10/18/2025
Unit-iii
Precipitation Reactions:
Precipitation reaction is the formation of a solid in a solution solid formed is called the
precipitate
A precipitation reaction occurs when water solutions of two different ionic compounds are
mixed and an insoluble solid separates out of solution. A- + B+ = AB (Precipitate)
The precipitate is itself ionic; the cation comes from one solution and the anion from another.
, 10/18/2025
Precipitation Titrations:
A reaction in which the analyte and titrant form an insoluble precipitate can also form
the basis for A titration. We call this type of titration A precipitation titration.
One of the earliest precipitation titrations, developed at the end of the eighteenth
century, was for the analysis of K2CO3 and K2SO4 in potash. Calcium nitrate, Ca(NO3)2,
was used as a titrant, forming a precipitate of CaCO3 and CaSO4.
The endpoint was signaled by noting when the addition of titrant ceased to generate
additional precipitate.
The importance of precipitation titrimetry as an analytical method reached its zenith
in the nineteenth century when several methods were developed for determining Ag+
and halide ions.
Precipitation Titrations:
Most of metallic halides are titrated by precipitation method. Silver Nitrate is most
important precipitating reagent.
Here separable solid compounds form during the course of the reaction. The titrating
agent, therefore, has to react with the analyte to produce a compound of limited
solubility.
Titrations involve Ag+ as either an analyte or titrant. Those titrations in which Ag+ is
the titrant are called argentometric titrations, from the Latin name for silver,
argentum.
The major precipitation reaction used is that of silver with a range of anions. These
anions include:
Halides (Cl- , Br- , I- )
Pseudohalides (S 2- , HS- , CN- , SCN- )
The reaction rates for the silver salt precipitation is rapid.
The reaction ratio is 1:1 and silver salts formed are generally quite insoluble.
, 10/18/2025
Precipitation Titrations: Argentometric methods can be divided into two types
1 - Direct determinations , in which a standard solution of silver nitrate is added to the
sample solution until the end point is reached .
2 - Indirect determinations , in which an excess of silver nitrate is added to the sample
solution and the excess silver nitrate is back titrated with another standard solution ,
usually ammonium or potassium thiocyanate .
Direct titrations are to be preferred but it is not possible to apply the method in all
cases .
•If the solubility of the precipitate formed during the titration is too great , the end
point cannot be detected . In this case , it is better to add an excess of the reagent
thereby simplify the solubility of the precipitate , and determine the excess by
another method (Indirect Method).
Precipitation Titrations:
The concentrations of A and B are varying through the titration process. When titrating B-
(analyte) with A+ (titrant).
a) in the beginning of titration, concentration of A + is zero and concentration of B- is
calculated from its salt
b) After addition of few milliliters of A+, B- will react with A+ forming AB precipitate,
concentrations of A+ will be calculated according to Ksp . B- will be excess.
c) At the end point, concentrations A+, B- will be calculated from Ksp .
d) After the end point A+ is the excess and B- will be calculated from Ksp .
Titration curve in the precipitation titration is a plot of
pA and/or pB versus
volume of titrant (A)
Unit-iii
Precipitation Reactions:
Precipitation reaction is the formation of a solid in a solution solid formed is called the
precipitate
A precipitation reaction occurs when water solutions of two different ionic compounds are
mixed and an insoluble solid separates out of solution. A- + B+ = AB (Precipitate)
The precipitate is itself ionic; the cation comes from one solution and the anion from another.
, 10/18/2025
Precipitation Titrations:
A reaction in which the analyte and titrant form an insoluble precipitate can also form
the basis for A titration. We call this type of titration A precipitation titration.
One of the earliest precipitation titrations, developed at the end of the eighteenth
century, was for the analysis of K2CO3 and K2SO4 in potash. Calcium nitrate, Ca(NO3)2,
was used as a titrant, forming a precipitate of CaCO3 and CaSO4.
The endpoint was signaled by noting when the addition of titrant ceased to generate
additional precipitate.
The importance of precipitation titrimetry as an analytical method reached its zenith
in the nineteenth century when several methods were developed for determining Ag+
and halide ions.
Precipitation Titrations:
Most of metallic halides are titrated by precipitation method. Silver Nitrate is most
important precipitating reagent.
Here separable solid compounds form during the course of the reaction. The titrating
agent, therefore, has to react with the analyte to produce a compound of limited
solubility.
Titrations involve Ag+ as either an analyte or titrant. Those titrations in which Ag+ is
the titrant are called argentometric titrations, from the Latin name for silver,
argentum.
The major precipitation reaction used is that of silver with a range of anions. These
anions include:
Halides (Cl- , Br- , I- )
Pseudohalides (S 2- , HS- , CN- , SCN- )
The reaction rates for the silver salt precipitation is rapid.
The reaction ratio is 1:1 and silver salts formed are generally quite insoluble.
, 10/18/2025
Precipitation Titrations: Argentometric methods can be divided into two types
1 - Direct determinations , in which a standard solution of silver nitrate is added to the
sample solution until the end point is reached .
2 - Indirect determinations , in which an excess of silver nitrate is added to the sample
solution and the excess silver nitrate is back titrated with another standard solution ,
usually ammonium or potassium thiocyanate .
Direct titrations are to be preferred but it is not possible to apply the method in all
cases .
•If the solubility of the precipitate formed during the titration is too great , the end
point cannot be detected . In this case , it is better to add an excess of the reagent
thereby simplify the solubility of the precipitate , and determine the excess by
another method (Indirect Method).
Precipitation Titrations:
The concentrations of A and B are varying through the titration process. When titrating B-
(analyte) with A+ (titrant).
a) in the beginning of titration, concentration of A + is zero and concentration of B- is
calculated from its salt
b) After addition of few milliliters of A+, B- will react with A+ forming AB precipitate,
concentrations of A+ will be calculated according to Ksp . B- will be excess.
c) At the end point, concentrations A+, B- will be calculated from Ksp .
d) After the end point A+ is the excess and B- will be calculated from Ksp .
Titration curve in the precipitation titration is a plot of
pA and/or pB versus
volume of titrant (A)
