Precipitation titration – Pharmaceutical Analysis 1 B. Pharma 1st semester

Precipitation titration


• Introduction

• Requirements for a precipitation titrations

• Mohr’s method

• Volhard’s method

• Modified Volhard’s method

• Modified Fajans method

• Application of precipitation titration


At the end of this
lecture, the student will be able to

• Define precipitation titration

• Classify precipitation titration

• Explain Mohr’s method of precipitation

• Explain Volhard’s method

• Explain Modified Volhard’s method

• Explain Modified Fajans method

• Outline the application of precipitation titration


Definition of
Precipitation Titration:

• Type of volumetric analysis that depends on the formation
of a precipitate

• Precipitating agent used in precipitation titration is
Silver Nitrate (Argentometric titration).

• Application:

-Used for determination of halides (Cl-, Br- & I-),

-Used for determination of thiocyanide and cyanide, –Some
compounds that form insoluble products (ppt) with Silver Nitrate

for a Precipitation Titrations

• The precipitate must be practically insoluble

• The precipitate should be rapid

• The reaction should go rapidly to completion and should be

• Only one reaction should take place

• The titration results should not be hampered by adsorption
(Co precipitate) effects

• The compounds must be ionic in nature

• There should be no change or any harm in detecting the end

Reason for
Less Popularity of This Titration

• Lack of suitable indicators to detect the end point in
precipitation titration

• The composition of the precipitate is not always known

• The cost silver nitrate is too high

Methods of
Determining the Equivalence Point in Precipitating Titration

• Mohr’s method – Formation of colored precipitate

• Volhard’s method- Formation of Colored complex ions
(Colored solution)

• Modified Volhard’s method

• Fajan or Adsorption indicator method- Depends on the
formation of Colored adsorption compound

Method (1856)

• Sample: Cl- ,
Br- (Not for I- or SCN-).

• Type of titration:
Direct titration.

• Standard Solution:
Standard silver nitrate solution.

• pH: Neutral or
slightly alkaline. (pH ≈ 6.5 -9).

• Indicator: 1 ml
of 5% Potassium Chromate (K2CrO4) soln.

• Color at end point:
Brick red ppt due to formation of Silver chromate (Ag2CrO4)

• Titration reaction:

+       Ag +         
AgCl ↓  Ksp = 1.8 x 10–10

sample            titrant        white ppt

• Endpoint reaction

2 Ag++     
CrO4– –          Ag2CrO4 ↓ Ksp = 1.2 x 10–12

titrant   indicator        brick red ppt

Limitations of Mohr’s

1. Effect of pH:
Mohr’s method should be done in neutral or slightly alkaline medium (pH ≈ 6.5
-9) because:

-At pH > 9: Ag+
will be precipitated as AgOH↓ (brown to black ppt):
leading to Consumption
of the titrant & Masking of the end point color.

Ag+ + OH → AgOH ↓ (brown to black

-While at pH <
6.5: The chromate ion (CrO4 –) changes into acid chromate (HCrO4 -) then to
dichromate (Cr2O7 –).
Both HCrO4 – & Cr2O7 — form soluble salts with
Ag+ & so no colored ppt will be formed at the end point.

2 CrO4 + 2 H+                      2 HCrO4                        Cr2O7 + H2O

chromate                              acid chromate             dichromate

2. Ag2CrO4 is more soluble than AgCl so that no Ag2CrO4 will
be precipitated until all Cl- ions have been precipitated as AgCl provided that
CrO4– concentration should be adjusted to make Ag2CrO4 formed only at the end
point and so to prevent error in the end point, 1 ml of 5% K2CrO4 solution is
suitable based on solubility product.

• High concentration
of K2CrO4
gives too
soon (early) E.P.
because Ag2CrO4 will be rapidly precipitated before E.P.

• Low concentration
gives too
late E.P
. because CrO4—will be insufficient and so a large amount of Ag+ (titrant)
will be needed to precipitate Ag2CrO4 and so the E.P. comes too late.

