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1. Gravimetric Analysis
Chapter 6
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2. 6.1. Principle and types of Gravimetric analysis
 Gravimetric Analysis: is a method 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
 Gravimetric analysis is potentially more accurate and precise than
volumetric analysis.
 Gravimetric analysis avoids problems with temperature fluctuations,
calibration errors, and other problems associated with volumetric analysis.
Advantages - when done correctly, it is highly accurate (most accurate of all
time); requires minimal equipment.
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Gravimetry: Any method in which the signal is a mass or
change in mass

3. Example:
 Determination of lead (Pb+2) in water
Pb+2 + 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
Disadvantage - requires skilled operator, slow operation.
Convert analyte into a solid; filter, weigh and calculate a mole
But there are potential problems with gravimetric analysis that must be
avoided to get good results.
Proper lab technique is critical
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4. Types of Gravimetry
1. Combustion (Volatilization) Analysis
 Technique is accurate and usable with a wide range of compounds.
 In volatilization methods, the analyte or its decomposition products are
volatilized at a suitable temperature. 5
 Gravimetric method types are:
Precipitation gravimetry: A gravimetric method in which the signal is the mass of a
precipitate.
Electrogravimetry: A gravimetric method in which the signal is the mass of an
electrodeposit on the cathode or anode in an electrochemical cell.
Volatilization gravimetry: A gravimetric method in which the loss of a volatile species
gives rise to the signal.
Particulate gravimetry: A gravimetric method in which the mass of a particulate
analyte is determined following its separation from its matrix.

5. 2. Precipitation Gravimetry
 The analyte is converted to a sparingly soluble precipitate.
 This precipitate is then filtered, washed free of impurities, converted to a product of
known composition by suitable heat treatment and weighed.
Example: - Determination of calcium in natural waters: In this method, an excess of oxalic
acid, H2C2O4, is added to a carefully measured volume of the sample. Ammonia is then
added to neutralize the solution and cause the calcium in the sample to precipitate as
calcium oxalate. The reactions are:
 The precipitate is filtered using a weighed filtering crucible and is dried and ignited. This
process converts the precipitate entirely to calcium oxide. The reaction is:
 After cooling, the crucible and precipitate are weighed and the mass of calcium oxide is
determined by subtraction of the known mass of the crucible.
Reagent + Analyte Solid Product (collect and measure mass)
2NH3 + H2C2O4 2NH4
+
+ C2O4
2-
Ca2+
(aq) + C2O4 (aq) CaC2O4 (s)
CaC2O4(s) CaO (s) + CO (g) + CO2 (g)
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6. 5.2.Properties of precipitates and precipitating agents
 Ideally, a gravimetric precipitation agent should react specifically, or if not, then
selectively with the analyte.
In addition to specificity or selectivity.
 the ideal precipitating reagent would react with the analyte to give a product that is:
 easily filtered and washed free of contaminants
 sufficiently low solubility so that no significance loss of the solid occurs
during filtration and washing
 unreactive with constituents of the atmosphere
 known composition after it is dried, or if necessary, ignited
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Specific reagents, which are rare, react only with a single chemical species.
Selective reagents, which are more common, react with a limited number of
species.

7. 6.3. Steps in gravimetric analysis
1. Preparation of the solution
2. Precipitation
3. Digestion
4. Filtration
5. Washing
6. Drying or igniting
7. Weighing
8. Calculation
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8. 1. Preparation of the Solution
 Gravimetric analysis usually involves precipitation of analyte from solution.
This may involve:
 to separate potential interferences before precipitating analyte
 adjustment of the pH of the solution in order for the precipitate to occur
quantitatively and get a precipitate of desired properties
 removing interferences
 adjusting the volume of the sample to suit the amount of precipitating agent to
be added.
 when possible, select precipitating agents that are selective (or specific, if
possible).
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9. 2. Precipitation
 This requires addition of a precipitating agent solution to the sample solution.
 Upon addition of the first drops of the precipitating agent, super saturation
occurs, and then nucleation starts to occur where every few molecules of
precipitate aggregate together forming a nucleolus.
 At this point, addition of extra precipitating agent will either form new nuclei
or will build up on existing nuclei to give a precipitate.
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The precipitate should
be insoluble, but not too insoluble
have large crystals
Easier to filter large crystals
be free of contaminants
The smaller the surface area the better

