2. SCOPE
This course deals with the fundamentals of
analytical chemistry, various techniques of
analysis, principles of electrochemical
analysis of drugs and their role in chemical
research. This lecture basically focuses on
precipitation titration.
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3. OBJECTIVES
Upon completion of this lecture you will be able to:
1. Know the concept of precipitation titration
2. Know about various methods involved in
precipitation titration.
3. Know about various application of precipitation
titrations.
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4. CONTENTS
• Introduction
• General principle and theory
• Mohr’s Method
• Volhard’s Method
• Fajan’s Method
• Applications
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5. Introduction
• Precipitation is the process in which there is combination
of ionic species to form insoluble product. This product is
called as precipitate or phenomenon is precipitation.
• These precipitation titrations involves formation of
precipitate at equivalence point which indicate completion
of the reaction. It is also known as argentometric titration.
Ideal properties of precipitation titrations :
• The precipitate must be insoluble in the liquid.
• The process of precipitation should be rapid.
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6. • The titration results should not be affected by adsorption
problems.
• It should be easy to detect the end point during the
titration.
• The indicator must be suitable to determine the end point.
• The choice of precipitating agent should depend upon
solubility of the drug inside the solvent used.
One of the main reasons for the limited use of such
titrations is the lack of suitable indicators to detect the end
point.
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7. Theory
• The precipitation of many products occurs mainly due to solubility. The
solute molecules are held together by inter molecular forces of
attraction.
• In order to dissolve a solid, these forces of attraction must be overcome
so that solute-solute attraction is replaced by solute-solvent attraction.
• The solvent should compete with crystal forces and overcome them,
which often means that the solvent environment must be similar to that
provided by the crystalline structure.
• During precipitation, the inter molecular forces between the molecules
of the product are high and solute-solute forces replace the solute –
solvent forces.
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8. The solubility mainly depends upon several factors such as ions, pH,
temperature and solvents etc.
1) Common ion effect: The solubility of any slightly soluble salt can be
decreased by adding an excess of either of its ions.
Ex.- The dissociation of a slightly soluble salt BA is
BA(S) ⇌ B+ + A-
At equilibrium,
Ksp = [B+] [A-]
Ksp = Solubility product which is a constant
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9. • If, an excess of either B+ or A- ions are added in the form of another salt
(whose solubility is greater than that of BA), then the product of ionic
concentrations [B+][A-] will exceed the solubility product and hence BA
will precipitate, The common ion effect provides a valuable method for
controlling (the concentration of the ions furnished by a weak
electrolyte.
• Effect of pH in solubility : The solubility of a salt will be decreased by
an increase in pH and vice versa and mainly depends on the anion of the
salt and it is a conjugate base of a weak acid or cation which is conjugate
acid of a weak base.
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10. Effect of temperature on solubility : The solubility of the precipitate
increases with the rise in temperature.
• The influence of temperature is small in case of some substances, but
with some substances it is quite appreciable.
• The solubility of AgCl at 10°C and 100°C is 1.72 and 21.1 mg / lit., while
that of BaSo4 is 2.2 and 3.9 mg/lit. respectively.
Effect of the solvent upon the solubility : The solubility of most
inorganic compounds is reduced by the addition of organic solvents
such as alcohols.
• Ex : The addition of above 20% by volume of ethanol renders the
solubility of lead sulphate practically negligible, thus permitting
quantitative separation
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11. Determination of end point
1. Mohr’s method- depends upon formation of colored precipitate.
2. Volhard’s method- depends upon formation of soluble colored
compound or colored complex ions (colored solution).
3. Fajan’s method- depends on the formation of colored adsorption
compound.
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According to the method of detection of endpoint, precipitation titration is
classified into 3 main types:
12. Principle : It is one of the important precipitation method used for
quantitative determination of halide ion. It was developed by Mohr in
1856.
• Silver nitrate is used as precipitated agent to precipitate the analyte
chloride ion (Cl-). Chromate solution (CrO4
-2) is used as indicator to
determine the end point.
