Gravimetric analysis involves determining the amount of analyte by measuring the mass of a pure substance containing the analyte. It has two main types - precipitation and volatilization. In precipitation, the analyte is converted to a solid precipitate using a reagent, then the precipitate is filtered, washed, purified and weighed. In volatilization, the analyte or its products are vaporized and collected or the mass loss is measured. Key steps in gravimetric analysis include preparation of the analyte solution, precipitation, digestion, filtration, washing, drying/ignition, and weighing to calculate the analyte amount. It is potentially more accurate and precise than volumetric analysis but requires proper technique.

1. Gravimetric AnalysisGravimetric Analysis

2. IntroductionIntroduction
The term gravimetric pertains to aThe term gravimetric pertains to a WeightWeight
Measurement.Measurement.
Gravimetric method is one in which the analysisGravimetric method is one in which the analysis
is completed by a weighing operation.is completed by a weighing operation.
Gravimetric Analysis is a group of analyticalGravimetric Analysis is a group of analytical
methods in whichmethods in which the amount of analyte isthe amount of analyte is
determined by the measurement of the mass ofdetermined by the measurement of the mass of
a pure substance containing the analyte.a pure substance containing the analyte.
Gravimetric Methods can also be defined asGravimetric Methods can also be defined as
quantitative methodsquantitative methods based on the determiningbased on the determining
the mass of a pure compound to which thethe mass of a pure compound to which the
analyte is chemically related.analyte is chemically related.
Analyte = constituents to be determined

3. There are two main types of gravimetric analyses
A) Precipitation
B) Volatilization
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.
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.

4. Example for Precipitation:-Example for Precipitation:-
Calcium can be determined gravimetrically byCalcium can be determined gravimetrically by
precipitation of calcium oxalate and ignition ofprecipitation of calcium oxalate and ignition of
the oxalate ion to calcium oxide.the oxalate ion to calcium oxide.
CaCa2+2+
+ C+ C22OO44
2-2-
→CaC→CaC22OO44
CaCCaC22OO44 → CaO + CO→ CaO + CO22 + CO+ CO
The precipitate thus obtained are weighed andThe precipitate thus obtained are weighed and
the mass of calcium oxide is determined.the mass of calcium oxide is determined.

6. Example for Volatilization:-Example for Volatilization:-
The analyte or its decomposition products are volatilised atThe analyte or its decomposition products are volatilised at
a suitable temperature. The volatile product is thena suitable temperature. The volatile product is then
collected and weighed, i.e. the mass of the product iscollected and weighed, i.e. the mass of the product is
indirectly determined from the loss in mass of the sample.indirectly determined from the loss in mass of the sample.
ExampleExample
Water can be separated from most inorganic compounds byWater can be separated from most inorganic compounds by
ignition, the evolved water can then be absorbed on anyignition, the evolved water can then be absorbed on any
one of several solid desiccants. The weight of water evolvedone of several solid desiccants. The weight of water evolved
may be calculated from the gain in weight of the absorbent.may be calculated from the gain in weight of the absorbent.

7. Gravimetric AnalysisGravimetric Analysis
Gravimetric analysis is potentially more accurateGravimetric analysis is potentially more accurate
and more precise than volumetric analysis.and more precise than volumetric analysis.

Gravimetric analysis avoids problems withGravimetric analysis avoids problems with
temperature fluctuations, calibration errors,temperature fluctuations, calibration errors,
and other problems associated withand other problems associated with
volumetric analysis.volumetric analysis.

But there are potential problems withBut there are potential problems with
gravimetric analysis that must be avoided togravimetric analysis that must be avoided to
get good results.get good results.

Proper lab technique is criticalProper lab technique is critical

8. Steps in a Gravimetric AnalysisSteps in a Gravimetric Analysis
1. Preparation of the solution1. Preparation of the solution
2. Precipitation2. Precipitation
3. Digestion3. Digestion
4. Filtration4. Filtration
5. Washing5. Washing
6. Drying or ignition6. Drying or ignition
7. Weighing7. Weighing
8. Calculation8. Calculation

9. 1.1. Preparation of analyte solutionPreparation of analyte solution

Gravimetric analysis usually involvesGravimetric analysis usually involves
precipitation of analyte from solutionprecipitation of analyte from solution..

