O5 = 2.1771 g x 141.95 x 1 mol
1876.5
2 mol
= 0.0823g
% P2O5 = 0.0823g x 100%
0.3588g
= 22.94%
Precipitation Reactions
Solubility of a compound = concentrations of a
soluble species at equilibrium with its insoluble
form.
If the compound is sparingly soluble, it will
produce cation & anion.
Eg PbCl2 slightly dissolved in water. So PbCl2
has a specific solubility, s = solid phase aq =
aqueous phase
PbCl2 (s)
Pb2+ (aq)+ 2Cl- (aq)
The equilibrium constant for the reaction is
known as solubility product constant.
Ksp (PbCl2) = [Pb2+][Cl-]2
Concentration of any solid (PbCl2) is
constant and is combined in the
equilibrium constant to give Ksp
Example
Calculate the concentration of Ag+ and Clin a saturated solution of AgCl, and the
molar solubility of AgCl. Ksp for AgCl at
25°C is 1.0 x 10-10.
Solution :
AgCl
Ag+ + Cls
s
s=solubility (M)
Ksp = [Ag+] [Cl-]
= (s)(s)
= 1.0 x 10-10
s = 1.0 x 10-5 M = [Ag+] = [Cl-]
Molar solubility of AgCl is equal to concentration of
[Ag+] or [Cl-] so molar solubility of AgCl= 1.0 x 10-5
M
Common ion effect
If there is an excess of one ion over the
other :
- the concentration of the other is
suppressed
- solubility of precipitate is decreased
Example
Calculate the molar solubility of AgCl in (a)
water and (b) in 0.10M KCl solution. Ksp for
AgCl is 1.7x10-10.
Solution
(a) Solubility of AgCl in water
AgCl
Ag+ + Cls
s
s=solubility(M)
Ksp = [Ag+ ] [Cl- ]
= (s)(s)
= 1.7x10-10
s = 1.3x10-5 M
Solubility of AgCl in water = 1.3x10-5 M
b) Solubility of AgCl in 0.10M KCl
AgCl
Ag+ + Cls
s+0.10
Ksp = [Ag+ ] [Cl- ]
= (s)(s+0.10)
= 1.7x10-10
Since s is small compared to 0.10M, it can be neglected
Ksp = (s)(0.10)
= 1.7x10-10
s = 1.7x10-9 M
Solubility of AgCl in the presence of 0.10M
KCl is 1.7x10-9
Precipitation only occur when ionic product
greater than Ksp value
If ionic product equal Ksp, all the ions will
be in the solution without formation of
precipitate