3. I& SCNcannot be determined
by Mohr’s method because the formed silver chloride & silver thiocyanate strongly
adsorb CrO4 on their surfaces and so the ppt formed at the end
point will be an adsorption compound which is less colored and so less sharp
endpoint will be obtained

Modification of Mohr’s

1. Silver chromate – soluble in acid, therefore the solution
to be titrated –neutral. If acidic, first neutralized –pure powdered calcium
carbonate and then indicator is added

2. Solution – alkaline – form ppt of silver hydroxide, first
neutralized by adding dilute nitric acid. Then the excess acid is neutralized –
pure powdered calcium carbonate

chloride estimation


Burette and stand

10 and 20 mL pipettes

100 mL volumetric flask

250 mL conical flasks

10 mL and 100 mL measuring cylinders

Solutions Needed:

Silver nitrate
(0.1 mol L−1) If possible, dry 5 g of AgNO3 for 2 hours at 100°C
and allow to cool. Accurately weigh about 4.25 g of solid AgNO3 and dissolve it
in 250 mL of distilled water in a conical flask. Store the solution in a brown

Potassium chromate
indicator solution:
(approximately 0.25 molL-1) Dissolve 1 g of K2CrO4 in
20 mL distilled


1. Dilute seawater by pipetting a 20 mL sample into a 100 mL
volumetric flask and making it up to the mark with distilled water.

2. Pipette a 10 mL aliquot of diluted seawater into a conical
flask and add about 50 mL distilled water and 1 mL of chromate indicator.

3. Titrate the sample with 0.1 mol L−1 silver nitrate solution.
Although the silver chloride that forms is a white precipitate, the chromate
indicator initially gives the cloudy solution a faint lemon-yellow colour

4. Repeat the titration with further aliquots of diluted seawater
until concordant results (titres agreeing within 0.1 mL) are obtained.

Result Calculation

1.  Determine the
average volume of silver nitrate used from your concordant titres.

2.  Calculate the
moles of silver nitrate reacting.

3.  Use the following
reaction equation to determine the moles of chloride ions reacting.

Ag+(aq) + Cl(aq)

4. Calculate the concentration of chloride ions in the
diluted seawater.

5.  Calculate the
concentration of chloride ions in the original undiluted seawater.

6.  Calculate the
concentration of sodium chloride in the seawater in molL−1, gL−1 and g/100 mL

Method (1874)

• Sample: Cl- , Br-  
I- or SCN-.

• Type of titration: Back titration.

• Standard Solution: Standard ammonium thiocyanate solution.

• pH: Acidic

• Indicator: ferric alum.

• Color at end point: Brick red ppt due to formation of ferrous
thiocyanate complex

Steps involved in
Volhard’s method

Step 1:   Adding
excess Ag+ into sample

Ag+ + Cl– → AgCl↓ + left Ag+

Step 2:   Removing
AgCl ↓ by filtration/washing

Step 3:   Adding Fe3+
into filtrate (i.e., the left Ag+)

Step 4:   Titrating
the left Ag+ by SCN–:

Ag+ + SCN–  → AgSCN ↓

Step 5:   End point
determination by red colored Fe(SCN)2+ complex. (when all Ag+ has been consumed,
SCN– reacts with Fe3+)

SCN– + Fe3+   → Fe(SCN)2+(aq)

Disadvantages of
Volhard’s method

• During the filtration process, the sample may be lost

• Filtration process has be performed in order to prevent
the interaction of precipitated silver chloride with thiocyanate ions.

• In case of iodide estimation, iron (III) indicator should
not be added until excess of silver is present, since the dissolved iodide
reacts with iron (III) ions

2 Fe3+ +  2I-   ó  2Fe2+ + I2

Volhard’s method

In order to prevent the reaction between precipitated silver
chloride and thiocyanate and to avoid the filtration process:

– A coating reagent is added in the reaction such as
nitrobenzene, dibutyl phthalate, potassium nitrate Such method is known as
modified Volhard’s method


• Fajan’s method is the most recent and most accurate silverhalide

• It is based on the adsorption of adsorption indicators on
the surface of the positively charged silver chloride particles formed in the
precipitation titration when Ag+ ion is in excess

• Sample:
Generally, it can be used for Cl-, Br-, I- & SCN-.

[Fluorescein ind. is used for Cl-, Br-, I- & SCN- but
Eosin ind. is used for Br-, I-& SCN-(Not for Cl-)]

• Type of titration:
direct titration.

