The document outlines the principles and methods of precipitation titration, including Mohr's method, Fajan's method, and Volhard's method, commonly used in pharmaceutical chemistry to determine ion concentrations. It describes how precipitation reactions occur based on the solubility of the analyte and titrant, and details specific procedures and indicators used in each method. The document serves as an educational resource for 1st-year pharmacy students presented by Ms. Shweta Singh.

1. PHARMACEUTICAL CHEMISTRY
CHAPTER-2
TOPIC: PRECIPITATION TITRATION
D. PHARM 1ST YEAR
Presented By:
MS. SHWETA SINGH
(Assistant Professor)
RBMI, Bareilly

2. CONTENTS
Principle of Precipitation titration
Mohr’s Method
Fajan’s Method
Volhard Method

3. PRECIPITATION TITRATION
 Precipitation is the formation of a solid in a solution and the solid formed is
called a precipitate.
 This type of volumetric method which involves the formation of precipitate
during the reaction is known as precipitation titration.
 The precipitation titration is also known as argentometric titration when silver
nitrate (AgNO3) is commonly used as a titrant.
 Eg: KCl + AgNO3 AgCl + KNO3
(Potassium chloride) (Silver nitrate) (Silver chloride) (Potassium nitrate)
(Analyte) (Titrant) (Precipitate)

4. PRINCIPLE OF PRECIPITATION TITRATION
 The titration in which the precipitation reaction takes place is called precipitation titration.
 Eg: Titration of AgNO3 with halide ions (Cl-/Br-/F-/ I-).
 Precipitation titration is based on the solubility of products.
 If the solubility of titrant and analyte is high, precipitate will not form.
 If the solubility of titrant and analyte is low, precipitate will form easily.
 Eg: NaCl + AgNO3 AgCl + NaNO3
(Sodium chloride) (Silver nitrate) (Silver chloride) (Sodium nitrate)
(Analyte) (Titrant) (Precipitate)
KBr + AgNO3 AgCl + KNO3
(Potassium Bromide) (Silver nitrate) (Silver chloride) (Potassium nitrate)
(Analyte) (Titrant) (Precipitate)

5. According to the end-point detection method, three main
procedures are widely used depending on the type of
application.
METHOD OF PRECIPITATION TITRATION

6. MOHR’S METHOD
 Karl Friedrich Mohr was the scientist who introduced this method.
 Titrant - 0.1 N AgNO3 (Silver nitrate)
 Analyte – 0.1 N NaCl (Sodium chloride)
 Indicator – 5% K2CrO4 (Potassium chromate)
Principle:
NaCl + AgNO3 AgCl + NaNO3 Ksp[AgCl]= 1.2X 10-10
K2CrO4 + AgNO3 Ag2CrO4 + 2KNO3 Ksp[AgCrO4]= 1.7X 10-12
 Chlorides and bromides are used as analytes in this method.
 The pH range in Mohr’s method is 6.5-7.5

7. Mohr titration is defined as the type of titration that helps in
determining the Chloride ion (Cl-) concentration when it is titrated
against Silver nitrate (AgNO3).
When we add Silver nitrate (AgNO3) to a chloride solution it forms
Silver Chloride (AgCl) precipitates.
• When all the Chloride ions are precipitated, the endpoint is reached.
Silver ions Ag+ that are left in the solution will react with the Chromate
ion (CrO4)2- of the indicator Potassium chromate K2CrO4 to form a red-
brown precipitate of Silver chromate Ag2CrO4.
2Ag+(aq) + (CrO4)2- (aq) → Ag2CrO4 (s)
• So by this method Chloride ion concentration can be determined in
any source like seawater, sewage, etc.

9. FAJAN’S METHODS
 K. Fajan introduced this method.
 In this titration adsorption indicators are used which are mainly dyes.
 At the endpoint the indicator is adsorbed by the precipitate, and during the process of
adsorption, a change occurs in the indicator which leads to the formation of substances of
different colour.
 Titrant - 0.1 N AgNO3 (Silver nitrate)
 Analyte – 0.1 N NaCl/NaBr/NaI (Sodium chloride, Sodium bromide, sodium
iodide)
 Indicator – Fluorescein, Eosin and Rhodamine dyes
 Fajan’s method is used to determine – Chlorides, Bromides and Iodides.

10. • The property of a colloidal precipitate is to adsorb its own ions which are in excess.
• When NaCl solution is titrated with AgNO3, the AgCl precipitate will adsorb chloride ions
which are initially in excess.
• Thus, the primary layer will be formed by chloride (Cl-) ions, which will attract a secondary
layer of oppositely charged Na+.
• At the endpoint, Ag+ ions are in excess, and hence AgCl adsorb Ag+ ions as primary adsorb
layer and NO3
- as secondary adsorb layer.
• If fluorescein indicator is also present in the solution then the negatively charged fluorescein
ions would be adsorbed as a secondary layer and this adsorption occurs along with a change
to a pink-colored complex of Ag+ and fluorescein ions.

11. VOLHARD METHOD
• Volhard Method is a back titration/ indirect titration and a precipitation
titration that depends on the formation of colored complex ions (colored
solution) at the endpoint.
• The titration is performed only in an acidic medium using nitric acid (HNO3).
• Fe3+ ion from ferric alum (Ferric ammonium sulfate) NH4Fe(SO4)2. 12H2O
acts as an indicator in this practical.
• Brick red color is observed at the endpoint.
 Titrant- AgNO3, Potassium thiocyanate (KSCN)/ Ammonium thiocyanate
(NH4SCN)
 Analyte – 0.1 N NaCl/NaBr/NaI (Sodium chloride, Sodium bromide, sodium
iodide)
 Indicator – (Ferric ammonium sulfate) NH4Fe(SO4)2. 12H2O /Iron alum.

12. PRINCIPLE
METHODOLOGY
• Procedure for determination of chloride ion concentration by titration (Volhard’s Method)
• Fill the burette with 0.1 M potassium thiocyanate solution and adjust the zero.
• Measure 10 ml of the sample and pour it into a conical flask.
• Add 1 ml of ferric ammonium sulfate solution as an indicator.

13. Using the potassium thiocyanate (KSCN) solution, titrate the unreacted silver ions.
• Because of the ferric thiocyanate complex, the endpoint is the appearance of a dark red color.
• Repeat the titration three times to get an accurate result.
• Calculate the moles of potassium thiocyanate used.