• The end point is activated when there is appearance of brick red
precipitate of silver chromate. The following reaction takes place
• AgNO3 Ag+ + NO3
-
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Mohr’s Method
13. Ag+ + Cl- AgCl (Ppt.)
2Ag+ + CrO4
-2 Ag2 CrO4 (ppt.)
• Firstly silver ions combine to chloride ions to form highly insoluble
AgCl precipitation.
• When all chloride ions consumed then excess silver ions will react
with chromate to form silver chromates.
• First of all AgCl precipitation occurs, sometimes even though silver
chromates appears then it immediately converts to AgCl.
Ag2CrO4 + 2 Cl- 2 AgCl + CrO4
-2
When the whole of chloride has been used up, silver chromate will be
formed and brick red coloration or precipitate will appear.
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14. Procedure
• Dissolve 1.69 g AgNO3 in little amount of distilled water. When it
dissolves completely then make it up to 100 mL with distilled water. It
will produce 0.1 N AgNO3 Solution.
• Consider that we have to determine the concentration of Cl- in a sample
NaCl or KCl. For this weigh accurately 1.2 g of NaCl or 1.6 g of KCl and
dissolve in distilled water and make it up to 250 mL with distilled
water.
• Transfer 50 mL of Chloride solution into conical flask & add 2 mL of
potassium chromate solution as indicator.
• Fill the burette with 0.1 N AgNO3 Solution.
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15. • Now start the titration by drop wise addition of AgNO3 from burette
with continuous shaking of conical flask.
• A time will reach when a brick redcolor of non uniform intensity will
produce. Continue the titration till uniform brick redcolor is produced.
Note the reading as this end point.
• Perform blank titration and subtract the blank volume from the
previous volume.
• To check the quantity of Cl- ions
• 1mL of 0.1 N AgNO3 is equivalent to 0.003558 g of Chloride.
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16. Main features of Mohr’s Method
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Sample Cl-, Br- (Not for I- or SCN-)
Type of titration Direct titration
pH Neutral or slightly alkaline
Indicator Potassium Chromate
Standard solution Standard AgNO3
Color at end point Brick red color
17. Limitations
1. pH- Mohr method should be done in neutral or slightly alkaline
medium (pH= 6.5-9) because:
• At acidic pH: The chromate ion changes into acid chromate (HCrO4
-)
and then to dichromate (Cr2O7
-). Both these form soluble salts with
silver ions and so no colored ppt will formed at the end point. To
overcome this sufficient CaCO3 is added to neutralize it.
• At basic pH (>9): If the solution is basic, then the AgNO3 forms silver
hydroxide. To overcome this dil. nitric acid is added to neutralize it.
Ag+ + OH- AgOH (Silver hydroxide)
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18. 2. This method is not suitable for iodides because the color of precipitate
of AgI is similar to potassium chromate solution. Due to this end point
detection is difficult to recognize.
3. For titrating chlorides and bromides of barium, the solution is treated
with a slight excess of potassium sulphate. This is due to the insoluble
nature of barium sulphate than silver chromate and will be precipitated
earlier.
4. Silver chromate is more soluble than silver chloride so that no silver
chromate be precipitated until all Cl- ions have been precipitated as
AgCl provided that the CrO4
- conc. should be adjusted to make silver
chromate formed only at the end point and so prevent error in the end
point. 1 ml of 5% K2CrO4 solution is suitable.
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19. Applications
• Some of the important drugs determine by this method are
NaCl and dextrose injection.
• Used for estimation of Cl- in body fluids.
• Used for estimation of Chromate ion
• It is useful for estimation of NaCl and KCl in electrolytic
combination.
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20. Volhard’s Method
• It was developed by Volhard in 1874. It was developed for estimation
of silver in the presence of dil. HNO3 by titrating against standard
thiocyanate (SCN) solution in presence of ferric salt as indicator (ferric
alum). It is carried out in acidic medium in order to prevent the
hydrolysis of ferric ion.
