PRECIPITATION
TITRATION
NFNF2813
Assoc. Prof. Dr. Khairana
Husain
INTRODUCTION
Quantitative Classical Chemical Analysis
Acid-base
Gravimetry
Titrations
Precipitation
Complexometric
Argentometric
Mohr
Kolthoff
Mercurimetric
Volhard
Redox
Fajans
INTRODUCTION
Precipitation reaction
▪Results in the formation of an
insoluble product or precipitate
▪Usually involve 2 ionic
compounds react to produce a
precipitate
EXAMPLE:
Pb(NO3)2 (aq) + 2NaI(aq)
PbI2(s) + 2NaNO3(aq)
These reactions occur when the
solute-solute attractions between
the ions in the precipitate are
stronger than the solvent-solute
attractions.
Precipitate:
▪An insoluble solid
compound formed during a
chemical reaction in solution
 An insoluble substance
which falls out of the solution
Solubility
▪The maximum amount
of solute that will
dissolve in a given
quantity of solvent at
specific temperature
PRECIPITATION EQUILIBRIA
Consider the general reaction:
AgCl (s)
AgCl (aq)
Ag+ +
Cl-
Ksp = [Ag+][Cl-]
Ksp : solubility product constant , or
solubility product
[ ] : molar concentration of ion product
PRECIPITATION: WILL IT OCCUR?
AgCl (s)

Ag+ +
Cl-
To determine whether a precipitate will
form→ compare Q and Ksp
Q →ion product
(original concentrations are used)
Q= [Ag+][Cl-]
PRECIPITATION: WILL IT OCCUR?
If : Q > Ksp : Supersaturated solution;
AgCl will precipitate out
Q < Ksp : Unsaturated solution
Q = Ksp : saturated solution
Q= [Ag+][Cl-]
Example
Write the solubility product
expressions :
PbI2(s)
(b) Pb3(AsO4)2 (s)
(a)
EXAMPLE
What are the molar concentration of
Ag+ and CrO42- in a saturated
solution of Ag2CrO4 at 25 C?
(For Ag2CrO4, Ksp= 1.9 x 10-12).
EXAMPLE
The Ksp value for copper(II)
iodate, Cu(IO3)2, is 1.4 x 10-7 at
25 C. Calculate its solubility
at 25 C.
SOLUBILITY PRODUCT CONSTANT
Solubility product of compound
 product of the molar concentrations of the constituent ions,
each raised to the power of its stoichiometric coefficient in the
equilibrium equation.
FACTORS INFLUENCING THE
SOLUBILITY
1.
2.
3.
4.
5.
complexation
acidity
common ion
temperature
Solvent properties
PRECIPITATION TITRATION
(PRECIPITATOMETRIC)
▪ Titrations between analytes and reagents
resulting in the formation of a precipitate.
➢ Or which is based on reactions that yield ionic
compounds of limited solubility
▪ It is one of the oldest analytical techniques.
▪ The most useful of these precipitating reagents is
silver nitrate.
PRECIPITATION TITRATION
•
Used for the determination of many
anions including:
➢
➢
➢
➢
halides (most popular)
divalent anions
mercaptans
certain fatty acids
PRECIPITATION TITRATION
• Precipitation titrations are based
SOLUBILITY PRODUCT of the salt, KSP.
on
the
• The smaller KSP, the less soluble the silver salt
and the easier it is to determine the endpoint
• Endpoint determination is by coloured
indicators (usually back titrations) or turbidity
methods.
The most accurate is the VOLHARD METHOD.
TITRATION CURVES
A plot of the progress of a titration as a
function of the volume of titrant added.
OR
▪ a result of plotting
postequivalence point region
p[X] (p[X]=-log [X])
versus the concentration
equivalence point
of the titrant
▪
•
preequivalence-point region
Shape: Sigmoidal curve
PRECIPITATION TITRATION CURVES
INVOLVING SILVER ION
▪ The most common method of determining the
halide ion concentration of aqueous solution is
titration with a standard solution of silver
nitrate.
▪ To construct titration curve three type of
calculation are required, each of which
corresponds to a distinct stage in the reaction:
（1）pre-equivalence,
（2）equivalence, and
（3）post-equivalence.
PRECIPITATION TITRATION CURVES
INVOLVING SILVER ION
Titration curves for 100 mL 0.1 M chloride solution
versus 0.1 M standard solution of AgNO3
(argentometric methods).
Prepared by plotting pCl- (-log [Cl-]) against the
volume of AgNO3
NaCl + AgNO3 → AgCl 
Ag+
+
+
NaNO3
Cl-
x = I-, Br -, Cl-
(a) pre/Behind (Beginning) Equivalence
Point Of The Titration:
▪ We have 0.1 M Cl-, and pCl is 1
▪ Titration continues, part of the Cl- is
removed from solution by precipitation as
AgCl.
pCl is determined by the concentration of
the remaining Cl-.
