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Precipitation Gravimetric
Method
Lismaryani Bertin
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Calculation
Heating
Washing
Filtration
Precipitation
Solubility
Prec Grav
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The precipitate must be
1. So insoluble
No loss in filtration
Quantity remaining in solution < 0,1 mg
2. Readily separated by filtration
Can be washed free of soluble impurities
The size is big enough
3. Convertible into a pure substance ofdefinite chemical
composition
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Solubility
Maximum amount of substance could
be dissolved (solute) in a volume of
solvent at a certain temperature
Mg/100 mL
Mole/Lg/L
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Factors
1. Temperature
2. pH
3. Complex formation
4. Common ion effect
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Solubility Product
Product of the total molecularconcentration of the ions is
constant atconstant temperature
Ksp = Constant of Solubility Product
AnBmn A+ +m B-
Ksp of AnBm = [A+]n [B-]m
Ksp is specific for every electrolyte
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Precipitation
In any concentration of electrolyte,
(the concentration unknown) product
of molecular concentration (hkk)
could be:
Hkk < Ksp
Hkk =Ksp
Hkk > Ksp
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What will happen?
Hkk < Ksp
The solution is unsaturated
Hkk = Ksp
The solution is saturated
Hkk > Ksp
The solution is supersaturated
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Saturated
Quantity of the solute is
exactly the same as thesolubility
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Unsaturated
Quantity of the solute is less
than the solubility, so thesolution can solve more
solute
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Supersaturated
Quantity of the solute is more than thesolubility (the condition
is abnormal),
the solution will produce precipitate ifthe condition change to
normal
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Exercise
1. Calculate the solubility product of
silver chloride. The solubility is
0,0015 gram/litre.
2. 100 mL of 0,01 M AgNO3 solution
mixed with 10 mL of 0,001 NaCl
solution. Find out if there is any
precipitate or no!
Ksp of AgCl is 10-10
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What is the correlation of v and the
size of precipitate?
Larger the v, less the size of
precipitate
To make the size bigger, we should
make v is less
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v will be least if :
(Q-S)/S is as small as possible
How?Make the Q very small (dilutesolution, add the precipitating
agentslowly, keep stirring)
Make the S very big (rise thetemperature)
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General Rules of
Conditions of Precipitation
1. Dilute solution ( solubility ofprecipitate, time for
filtration,minimize co precipitation)
2. Mixed slowly with constant stirringand small excess (keep the
degreeof super saturation small, largecrystal)
3. Hot solution (solubility, coagulation,velocity of
crystallization
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Addition
4. Digesting for crystalline precipitate
5. Washing the precipitate with
appropriate solution6. Re-precipitation if needed