• Standard Solution:
Standard silver nitrate solution.

• Indicator:
Adsorption indicators like eosin, fluoroscein…

• Color at end point:
Depending upon the use of indicator


• Based on the properties of colloids adsorption indicators act

• An adsorption indicator is an organic compound (dye) that
tends to be adsorbed onto the surface of the solid in a precipitation

• 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).


Acid dyes: Fluorescein, eosin as sodium salts

Basic dyes: Rhodamine 6G and Phenosafranine as halogen salts

Theory of Adsorption

• The action of adsorption indicators depends on the electrical
phenomena associated with the formation of colloidal precipitates

• Colloidal ppt tends to form an electrical double layer
around its particles when it is in contact with a solution. This electrical
double layer consists of:

a) Primary adsorption layer: in this layer, the ppt tends to
adsorb one of its own ions (the ion present in excess).

b) Secondary adsorption layer: a layer of opposite charge
(counter-ion layer) containing oppositely charged ions from the surrounding

of sodium chloride by using adsorption indicator method

• It depends on the titration of NaCl sample with standard AgNO3
using fluorescein indicator.

• At the endpoint, the formed AgCl↓ ppt becomes pink.

• Titration reaction:

NaCl + AgNO3 fluorescein ind AgCl ↓ + NaNO3


and Standardization of Silver Nitrate Solution


• Silver Nitrate, 0.1 M: Dissolve 17.0 g in sufficient water
to produce 1000 ml.


• Weigh accurately about 0.1 g of sodium chloride,
previously dried at 110° for 2 hours and dissolve in 5 ml of water.

• Add 5 ml of acetic acid, 50 ml of methanol and 0.15 ml of
eosin solution. Stir, preferably with magnetic stirrer, and titrate with the
silver nitrate solution.

I.P factor: 1 ml of 0.1 M silver nitrate is equivalent to
0.005844 g of NaCl.

and Standardization of Ammonium Thiocyanate


• Ammonium Thiocyanate, 0.1 M: Dissolve 7.612 g of ammonium
thiocyanate in sufficient water to produce 1000 ml.


• Pipette 30.0 ml of 0.1 M silver nitrate into a
glass-stoppered flask

• Dilute with 50 ml of water, add 2 ml of nitric acid and 2
ml of ferric ammonium sulphate solution and

• Titrate with the ammonium thiocyanate solution to the first
appearance of a red-brown colour.

I.P factor: 1 ml of 0.1 M silver nitrate is equivalent
0.007612 g of NH4SCN.

Assay of
Sodium Chloride Based on Modified Volhard’s Method

Step 1:   Adding
excess Silver nitrate solution into sample in acidic media using nitric acid,
ferric alum and dibutyl phthalate:

AgNO3 + NaCl → AgCl↓ + NaNO3 
+ AgNO3

Step 2:   Titrate the
left out/unreacted silver nitrate with ammonium thiocyanate :

AgNO3 + NH4 SCN → AgSCN ↓+ NH4NO3

Step 3:   End point
determination by red colored Fe(SCN)2+ complex. (when all Ag+ has been consumed,
SCN– reacts with Fe3+)

SCN– + Fe3+  

procedure as per I.P 2007


• Weigh accurately about 0.1 g and dissolve in 50 ml of water
in a glass-stoppered flask.

  Add 50.0 ml of 0.1
M silver nitrate, 5 ml of 2 M nitric acid and 2 ml of dibutyl phthalate, shake
well and

• Titrate with 0.1 M ammonium thiocyanate using 2 ml of
ferric ammonium sulphate solution as indicator, until the colour becomes
reddish yellow.

I.P.Factor:  1 ml of 0.1 M silver nitrate is equivalent to
0.005844 g of NaCl.


• Type of volumetric analysis that depends on the formation
of a precipitate

• Mainly used for estimating halides with the help of silver
nitrate or ammonium thiocyanate

• Different methods: Mohr’s, Volhards, Fajans

• Different methods: Volhards, Fajans

• Volhards method are carried out in   acidic media with back titration process

• Fajans method is based on adsorption indicator method

• Application mainly used to estimate sodium chloride and
potassium chloride in electrolytes

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