The contribution of Cl- from dissocciation
of precipatate is negligible (coz very small)
AgCl
Ag+ + Cl-
(b) Equivalence Point
▪A saturated solution of AgCl will formed
Ksp = [Ag+][Cl-]
Notice that : [Ag+]= [Cl-]= x
Ksp = [Cl-] [Cl-]
= [Cl-]2
 Ksp = [Cl-]
(c) Post/Beyond The Equivalence Point
▪Excess Ag+
 the Cl- concentration is
determined from the concentration
of Ag+ and Ksp of AgCl.
FACTORS INVOLVE IN TITRATION CURVES
(1) Solubility
▪ The smaller Ksp, the larger the
break pX at the equivalence
point
(X: Cl-, Br -, I-)
Example Ksp value of :
AgCl
1 x 10-10
AgBr
4 x 10-13
AgI
1 x 10-16
(1) SOLUBILITY
▪ at equivalence point, [X-] is
smaller for the smaller Ksp
values.
pX is larger for a saturated
solution of the salt (AgX).
Cont.
[X ]
= Ksp
pX = -log
[X ]
Cont.
(1) SOLUBILITY
▪Beyond the equivalence
point, [X ] is smaller when
Ksp is smaller.
▪Overall effect: is a larger
pX at the equivalence
point
(2) CONCENTRATION
▪Concentration of analyte and titrant will
involve in magnitude of pAg at
equivalence point area
▪If concentration of analyte and titrant
that have been used more dilute,
resulting in smaller pAg at the
equivalence point
 To choice indicator : more difficult.
PRECIPITATOMETRIC TITRATION
Three types:
(1) Argentometric
• Involve with silver ion
(2) Mercurimetric
Hg2+ + 2Cl-
HgCl2 (as for other halides)
(3) Kolthoff
2 K4Fe(CN)6 + 3Zn2
K2Zn3[Fe(CN)6]2 + 6K+
ARGENTOMETRIC TITRATION
▪ most usefull method of precipitatometric
titration, as it caused of very low solubility
product of halide (or pseudohalide)
salts,i.e.
Ksp AgCl = 1.82 x 10-10
Ksp AgCN = 2.2 x10-16
Ksp AgCNS = 1.1 x 10-12
Ksp AgI = 8.3 x10-17
Ksp AgBr = 5.0 x10-18
ARGENTOMETRIC TITRATION
• Titrations involving silver or Argentum.
• The major precipitation reaction used is
that of silver with a range of anions
including:
➢ Halides (Cl-, Br-, I-)
➢ Pseudohalides (S2-, 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.
Indicators for Argentometric Titrations
❑Three types of end points are encountered in
titrations with silver nitrate
(1) chemical (coloured indicators)
（2）potentiometric (electrochemical techniques)
（3）amperometric
❑ The end point produced by a chemical indicator
consists of a color change or occasionally, The
requirements for an indicator are the color change
should :
(1）occur over a limited range in p- function of the
reagent or the analyte .
（2）take place within the step portion of the titration
curve for the analyte.
DETECTION OF THE END POINT:
INDICATORS
Three chemical techniques of end point determination:
(1) Mohr (indicator : chromic potassium)
(2) Volhard (indicator : ferric salt)
(3) Fajans (indicator : fluorosceince)
(1) MOHR METHOD
▪ Introduced by : K.F. Mohr (Germany/ 1865)
▪ useful for determining Cl- in neutral or unbuffered
solutions such as drinking water.
▪ This direct method uses potassium chromate (chromate
ions (CrO4 2-) ) as an indicator in the titration of (Cl-, Br-,
and CN- )ions (analyte) with a silver nitrate standard
solution (titrant).
▪ Eg:
Chloride is titrated with standard AgNO3 solution.
titration reaction:
Ag+ + ClAgCl(s)
white precipitate
Ksp = 1.8 x 10–10
(1)MOHR METHOD
▪ A soluble chromate salt(Na2CrO4) is added
as the indicator. This produces a yellow
color solution.
▪ When the precipitation of the chloride is
complete, the first excess of Ag+ reacts with
the indicator to precipitate red silver
chromate, Ag2CrO4(s)
Indicator reaction:
2 Ag+(aq) + CrO42–(aq) →Ag2CrO4(s)
excess
Yellow
red ppt
Ksp = 1.2 x 10–12
Conditions For Mohr’s Method
▪ The titrations are performed only in neutral
or slightly basic (pH 8) medium to prevent
silver hydroxide formation
(1) at pH > 10
(2) Or the formation of chromic acid at pH < 7.
• Reducing [CrO4 2- ] will delay the formation of the precipitate
although more Ag+ to be added to reach end point, which
cause error.
(2) VOLHARD TITRATION
▪ Introduced by : Jacob Volhard (Germany/ 1874)
▪ indirect (back titration) technique which is used if
reaction is too slow or if there is no appropriate
indicator selected for determining the equivalent
point.
