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General Information
15
Chapter
Volumetric Analysis
3.1 ANALYSIS
An analysis is the process of breaking down a complex structure into smaller ones using
various analytical tests.
ana – crystal/structure lysis – breakdown
3.2 QUALITATIVE ANALYSIS
Qualitative analysis is the determination of the identity of a particular analyte.
E.g., Flame test
3.3 QUANTITATIVE ANALYSIS
Quantitative analysis is the determination of the absolute/quantitative concentration of
analyte present in a given sample.
E.g., Determination of assay in percent.
3.4 VOLUMETRIC ANALYSIS
It involves estimation of a substance or analyte in solution by neutralization, precipitation,
oxidation, or reduction reaction using another solution of accurately known concentration or
strength.
E.g., Acid-base titration.
The volumetric analysis depends on measurements of the volumes of solutions of the
interacting substances. A measured volume of the solution of a substance A is allowed to react
completely with the solution of definite strength of another substance B. The volume of B is
noted. Thus, we know the volume of the solutions A and B used in the reaction and the strength
3
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A Textbook of Pharmaceutical
Chemistry
of solution B, on this basis the strength of solution A is calculated. The amount or concentration
of the dissolved substance in the volumetric analysis is usually expressed in terms of normality.
Formula: Weight of the substance (g per litre) = Normality × gram equivalent
weight of the substance.
Conditions for Volumetric Analysis
(i) The reaction between the titrant and titer must be expressed.
(ii) The reaction should be practically instantaneous.
(iii) There must be a marked change in some physical or chemical property of the solution at
the endpoint of reaction.
(iv) An indicator should be available which should sharply define the endpoint.
Molecular Weight
The molecular weight is the mass of one mole of a substance in a particular molecule.
E.g., Molecular weight of NaOH: 23 + 16 + 1 = 40
Equivalent Weight
The equivalent weight of a compound that contains one equivalent of a proton (for acid).
E.g., (a) Equivalent weight of NaOH: 40.
(b) Equivalent weight of H
2
SO
4
?
Molecular (atomic) weight of H
2
SO
4
: 98, but there are two acidic hydrogen atoms.
Equivalent weight of H
2
SO
4
: 98/2 = 49
Normal Solution
Normal solution is a solution that contains one gram equivalent weight of the substance in
one litre of solution.
Molar Solution
Molar solution is a solution that contains one gram mole of the substance in one litre of
solution.
Normality
Normality is a measure of concentration equal to the gram equivalent weight per liter of
solution. Gram equivalent weight is the measure of the reactive capacity of a molecule. Normality
is also known as the equivalent concentration of a solution.
17
Volumetric Analysis
E.g., 40 gm of NaOH in 1000 mL of water: 1M/1N NaOH solution.
Molarity
Molarity is defined as the number of moles of solute divided (dissolved) by the number of
liters of solution. It is expressed in terms of moles per litre (mol/L).
E.g., 40 gm of NaOH in 1000 mL of water: 1M/1N NaOH solution.
Molality
Molality is a measure of the concentration of a solute in a solution in terms of the amount of
substance in a specified amount of mass of the solvent.
Formality
It is used to express the concentration of the ionic solids which do not exist as molecules but
as a network of ions.
Standard Solution
A standard solution is one whose concentration is precisely known.
Test Solution
A test solution is one whose concentration is unknown or to be estimated.
Titrant
The titrant is the standardized solution that is added during titration from the burette.
Titre
Titre is the test solution in a conical flask taken for titration.
Titration
The word titration comes from the Latin word “titulus”, which means inscription or title. A
titration is a technique where a solution of known concentration is used to determine the
concentration of an unknown one.
Analyte
An analyte is the test substance whose concentration is being determined in titration.
Equivalence Point
The equivalence point is the point at which added titrant moles are stoichiometrically equal
to the moles of analyte in the sample.
Endpoint
An endpoint is a point at which reaction is completed or an equivalence point is achieved.
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A Textbook of Pharmaceutical
Chemistry
Indicator
It is a substance that gives the color change to the solution at the equivalence point (endpoint)
of titration or completion of a chemical reaction.
Solution
A solute dissolved in a solvent is known as a solution.
Solute
The substance which is dissolved in a solvent is known as solute.
Solvent
The substance in which solute is dissolved is called a solvent
Aqueous Solution
A solution prepared in water is called as an aqueous solution.
Primary Standard
A primary standard is a standard of high purity which is used to calibrate other standards
(secondary).
