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.
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
 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
 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
- adjustment of solution condition
(pH/temp/vol/conc of test substance) to maintain
low solubility of precipitate & max 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 (KCl) to form AgCl
precipitate,
1) Supersaturation : the solution phase contains
more dissolved salt than at equilibrium. The
driving force will be for the system to approach
equilibrium (saturation).
2) Nucleation : initial phase of precipitation. A
min number of particle will gather together to
form a nucleus of particle or 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.
3) Particle growth : particle enlargement process.
Nucleus will grow by deposition of particles
precipitate onto the nucleus 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
 S = solubility of precipitate at equilibrium
Keep it high with high temperatures, adjusting
pH
 Q = concentration of reagents 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
 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
weighing differ by 0.3 mg or less.
chapter 2
5. Washing
 Co precipitated impurities esp those on
surface, 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 non-
hygroscopic 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 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
1g of AgCl is?
F wt Cl = 35.453
F wt Ag = 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/mol precipitate)
f wt precipitate (g/mol) b
= g analyte / g precipitate
g Cl2 = g AgCl x f wt analyte (g/mol) x a
f wt precipitate (g/mol)
b
= 1 AgCl x 70.906 g/mol
x 1 mol
143.321 g/mol
2 mol
= 0.2474 g
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 0.3516g 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 100ºC. This residue
weighed 0.2161g. 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 %
EXAMPLE 3
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 g/mol x 1 mol
1876.5 g/mol 1 mol
= 0.0359g
% P = 0.0359g x 100%
0.3588g
= 10.01 %
g P2O5 = 2.1771 g x 141.95 g/mol x 1 mol
1876.5 g/mol
2 mol
= 0.0823g
% P2O5 = 0.0823g x 100%
0.3588g
= 22.94%