Precipitation Titration

1. Shree WaranaVibhag Shikshan Mandal's
Tatyasaheb Kore College of Pharmacy,
Warananagar
Created By
Miss. A. B. Patil
M. Pharm (Pharmaceutical Chemistry)
Assistant Professor 1
PRECIPITATION TITRATION

2. Contents
• Introduction
• Principle
• Detection of End Point
• Application of Precipitation Titration
• Limitations of Precipitation Titration

3. Introduction
• Precipitation titration definition is the titration in which the titrant
reacts with the analyte to form an insoluble precipitate as the end point.
• Precipitate- In precipitation titration the titrant reacts with analyte and
forms an insoluble substance called precipitate.
• Precipitating agent- The chemical responsible for causing a precipitate
to form is known as the precipitating agent. Ex- Silver Nitrate( AgNO3).
• Argentometric Titrations: Argentometry is derived from a latin word
Argentum which means silver .
• The most widely applicable precipitation titrations involve the use of
silver nitrate with chlorides, bromides, iodides, and thiocyanate. Since
silver is always there, precipitation titrations are referred to as
Argentometric titrations.

4. Principle
• Formation of an insoluble product by the combination of two ionic species is
known as precipitation.
• In this titration the titrant react with analyte and forms an insoluble substance
called precipitate.It continues till the last amount of analyte is consumed.
• End Point can be determined using a suitable indicator which forms a colored
precipitate at the end point.
• Ex- Silver nitrate reacts with sodium chloride in presences of suitable
indicator which forms white color ppt of silver chloride and sodium nitrate as
by product.
AgNO3 + NaCl AgCl + NaNO3
Silver nitrate sodium chloride silver chloride sodium nitrate
AgNO3 + Indicator Ag Indicator Complex (Coloured ppt)

5. Detection of End Point
• According to end point detection method, following procedures
are widely used depending on the type of application.
• These are:
1. Formation of colored precipitated i.e. Mohr’s method
2. Formation of soluble colored compound i.e. Volhard Method
3. Use of adsorption Indicators i.e Fagan’s Method
4. Turbidity Method i.e. Gay Lussa’s Method

6. Mohr’s method
• In the Mohr titration, the given chloride solution (eg. NaCl) is titrated
with a standard AgNO3solution when Cl-ions are precipitated as
AgCl.
• AgNO3(aq) + NaCl(aq) AgCl(s) + NaNO3
• This method utilizes chromate as an indicator. Chromate forms a
precipitate with Ag+ but this precipitate has a greater solubility than
that of AgCl, for example.
• Therefore, AgCl is formed first and after all Cl- is consumed, the first
drop of Ag+ in excess will react with the chromate indicator giving a
reddish precipitate.
• 2 Ag+ + CrO4
2-
Ag2CrO4

7. Mohr’s method
• In this method, neutral medium should be used since, in alkaline
solutions, silver will react with the hydroxide ions forming
AgOH.
• In acidic solutions, chromate will be converted to dichromate.
• Therefore, the pH of solution should be kept at about 7.
• There is always some error in this method because a dilule
chromate solution is used due to the intense color of the
indicator.
• This will require additional amount of Ag+ for the Ag2CrO4 to
form.

8. Mohr’s method
Limitations:
• Ag2CrO4is soluble in acid so, the solution should be always neutral.
• If the solution is acidic, first neutralize with CaCO3to make solution
neutral, then start the titration process.
• If the solution is basic, AgNO3is precipitated as Silver hydroxide.
So, first neutralize with HNO3and then start the titration.
• The titration is not suitable for iodides. For chloride and bromide the
method is applicable and suitable.
• If BaCl2/BaBr2is used in the process as a sample, slight excess of
K2SO4is added, otherwise BaSO4 will be precipitated.

9. Mohr’s method
• Determination of Normality of the given chloride solution by
Mohr’s Method
• Preparation of 0.1N AgNO3 solution:
Dissolve 8g AgNO3 in 1000mL water.
• Titration:
1. Standard Solution: 0.1N AgNO3in burette.
2. Sample Solution : 25 ml NaCl in clean conical flask.
3. Indicator: 1 ml K2CrO4(Potassium Chromate) in conical flask
4. End Point: Brick Red Precipitate
5. Blank Titration: Perform the blank titration taking 25 ml water in place of
sample.

10. Volhard Method
• The method is developed by Volhard (1874) for the estimation of silver in
dilute nitric acid solution by titrating against a standard thiocynate solution
in the presence of ferric salt as indicator.
• When a standard solution of ammonium or potassium thiocynate is added
from the burette to a solution of silver salt in the presence of nitric acid and
ferric indicator in the titration flask a precipitate of silver thiocynate
continues to be formed till the silver gets completely precipitated.
• For a halide solution having acidic medium potassium chromate
can not be used as indicator.
• Volhard’s Method is used for acidified halide solution.