Ideal Properties of Primary Standard
(i) It should be pure (purity greater than 99.99 %).
(ii) It should be stable.
(iii) It should be non-hygroscopic in nature.
(iv) It should be non-toxic.
(v) It should be economical.
(vi) It should have a high molecular weight.
E.g., Arsenic trioxide, Sodium chloride, Sodium carbonates, Benzoic acid, Potassium
hydrogen phthalate (potassium biphthalate), Sulphanilic acid, Potassium bromate, etc.
Secondary Standard
A secondary standard is a standard that is prepared in the laboratory for analysis purposes
and it is standardized against a primary standard.
E.g., Sodium hydroxide, Silver nitrate, etc.
Sodium hydroxide is a secondary standard; justify.
Sodium hydroxide is hygroscopic in nature.
19
Volumetric Analysis
It does not fulfill the criteria of the primary standard (Ideal properties).
Buffer Solution
It is an aqueous solution of a mixture of a weak acid or base and its salt. It resists changes in
the activity of an ion on the addition of a substance. They are used as a means of keeping pH at a
nearly constant value.
Buffer Capacity
Buffer solution can resist the change in pH upon the addition of a small amount of acid or
base. It is also defined as the amount of strong acid or base in gram equivalent that may be added
to a litre of the solution to change its pH by one unit.
General Information
20
Chapter
Titrations
The word titration comes from the Latin word “titulus”, which means inscription or title. A
titration is a technique where a solution of known concentration is used to determine the
concentration of an unknown one.
Titre
Titrant
Color change
Titration vessel
4.1 ACID-BASE TITRATION
An acid-base titration is also known as neutralization titration because in which acid is
neutralized by base and base is neutralized by acid.
4
Titrant
Titration vessel
Titre
Color change
21
Titrations
This titration is used to determine the strength of acid or base by titrating against base or acid
of known concentration. It is a quantitative type of analysis for acids and bases.
Principle
It involves the reaction of H
3
O
+
ions (hydronium ion) with OH
–
ions (hydroxyl ion) to form
water and simultaneously the counter ions form the salt.
The titration progress can be monitored by visual indicators, pH electrodes, or both.
Acid-base titrations can also be used to determine the purity of chemicals.
A strong acid will react with a strong base to form a neutral (pH = 7) solution.
A strong acid will react with a weak base to form an acidic (pH < 7) solution.
A weak acid will react with a strong base to form a basic (pH > 7) solution.
3 2 3 3
CH COOH + H O CH COO + H O
2
HCl + NaOH NaCl + H O
Reaction
Visual indicators are used to determine the progression of titration because they change their
color along with the change in pH range. Some common indicators and their respective color
changes are given below.
Indicators Color in Acidic Medium pH Range of Color
Change
Color in Basic Medium
Methyl violet Yellow 0.0 - 1.6 Violet
Bromophenol blue Yellow 3.0 - 4.6 Blue
Methyl orange Red 3.1 - 4.4 Yellow
Methyl red Red 4.4 - 6.2 Yellow
Litmus Red 5.0 - 8.0 Blue
Bromothymol blue Yellow 6.0 - 7.6 Blue
Phenolphthalein Colorless 8.3 - 10.0 Pink
Alizarin yellow Yellow 10.1 - 12.0 Red
Strong Acid against Strong base Titration
Let us consider the titration of hydrochloric acid and sodium hydroxide.
An indicator is suitable only if it undergoes a change in color at the pH near the endpoint.
Thus the indicators which show the color change in the pH range of 4 to 10 are suitable for this
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A Textbook of Pharmaceutical
Chemistry
type of titration. Thus, in strong acid-base titrations, any one of the following indicators can be
used.
(a) Methyl orange
(b) Methyl red
(c) Phenolphthalein
Weak Acid against Strong base Titration
Let us consider the titration of acetic acid against sodium hydroxide. This titration will have
endpoint in the range of pH 8 to10. This is due to the formation of sodium acetate.
(a) Phenolphthalein is a suitable indicator (pH range is 8 to 9.8)
(b) Methyl orange is not suitable (pH range is 3.1 to 4.5)
Strong Acid against Weak base Titration
Let us consider the titration of ammonium hydroxide against hydrochloric acid. This titration
will have endpoint in the range of pH 6 to 4. This is due to the formation of ammonium chloride.
(a) Methyl orange is a suitable indicator
(b) Phenolphthalein is not suitable.
Examples of weak/strong acids and bases.