11stst
step – prepare the analyte solutionstep – prepare the analyte solution

May need preliminary separation to separateMay need preliminary separation to separate
potential interferences before precipitatingpotential interferences before precipitating
analyteanalyte

May need adjustment of solution condition toMay need adjustment of solution condition to
maintain low solubility of precipitatemaintain low solubility of precipitate

May require pH/temp/volume (of sol.) and/orMay require pH/temp/volume (of sol.) and/or
other adjustments to maximize precipitateother adjustments to maximize precipitate
formation. Eg Calcium oxalate insoluble in basicformation. Eg Calcium oxalate insoluble in basic
mediummedium

10. 2. Precipitation2. Precipitation
 The precipitating reagent is added at aThe precipitating reagent is added at a
concentration that favors the formation of a "good"concentration that favors the formation of a "good"
precipitate.precipitate.
 This may require low concentration, extensiveThis may require low concentration, extensive
heating (often described as "digestion"), or carefulheating (often described as "digestion"), or careful
control of the pH.control of the pH.
 The precipitate shouldThe precipitate should

Be sufficiently insolubleBe sufficiently insoluble

Have large crystals (Easier to filter largeHave large crystals (Easier to filter large
crystals)crystals)

Be free of contaminantsBe free of contaminants

11. Precipitation process :Precipitation process :
When solution of precipitating agent (AgNOWhen solution of precipitating agent (AgNO33))
added into testing solution (chloride solution) toadded into testing solution (chloride solution) to
form AgCl precipitate,form AgCl precipitate,
SupersaturationSupersaturation : the solution phase contains: the solution phase contains
more dissolved salt than at equilibrium. Themore dissolved salt than at equilibrium. The
driving force will be for the system to approachdriving force will be for the system to approach
equilibrium (saturation).equilibrium (saturation).
- Nucleation- Nucleation : initial phase of precipitation. A min: initial phase of precipitation. A min
number of particle precipitate will gathernumber of particle precipitate will gather
together to form a nucleus of particle precipitatetogether to form a nucleus of particle precipitate
(solid phase). Higher degree of supersaturation,(solid phase). Higher degree of supersaturation,
the greater rate of nucleationthe greater rate of nucleation
Ag+
Cl-
Ag+
Cl-Ag+Cl-
Ag+
Cl-Ag+Cl-Ag+Cl-
Ag+Cl-Ag+Cl-Ag+
Cl-

12. nucleation involves the formation of ion pairs andnucleation involves the formation of ion pairs and
finally a group of ions formed.finally a group of ions formed.
-- Particle growthParticle growth : particle enlargement process.: particle enlargement process.
Nucleus will grow and forming a crystal of aNucleus will grow and forming a crystal of a
specific geometric shape.specific geometric shape.
Von weimarn discover – the particle size ofVon weimarn discover – the particle size of
precipitates is inversely proportional to theprecipitates is inversely proportional to the
relative supersaturation of the sol. during therelative supersaturation of the sol. during the
precipitation process.precipitation process.

13. 3. Digestion of the Precipitate3. Digestion of the Precipitate
Let precipitate stand in contact with motherLet precipitate stand in contact with mother
liquor (the solution from which it wasliquor (the solution from which it was
precipitated), usually at high temp.precipitated), usually at high temp.
This process is calledThis process is called digestiondigestion, or, or OstwaldOstwald
ripeningripening. The small particles tend to dissolve. The small particles tend to dissolve
and precipitate on the surfaces of the largerand precipitate on the surfaces of the larger
crystalscrystals
Large crystal (small surface area) have lowerLarge crystal (small surface area) have lower
free energy than small crystal (large surfacefree energy than small crystal (large surface
area)area)
Digestion make larger crystals, reduce surfaceDigestion make larger crystals, reduce surface
contamination, reduce crystals imperfectioncontamination, reduce crystals imperfection

15. 4. Filtration4. Filtration
Sintered glass crucibles are used to filter theSintered glass crucibles are used to filter the
precipitates.precipitates.
The crucibles first cleaned thoroughly and thenThe crucibles first cleaned thoroughly and then
subjected to the same regimen of heating andsubjected to the same regimen of heating and
cooling as that required for the precipitate.cooling as that required for the precipitate.
This process is repeated until constant massThis process is repeated until constant mass
has been achieved, that is, until consecutivehas been achieved, that is, until consecutive
weighings differ by 0.3 mg or less.weighings differ by 0.3 mg or less.
(you may see the proper technique of filtration in(you may see the proper technique of filtration in
chapter 2)chapter 2)