11. Volhard Method
AgNO3 + NaCl AgCl + NaNO3
Silver nitrate sodium chloride Silver Chloride Sodium Nitrate
AgCl + NH4SCN AgSCN + NH4NO3
Silver Chloride Ammonium Thiocynate Silver Thiocynate Ammonium Nitrate
AgNO3 + NH4SCN AgSCN + NH4NO3
Silver nitrate Ammonium Thiocynate Silver Chloride Ammonium Nitrate
Fe3+ + NH4SCN Fe(SCN)2+ + NH4
+
Ferric Ammonium Thiocynate Ferric Thiocynate Ammonium

12. Volhard Method
• This is an indirect method used for determining the anions that precipitate
with silver e.g Cl- , Br- and I .
• For example an excess amount of standard Ag+ is added to the chloride
unknown solution containing Fe3+ as an indicator.
Ag+ + Cl-
↔ white precipitate of AgCl
• The excess Ag+ is then titrated with standard NH4SCN-
solution
Ag+ + NH4SCN ↔ white precipitate of AgSCN
• Until a reddish color due to Fe(SCN)2+
complex formation is obtained
• Which results from the reaction:
Fe3+ + SCN-
reddish complex Fe(SCN)2+
• The indicator system is very sensitive and usually good results are obtained.

13. Volhard Method
• The medium should be acidic to avoid the formation of Fe(OH)3.
• If AgX is less soluble than AgSCN as in the case of I- and Br- ,
the excess Ag+ can be titrated in the presence of AgX precipitate.
But in the case of AgCl when SCN-
is added just after the
equivalent point the following reaction will take place:
AgCl + NH4SCN ↔ AgSCN + NH4Cl-
• This means that, SCN-
instead of reacting with the indicator it
will react with the AgCl precipitate.

14. Volhard Method
• We have two ways to overcome this problem :
• The first includes addition of some organic solvent which is
heavier and immiscible with water such as nitrobenzene or
chloroform, which covers the AgCl precipitate in the bottom of
conical flask and shields it from the aqueous medium which
contains the excess Ag+ that can be titrated with NH4SCN.
• The second procedure involves filtration of the precipitate
directly after precipitation, which protects the precipitate from
coming in contact with the added NH4SCN solution and titrating
the excess Ag+ in the filtrate.

15. Volhard Method
Procedure:
• Titration:
• Standard Solution: NH4SCN (8g in 1 lit) solution (0.1M) in burette.
• Sample Solution: 25 ml 0.1N AgNO3 in clean conical flask.
• Acidic Medium: Add 5ml 6N HNO3 in conical flask.
• Indicator: 1 ml Ferric ammonium sulphate (40%) in conical flask
• End Point: During titration at first white color precipitate of AgSCN is
formed (the solution appears milky white). At the end point blood red/
brown color appears.

16. Volhard Method
Advantages of Volhard’s method :
• The acidic environment give advantage for halide analysis
because anions such as carbonate , oxalate and arsenate that do
not form precipitate with silver in acidic medium ( but they do in
basic medium ) will not interfere with halides.
• Give accurate results due to back titration.
Limitations of Volhard’s method :
• Can not be used in neutral or basic medium.
• Time consuming.

17. Fajan’s Method
• This method involves the titration of AgNO3with any halide
solution in the presence of Adsorption indicator.
• Standard solution of AgNO3is added into unknown
concentration of halide (i.e. NaCl) and a white precipitate of
AgCl is formed.
AgNO3 + NaCl AgCl + NaNO3
Silver nitrate sodium chloride silver chloride sodium nitrate

18. Fajan’s Method
• 1stStage: At this stage Cl- ions will make a primary adsorption
layer over the precipitate of AgCl and Na+ will make a
secondary adsorption layer.
Primary Adsorption Layer
Secondary Adsorption Layer

19. Fajan’s Method
• 2nd
Stage: Addition of AgNO3 make reaction mixture fuel with
Ag+ and NO3 -ion.
• Now, Ag+ ion will make primary adsorption layer and NO3- ion
will make a secondary adsorption layer.
Primary Adsorption Layer
Secondary Adsorption Layer

20. Fajan’s Method
• 3rdStage: Fluoescein(Adsorption indicator) indicator forms F-(Fluoreceinateion) and
replace NO3-ion over surface of AgCl.
• After the end point, the surface of the precipitate carries a positive surface charge
due to the adsorption of excess Ag+. Dichlorofluoroscein now adsorbs to the
precipitate’s surface where its color is pink. This change in the indicator’s color
signals the end point.
Primary Adsorption Layer
Secondary Adsorption Layer

21. Fajan’s Method
Procedure:
• Titration:
• Standard Solution: 0.1N AgNO3in burette.
• Sample Solution: 25 ml halide solution in clean conical flask.
• Indicator: 8-10 drops Fluorescein in conical flask
• End Point: During titration at first white colour precipitate is
formed. At the end point pink colour appears.

22. Application of Precipitation Titration
• Determination of the chloride in natural water: Mohr and
Volhard
• Determination of Silver content in silver alloy: Volhard
method
• Determination of halogen content in organic compounds such
as food, organochlorine pesticides: Volhard method.

23. Limitations of Precipitation Titration
• A few number of ions such as halide ions (Cl-, Br-, l-) can be
titrated by precipitation method.
• Co-precipitation may be occurred.
• It is very difficult to detect the end point.

24. Thank You…