Types Examples
Strong acids Hydrochloric acid, sulfuric acid, nitric acid
Weak acids Acetic acid, hydrofluoric acid (HF), oxalic acid
Strong bases Sodium hydroxide, potassium hydroxide, lithium hydroxide
Weak bases Ammonium hydroxide, ammonia
Preparation of 1N HCl Solution
Dilute 85 mL of concentrated hydrochloric acid to 1000 mL with distilled water.
Standardization of 1N HCl Solution
Weigh accurately 5.0 g of tromethamine (Tris) previously dried at 150°C for 3 hours.
Dissolve in 50 mL carbon dioxide-free water, add two drops of bromo-cresol green as an
indicator and titrate with 1N HCl solution to determine the endpoint.
Endpoint of titration: Green color to pale yellow color.
Factor: 0.12114 g of tromethamine is equivalent to each mL of 1N HCl.
23
Titrations
N =
______ g of tromethamine × 1000
121.14 × ______ mL of HCl
Preparation of 1M Sulfuric Acid
Dilute 27 mL of concentrated sulfuric acid to 1000 mL with distilled water. (Precaution: The
reaction is highly exothermic; therefore always add concentrated sulfuric acid slowly in water
with stirring)
Standardization of 1M Sulfuric Acid Solution
Refer standardization procedure of HCl solution.
Preparation of 1N/1M Sodium Hydroxide Solution
Dissolve about 40 g of sodium hydroxide flakes or pellets in 1000 mL of distilled water.
Standardization of 1N/1M Sodium Hydroxide Solution (as per USP)
Weigh accurately 5.0 g of potassium hydrogen phthalate (potassium biphthalate) previously
dried at 120C for about 2 hours and dissolve in 75 mL carbon dioxide-free water and titrate with
1N sodium hydroxide solution using phenolphthalein indicator.
Endpoint of titration: Colorless to permanent pink color.
Factor: 0.20422 g of potassium biphthalate is equivalent to each ml of 1N NaOH.
N =
8 4 4
______ g of KHC H O × 1000
204.22 × ______ mL of NaOH
4.2 PRECIPITATION TITRATION
Titration which is based on the formation of a slightly soluble precipitate is called
precipitation titration. When silver nitrate is used in the formation of a precipitate, the method is
called argentometric titration. (argento – silver)
Reaction
3
dil.HNO
aq. 3 3
aq.
Cl + AgNO AgCl + NO
2– + –
4 4
2–
2 7 2
2CrO + 2H 2HCrO
2Cr O + H O
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A Textbook of Pharmaceutical
Chemistry
In the titration of a neutral solution of chloride ions with silver nitrate, a small quantity of
potassium chromate solution is added as an indicator. At the endpoint the chromate ions combine
with silver ions to form the sparingly soluble brick-red silver chromate complex.
Nitric acid helps the coagulation of silver chloride precipitate and that facilitates its filtration.
Endpoint Determination in Precipitation Titration
Mohr’s Method
Mohr’s method was first published in 1855 as a method for chloride analysis.
In the precipitation of chloride by silver ion, chromate ion
2–
4
(CrO )
is used as an indicator
which forms Ag
2
CrO
4
complex, a reddish-brown precipitate formed when excess Ag
+
ions are
present. The formation of colored precipitate is the endpoint of titration.
+ –
(s)
Analyte
Titrant
White PPT
Ag + Cl AgCl
The concentration of titrant rises sharply near the endpoint. The titration is performed only in
a neutral or slightly acidic medium to prevent silver hydroxide formation (pH> 10).
Volhard’s Method
Volhard’s method is used for the determination of chloride by titration of a silver ion with
thiocyanate using ferric ion as an indicator. This method was coined by Volhard in 1874. The
formation of a soluble colored complex is the endpoint of the titration.
+ –
(s)
Titrant
White PPT
Ag + SCN AgSCN
+ –
2
Indicator
Red PPT
Fe + SCN FeSCN
The endpoint is routinely used for halide determinations where a known excess of silver ion
is added to precipitate the halide ion, and the excess silver ion is back titrated using the
thiocyanate/iron (III) as an indicator.
The silver chloride precipitate is filtered, and the excess silver ion is titrated with thiocyanate
producing a white precipitate of silver thiocyanate (AgSCN).
Once the silver is consumed, the excess thiocyanate reacts with the iron (III) ion-producing a
red FeSCN
2+
complex. Thus, the appearance of the red color at the endpoint was observed.
The titration is usually done in an acidic medium to prevent precipitation of iron hydroxides
[Fe (OH)
3
].
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