Precipitimetry
Dr M. AFROZ BAKHT
Precipitimetry
It is a type of quantitative analysis in which the compound
to be analyzed is able to form insoluble product
precipitate (ppt.) when treated with certain reagents
known as precipitating agents.
The precipitate must be sparingly soluble and
formed quickly.
-To understand precipitation reactions the following topics
should be studied.
Solubility product (Ksp).
Precipitate formation.
Fractional Precipitation.
Common ion effect.
Solubility Product
Solubility product(sp) is the product of the molar
concentrations of the ions, each concentration being raised
to a power equal to the number of ions derived from the
ionization of one molecule of the original substance.
-Examples: AgCl =Ag+ + ClSolubility product (Ksp) AgCl = [Ag+] [Cl-]
Ag2CrO4 = 2Ag+
+ CrO4-2
(Ksp) Ag2CrO4 = [Ag+] 2 [CrO4-2]
-Calculate the Ksp of AgCl if you know that the solubility of it
in water
is 0.0015 g/l and its M.wt. = 143.5 g/mol
Molar concentration = 0.0015 / 143.5
= 1.05 x 10-5
(Ksp) AgCl = [Ag+] [Cl-]
= (1.05 x 10-5 ) (1.05 x 10-5 )
= 1.1 x 10-10
Solubility Product
• Calculate the solubility of Ag2CrO4. (Ksp = 2.4 x
10-12, M.wt. 331.8).
•
- Ag2CrO4
2Ag+ + CrO42• if A is the molar solubility
•
A
2A + A
•
Ksp = [2A]2 [A]
•
Ksp = 2[A]2 [A] = 2.4 x 10-12
• = 4A3
= 2.4 x 10-12
2.4x10
-5 mol/L
•
A=
=
8.4
x
10
4
• The solubility = 8.4 x 10-5 x 331.8 = 0.0279 gm/L
12
3
Solubility Product
• Calculate the Ksp of pb3(PO4)2 if you know
that the solubility = 0.00014 g/l (M.wt. =
811.7 g/mol)
• Calculate the solubility of AgI (Ksp = 0.9x10-16,
M.wt.=235 g/mol).
Precipitate Formation
The ppt. is formed when the product of ions concentrations
exceed the solubility product at given temperature.
-Dissolving the formed ppt.:
The ppt. is dissolved by making the product of ions
concentration below the solubility product of the precipitate.
Fractional Precipitation
-When a precipitating agent is added slowly to a solution
containing two ions capable of being precipitated by the
agent, the substance with the lower Ksp will precipitate first
followed by the one having higher Ksp.
Example:
Solution of Ba+2 and Sr+2 ions, upon the addition of H2SO4,
BaSO4 of (lower Ksp) will precipitate first, then SrSO4 (higher
ksp)
Common Ion Effect
- Common ion is an ion similar to one of the ions of weak electrolyte present in
solution.
- Common ion has a pronounced effect on the weak electrolytes.
A -Effect on ionization:
Ionization of weak electrolyte may be strongly reduced by the presence of a
common ion.
-Example:
CH3COOH =
H+ + CH3COOHCl
→ H+ + ClIonization of CH3COOH decreases by adding H+ or CH3COO- and the equilibrium
shifted to left.
B -Effect on Precipitation:
It is clear that the addition of common ion will decrease the solubility of the
salt and increase its precipitation.
e.g. Addition of Ag+ or Cl- to a saturated solution of AgCl
AgCl =
Ag+ + Clppt
ions (soluble)
-The presence of Ag+ or Cl- in the solution as common ion will shift the reaction
to left and accordingly the concentration of ionized part of AgCl decreased.
Detection of End Point in Precipitimetry
I. No indicator method
A- Disappearance of ppt.
Principle :The ppt. formed during titration is redissolved by the titrant.
The end point is detected by disappearance of turbidity.
Example:
Det. of Ag+ by standard CNThe first drop of addition of st. CN- titrant  AgCN is formed.
Precipitation is continued until all Ag+ is ppted. as AgCN.
Adding more CN- will form the soluble argento cyanide complex.
Ag+ + CN-  AgCN
AgCN + CN=
[Ag(CN)2]The process is continued until all AgCN is dissolved as soluble
complex.
The end point is the disappearance of last trace of turbidity.
Detection of End Point in Precipitimetry
B- Appearance of ppt. (Liebeg’s method)
Principle :This method depends on formation of complex. The end
point is detected by the appearance of the first turbidity.
Example:
Det. of CN- by st. Ag+
1) st. Ag+ titrant is added to CN- soln.  AgCN ppt.
2) The ppt. then redissolve as soluble complex.
3) Titration is continued till all CN- form soluble
argentocyanide.
AgCN + KCN = K[Ag(CN)2]
4) The first xss. of Ag+ added will form silver argentocyanide.