▪ Used for determining anions that precipitate with
silver (Cl-, Br-, I-, SCN-) in acid medium.
▪ performed in HNO3 solution (acidic solution) to
prevent precipitation of iron (III) as the hydrated
oxide
(2) VOLHARD TITRATION
▪ Ag+ is titrated with standard SCN- solution (in
HNO3 solution) while Fe3+ used as indicator
Ag+(aq) + Cl–(aq) → AgCl(s) + excess Ag+
▪ the excess silver ion is determined by
back-titration with a standard potassium
thiocyanate solution
excess Ag+(aq) + SCN–(aq) → AgSCN(s)
(2) VOLHARD TITRATION
▪ The endpoint is detected by adding iron III (Fe3+)
as ferric ammonium sulfate which forms a soluble
red complex with the first excess of titrant.
Fe3+(aq) + SCN–(aq) → [FeSCN]2+(aq)
(red)
(3) FAJAN’S METHOD
▪Used adsorption indicator:
an organic compound that tends to
be adsorbed onto the surface of the
solid in a precipitation titration.
Ideally, the adsorption occurs near
the equivalence point and results not
only in a color change but also in a
transfer of color from the solution to
the solid (or the reverse).
(3) FAJAN’S METHOD
▪ uses an adsorption indicator: Fluorescein,
Dichlorofluorescein or Eosin.
▪ The indicator adsorb onto the surface of the silver salt
precipitate at the endpoint.
▪ The adsorption process causes a change in the color of the
indicator.
▪ Common Fajans adsorption indicators are weakly acidic
organic compounds and in alkaline conditions will exist as
the conjugate base.
▪ This form of the indicator which interacts with the
precipitate
(3) FAJAN’S METHOD
Indicators have different colour in
the free and absorbed form.
Changes from greenish yellow to pink
Sharper color transition, binds to tightly to Cl-
(3) FAJAN’S METHOD
▪ Fluorescein: a typical adsorption indicator
useful for titration of chloride ion with silver
nitrate.
▪ In aqueous solution, fluorescein partially
dissociates into hydronium ions and
negatively charged fluoresceinate ion that
are yellow-green.
▪ The fluoresceinate ion forms an intensely
red silver salt.
Example : Fluoresence in form of its fluorescenate (yellowish
green) anion react with Ag+ to form an intensive red
precipitate which is adsorbed to AgCl precipitate surface
caused by ionic pair interaction.
Fluoroscein
Cl- with Ag+
Dichlorofluorosce Cl- with Ag+
in
Eoscein
Br-, I-, SCN- with
Ag+
Bromophenol
Hg2+ with Clblue
Orthocrom T
Pb2+ with CrO42-
EXAMPLE
The titration of Cl- with Ag+ as titrant:
Na+ Cl- + Ag+
AgCl
+ Na+
(a) Before the equivalence point
▪ Cl- is in excess, and the primary adsorbed layer is AgCl
will be precipitate.
AgCl : Cl
1 layer
▪ The primary adsorbed (Cl-) will now attract the cations
(Na+) to form secondary layer of adsorbed ions
AgCl:Cl-::Na+

2 layer/counter ion layer
▪ Indicator gives yellow-green colour to solution
(b) Postequivalence point region
▪ Ag+ is in excess,  the surface of the
precipitate becomes positively charged,
with the 1 layer being Ag+
▪ This will now attract the indicator anion and
adsorb it in the 2 layer
(Indicator is adsorbed on surface of ppt)
AgCl: Ag+::In 
1 2
BRITISH PHARMACOPEIA
0.1M Silver Nitrate
Dissolve 17.0 g of silver nitrate R in water R and dilute to 1000.0 mL with
the same solvent.
Standardisation. Dissolve 50 mg of sodium chloride RV in water R, add
5 mL of dilute nitric acid R and dilute to 50 mL with water R. Titrate with
the silver nitrate solution, determining the end-point potentiometrically
(2.2.20).
1 mL of 0.1 M silver nitrate is equivalent to 5.844 mg of NaCl.
Storage: protected from light.
BRITISH PHARMACOPEIA
Silver Nitrate Solution, Ammoniacal
Dissolve 2.5 g of silver nitrate R in 80 mL
of water R and add dilute
ammonia R1 dropwise until the precipitate
has dissolved. Dilute to 100 mL with water R.
Prepare immediately before use
BRITISH PHARMACOPEIA
0.1M Ammonium Thiocyanate
Dissolve 7.612 g of ammonium thiocyanate R in water R and dilute to
1000.0 mL with the same solvent.
Standardisation. To 20.0 mL of 0.1 M silver nitrate add 25 mL
of water R, 2 mL of dilute nitric acid R and 2 mL of ferric ammonium
sulfate solution R2. Titrate with the ammonium thiocyanate solution
until a reddish-yellow colour is obtained.