• Volhard, a process for assessing chlorine, bromine, and iodine in the
form of halides by precipitating them with excess silver nitrate and
using a thiocyanate solution to titrate excess.
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21. Principle :
• Analyte is treated with the measured excess of Silver nitrate:
X- + Ag+ AgX + Ag+ (excess)
• The unreacted Silver ions are titrated with a standard solution of
Thiocyanate ion, using Fe(III) as indicator:
Ag+ + SCN- AgSCN
• When all the silver has been consumed then upon further addition of
SCN, reaction takes place between ferric ion and SCN ions to form red
colored ferric thiocyanate complex.
Fe3+ + 3SCN- Fe(SCN)3 (red color)
• The formation of red color complex denotes the end point.
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22. Preparation of 0.1 N AgNO3 solution
• Dissolve 1.69 g AgNO3 in little amount of distilled water. When it
dissolves completely then make it up to 100 mL with distilled water. It
will produce 0.1 N AgNO3 Solution.
Preparation of 0.1 N KSCNsolution
• Weigh accurately 0.97 g KSCN and dilute upto 100 mL with distilled
water.
Procedure
• Fill the burette with KSCN solution. Transfer 25 mL or excess of 0.1 N
AgNO3 solution into conical flask and add 10 mL of dil. HNO3 solution.
• Now add 1 mL of ferric salt to conical flask & titrate the contents with
titrant (KSCN) from burette.
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23. • Add few mL of nitrobenzene to prevent the interference
with chloride impurities. When there is permanent
appearance of red color then stop the titration & note the
reading.
• Perform blank titration and subtract the blank volume
from the previous volume and calculate the conc. of Ag+
present in the sample.
• Calculate the percentage of silver in the sample
• 1mL of 0.1 N KSCN is equivalent to 0.0108 g of Silver.
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24. Main features of Volhard’s Method
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Sample All halogens and
thiocyanates
Type of titration Indirect titration
pH Acidic pH
Indicator Iron(III) alum
Standard solution Ammonium thiocyanate
Color at end point Appearance of red
complex with Fe3+ ions
• The Volhard method is an indirect or back titration method in which an excess
of a standard solution of silver nitrate is added to a chloride containing sample
solution.
• The excess silver is then back titrated using a standardized solution of
potassium or ammonium thiocyanate with ferric ion as an indicator. The
amount of silver that is precipitated with chloride in the sample solution is
calculated by subtracting the excess silver from the original silver content.
25. Modified Volhard’s method
• When chlorides are analyzed, the Volhard method has to be slightly
modified. During titration, the solution is in contact with two
precipitates simultaneously AgCl and AgSCN, which have different
solubility (AgSCN is less soluble than AgCl).
• So, after titration, the excess SCN- ions react with the AgCl in
precipitate, and the red color of the iron-thiocyanate complex gradually
disappears.
• Addition of next portion of SCN- restores it, but after short time the
process repeats. This could lead to huge error in analysis.
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26. • To avoid this, before titration with thiocyanates, add a solution of few
mL of an organic liquid (chloroform, nitrobenzene etc.) which does not
mix with water.
• This compound will moisten the surface of the precipitate forming on it
a film (layer) insoluble in water and thus isolating it from the solution.
This effectively prevents any exchange of ions between precipitate and
solution.
Limitations of Volhard's method
• Can not be used where the solution has to be neutral.
• Time consuming
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27. Applications
• Sulphanomide derivatives can be assayed by Volhard method.
• This method has applications in estimation of ferric ions in
haematinics.
• Modified volhard’s method can be used for estimating following drugs
a) Aminophylline ( Bronchodilator)
b) Aminophylline injection
c) Aminophylline tablet
d) NaCl injection.
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28. Fajan’s Method
• It was developed by Fajan in 1923 -1924. This method employs
adsorption indicator for detection of end point.
• Principle : The action of adsorption indicator is based upon the fact
that at the end point, the indicator get adsorbed on the surface of the
precipitate and there is color change.