K[Ag(CN)2] + Ag+ = Ag [Ag(CN)2] + K+
Insoluble or 2 AgCN
Liebeg's Method
KCN + AgNO3  AgCN + KNO3
AgCN + KCN =
K[Ag(CN)2]
-------------------------------------------------------2 KCN + AgNO3 = K[Ag(CN)2]+ KNO3
K[Ag(CN)2 ]+ AgNO3  2 AgCN + KNO3
5) The end point is detected by the appearance of the first
turbidity.
To make end point more clear we add I- which forms with
precipitating agent more insoluble salt. This modification is
known as Denige’s Modification.
Add I- in ammoniacal medium to detect the end point as
yellow turbidity of AgI because AgI is insoluble in ammonia
and has lower Ksp than AgCN i.e. AgI is ppted. at the end
point in place of AgCN
Detection of End Point in Precipitimetry
II –Using indicator methods:
-Formation of colored ppt.: e.g. Mohr’s method
-Formation of colored soln.: e.g. Volhard's method
-Using Adsorption Indicators.: e.g. Fajan’s method
-Mohr’s method
-It is a direct method used for determination of soluble chloride
and bromide salts by titration with standard solution of AgNO3 ,
in neutral or slightly alkaline medium at pH 7-9, using K2CrO4
solution as indicator.
-End point
After complete precipitation of halide with AgNO3, the first excess
drop of AgNo3 will precipitate CrO4-as brick red ppt. of Ag2CrO4
NaCl + AgNO3
→
AgCl (ppt.) +NaNO3
K2CrO4 + 2AgNO3
→
Ag2CrO4
+NaNO3
Mohr's Method
The medium must be neutral or slightly alkaline (pH 7-9).
• In acidic medium: formation of soluble acid chromate
and dichromate
• 2 CrO42- + 2 H+  2 HCrO4- = Cr2O72- + H2O
• In strongly alkaline medium: silver ppted. as its oxide
• 2 Ag+ + 2 OH-  2 AgOH = Ag2O + H2O
• It is not used for I- & CNS- which adsorb CrO42- (no
Ag2CrO4 is formed at E.p.), and yellow colour of AgI
render the end point difficult.
• It is not used for det. of halides in presence of CO32- ,
PO43- & oxalate which are ppted. in neutral medium.
• Titration should be carried out at room temp. because
Ag2CrO4 is more soluble in hot water.
Detection of End Point in Precipitimetry
Volhard’s method
It is an indirect method for determination of halides in an
acidic medium (HNO3 ).
A known excess of standard AgNO3 solution is added to the
soluble halides, filter and acidify the filtrate with HNO3 .
Residual AgNO3 is back titrated with standard SCN- in presence
of (Fe+3) ferric alum as indicator.
End point is detected by the appearance of red color of
Fe(SCN)3 or [Fe(SCN)6]-3
NaCl
+ AgNO3
→
AgCl (ppt.)
+NaNO3
AgNO3 (xss) + KSCN
→
AgSCN (ppt.) + KNO3
• At end point : Fe+3 + 6 SCN- →
[Fe(SCN)6]-3 (red color)
• The method can be used for direct determination of Ag+ by
SCN- and SCN- by Ag+ in an acidic medium.
Volhard's Method
The titration must be carried out in acidic medium (HNO3)
• (i) the formed ppt. is insoluble in HNO3.
• (ii) prevent pptn. of Ag+ and Fe3+ as AgOH or Fe(OH)3 in
alkaline medium.
• Filtration is very necessary to prevent attacking the formed
AgCl ppt. by CNS- due to the Ksp of AgCNS (Ksp = 7x10-13) is
less than that of AgCl (Ksp = 1.2x10-10). Therefore after the
CNS- reacts with all Ag+, it will displace the Cl- from AgCl to
form the more insoluble AgCNS.
• AgCl + CNS  AgCNS + Cl• This problem can also be avoided by adding nitrobenzene
which surround the particles of ppt. by a thin film which
protect the ppt. from attacking by CNS• Volhard method was used for the det. of Ag+ by direct
titration with CNS- in the presence of HNO3 using ferric
alum as indicator.
Detection of End Point in Precipitimetry
Adsorption Indicators
Fajan in 1889 discovered that, certain dyes change
their color when adsorbed to precipitate. Such
compounds are called adsorption indicators.
-They are either weak acids e.g. Fluorescein and
Eosin or weak bases e.g. Rhodamine 6G
-Examples of Adsorption Indicators
-Fluorescein: used for determination of Cl- and Br-,
color change form yellowish green to pink.
-Eosin: used for determination of Cl-, Br- and I-,
color change form orange red to bluish red.
-Rhodamine: used for determination of Ag+, the
color change from orange red to violet.
Fajan's Method
It is a direct method used for determination of
soluble Cl-, Br- , I- and SCN- in neutral medium by
titration with standard solution of AgNo3 , in
presence of an adsorption indicator as
fluorescein.
-The method is used for determination of Brand I- in an acidic medium, using eosin as
indicator.
-The method is used for determination of Ag+ by
titration with standard solution of Br- in an acidic
medium using Rhodamine as indicator