10. 3. Digestion of the Precipitate
 The precipitate is left hot (below boiling) for 30 min to 1 hour in order for the
particles to be digested.
 Digestion involves dissolution of small particles and re-precipitation on larger
ones resulting in particle growth and better precipitate characteristics.
 Digestion forces the small colloidal particles to agglomerate, which decreases
their surface area and thus adsorption.
 Let precipitate stand in contact with solution, usually at high temperature
 Large crystals (small surface area) have lower free energy than small crystals
(large surface area) 15
Generally, during digestion at elevated temperature:
Small particles tend to dissolve and re-precipitate on larger ones.
Individual particles agglomerate.
Adsorbed impurities tend to go into solution.

11. 4. Filtering the Precipitate:
Filtration should be done in an appropriate sized Goosh or ignition filter paper.
 Colloidal precipitates present filtering problems if particles are too small
– Can plug filter paper or glass or pass right through the filter if not
coagulated well
– Hydrophobic colloids generally filter better than hydrophilic colloids
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12. 5. Washing the Precipitate
 washing the precipitate very well in order to remove all adsorbed species which
will add to weight of precipitate.
 Washing removes the mother liquor
 Washing may also remove some of the coprecipitated compounds
 Washing can cause peptization of colloids
 Diluting the counter-ion layer causes it to get larger, forcing coagulated
colloidal particles apart
 And THERE THEY GO right through the filter
 Wash with a solution of a volatile electrolyte that will be removed in drying
step
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Peptization: The reverse of coagulation in which a coagulated precipitate
reverts to smaller particles.

13. 6. Drying or Igniting
 Dry the precipitate to remove water and volatile electrolytes from wash
solution
 Ignition (very high-temperature drying) converts precipitates to compounds
more suitable for weighing
 Removes water of hydration
 Converts hygroscopic compound to non-hygroscopic compound
 Sometimes it even converts the filter paper to ash
7. Weighing
• Properly calibrated analytical balance
• Good weighing technique
• Avoid static electricity
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14. 6.4. Gravimetric Calculation
 The results of a gravimetric analysis are generally computed from two experimental
measurements:
 the mass of a sample and the mass of a product of known composition. When the
product of the analyte X, the concentration is given by the equation :
Example 1: Determine the amount of iron in 2.5 g Fe2O3.
Solution: To determine how many grams of iron are contained in 2.5 g Fe2O3, one could
simply multiply the weight given by the fraction of it that is iron.
Gram of Fe = 2.50 x 2 Fe/Fe2O3
 Thus, the ratio 2Fe/Fe2O3 is called a gravimetric factor.
Percent X =
mass of X
mass of sample
x100% , where X represents the analyte
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Gravimetric factor:It is a ratio of atomic or molecular weights that will convert
the weight of some pure substance into the weight of some other substance that is
stoichiometrically related to it.

15. Example: The calcium in a 200.0 ml sample of a natural water was determined by
precipitating the cation as CaC2O4. The precipitate was filtered, washed, and
ignited in a crucible with an empty mass of 26.6g. The mass of the crucible plus
CaO (56.078 g/mol) was 26.7g. Calculate the concentration of Ca(40.078 g/mol)
in water in units of gram per 100 ml of the water.
Solution
 The mass of CaO = 26.7-26.6 = 0.1 g
The number of moles Ca in the sample is equal to the number of moles of CaO or
Amnt of Ca =0.1gCaO x 1molCaO/ 56.07gCaO x 1mol Ca/ molCaO
= 1.78 x 10-3 mol Ca
Conc.Ca = 1.78x10-3 mol Ca x 40.078g Ca/mol x 100ml
200ml sample
= 0.0714 g/100ml 21