• Various indicators like flourescein, Eosin, Dichlorofluorscein can be
used to detect the end point.
• Mechanism of Adsorption: Let us consider the titration between
sodium chloride and silver nitrate. When a Cl- salt reacts with Ag+ ions
there is formation of insoluble AgCl precipitates. The ions surrounding
the layer of precipitate constitute primary adsorbed layer.
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29. • As the precipitation proceeds & reaches near completion. There is
decrease in Cl- ions concentration & the ion in the primary layer are
replaced by opposite charges. The newly formed layer is called as
secondary adsorbed layer.
• When all the Cl- ions has been converted to AgCl then the indicator gets
adsorbed to secondary layer and causes color change. This point is
called end point.
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30. Procedure
• Prepare 0.1 N standard NaCl solution.
• Prepare 0.1 N AgNO3 solution and fill it with burette.
• Transfer 25 mL of 0.1 N NaCl standard solution to a conical flask and
add 10 drops of fluorescein indicator.
• Add 0.1 g dextrin to prevent the coagulation of AgCl.
• Now start the titration by drop wise addition of AgNO3 solution from
burette and continue till the color changes from yellow to pink.
• Repeat the titration for 3 times and calculate the average value & then
determine the conc. of Cl- ion and exact normality of AgNO3.
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Main features of Fajan’s Method
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Sample Cl-, Br-
Type of titration Direct titration
pH Neutral
Indicator Dichlorofluorescein
(Adsorption indicator)
Standard solution Standard AgNO3
Color at end point Appearance of pink color
32. Indicators used in Fajan’s method
Name of indicator Experimental
conditions
Color change at
end point
Dichlorofluorescein pH range 4.4 to 7 Yellow green to
red
Tartrazine Back titration Colorless to green
Fluorescein Neutral or weakly
basic solution
Yellow green to
pink
Tetrabromo
fluorescein
Ethanoic acid
solution
Pink to red violet
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33. Limitations
• Low background levels of non-reacting ions to ensure that coagulation
does not occur.
• Will not work with very low levels as there will not be enough
precipitate to allow the color change to be observed.
• Method is pH dependent as the indicator must be in the ionized form.
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34. Applications
• It is used for the estimation of NaCl
• It is also used for the estimation of Cl- ion in binary mixtures &
biological fluids.
• It is also used for standardization of AgNO3 solution.
Estimation of NaCl
• Estimation of NaCl ( Sodium chloride) or Assay of Sodium chloride can
be done by Mohr’s Method or Fajan’s Method ( Here you can write
any one method either Mohr’s or Fajan’s method ).
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35. Comparison of silver titration methods
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Method Advantages Disadvantages
Mohr’s • Simple • For alkaline solutions
only
• Not suitable for iodides
Volhard’s • Capable of direct silver and
indirect halide analyses
• Very clear color change
• Must be 1M nitric acid
solution.
• Some problems with
specific ions.
Modified
Volhard’s
• It minimize the error
occurred in Volhard’s
method due to the addition
of nitrobenzene solution
• Must be 1M nitric acid
solution.
Fajan’s • Capability for different pH
ranges and selectivity with
different indicators
• Difficulty with dilute
solutions
• Should not be a high
background ionic level.
36. References
1. G. Vidyasagar “Text Book of Pharmaceutical Analysis”,
Kalyani publishers, Vol 1, Ist edition, 2005, Page no. 93-101
2. R. M Verma Textbook of Analytical Chemistry, CBS
Publishers, 3rd edition, 2008
3. Ashutosh Kar “Pharmaceutical Analysis” CBS Publishers,
Volume 1, Ist edition, 2010
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37. Questions
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1. What is precipitation titration?
2. Give the ideal characteristics for precipitation titration.
3. Describe in detail the influence of a) Acid b) Temperature c) Solvent
4. Explain in detailed about the following precipitation reactions,
namely
i) Volhard’s method
ii) Mohr’s method
iii) Fajan’s method
5. Enumerate the limitations of mohr’s method.
6. Write down the applications of precipitation titrations.