17. 5. Washing5. Washing
Coprecipitated impurities esp those onCoprecipitated impurities esp those on
surface can be removed by washing thesurface can be removed by washing the
precipitateprecipitate
Wet precipitate with mother liquor andWet precipitate with mother liquor and
which will also be remove by washingwhich will also be remove by washing
Need to add electrolyte to the wash liquidNeed to add electrolyte to the wash liquid
bcoz some precipitate cannot be washbcoz some precipitate cannot be wash
with pure water, peptization occur.with pure water, peptization occur.
Eg HNOEg HNO33 for AgCl precipitatefor AgCl precipitate

18. 6) Drying or ignition6) Drying or ignition
To remove solvent and wash electrolytesTo remove solvent and wash electrolytes
Done by heating at 110 to 120°C for 1 to 2Done by heating at 110 to 120°C for 1 to 2
hrs.hrs.
Converts hygroscopic compound to non-Converts hygroscopic compound to non-
hygroscopic compoundhygroscopic compound
May used high temp if precipitate must beMay used high temp if precipitate must be
converted to a more suitable form beforeconverted to a more suitable form before
weighingweighing
Eg MgNHEg MgNH44POPO44 convert to pyrophosphateconvert to pyrophosphate
MgMg22PP22OO77 by heating at 900°C.by heating at 900°C.

19. 7) Weighing7) Weighing
After the precipitate is allowed to coolAfter the precipitate is allowed to cool
(preferably in a desiccator to keep it from(preferably in a desiccator to keep it from
absorbing moisture), it is weighed (in theabsorbing moisture), it is weighed (in the
crucible).crucible).
Properly calibrated analytical balanceProperly calibrated analytical balance
Good weighing techniqueGood weighing technique

20. 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 Analysis: WeightGravimetric Analysis: Weight
RelationshipRelationship

21. 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

22. 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

23. 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 a
f wt precipitate (g/mol) b
= 1 AgCl x 70.906 x 1
107.8 2
= 0.32g

24. percent composition by weight of the analytepercent composition by weight of the analyte
in the sample :in the sample :
% A = g% A = gAA x 100%x 100%
ggsamplesample
ggAA – grams of analyte (the desired test substance)– grams of analyte (the desired test substance)
ggsamplesample – grams of sample taken for analysis– grams of sample taken for analysis

25. 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.

26. g P = 0.2161 g x 30.974 x 2g P = 0.2161 g x 30.974 x 2
222.57 1222.57 1
= 0.0601g= 0.0601g
% A = g% A = gAA x 100%x 100%
ggsamplesample
= 0.0601 g x 100%= 0.0601 g x 100%
0.3516 g0.3516 g
= 0.1709 %= 0.1709 %
GF = f wt analyte (g/mol) x a (mol analyte/mol precipitate)
f wt precipitate (g/mol) b
= g analyte / g precipitate

27. Orthophosphate (POOrthophosphate (PO44
3-3-
) is determined by) is determined by
weighing as ammonium phosphomolybdateweighing as ammonium phosphomolybdate
(NH(NH44)PO)PO44.12MoO.12MoO33. Calculate the percent P. Calculate the percent P
and the percent Pand the percent P22OO55 if 2.1771g precipitateif 2.1771g precipitate
(ppt) were obtained from a 0.3588g sample.(ppt) were obtained from a 0.3588g sample.
[F wt P = 30.97], [F wt P.molybdate = 1876.5],[F wt P = 30.97], [F wt P.molybdate = 1876.5],
[F wt P[F wt P22OO55 = 141.95]= 141.95]

28. g P = 2.1771g x 30.97 x 1 molg P = 2.1771g x 30.97 x 1 mol
1876.5 1 mol1876.5 1 mol
= 0.0359g= 0.0359g
% P = 0.0359g x 100%% P = 0.0359g x 100%
0.3588g0.3588g
== 10.01 %10.01 %

29. g Pg P22OO55 = 2.1771 g x 141.95 x 1 mol= 2.1771 g x 141.95 x 1 mol
1876.5 2 mol1876.5 2 mol
= 0.0823g= 0.0823g
% P% P22OO55 = 0.0823g x 100%= 0.0823g x 100%
0.3588g0.3588g
== 22.94%22.94%