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25
Titrations
Fajan’s Method
Fajan’s method is also known as the adsorption indicator method. Adsorption indicators are
organic compounds that tend to be adsorbed onto the surface of the solid precipitate in a
precipitation titration.
Adsorption indicators act best when……..
They do not precipitate out silver ions when the indicators are at low concentration.
They bind to the precipitate only when the excess silver ion is present to produce color.
E.g., Fluorescein.
Only the ionized fluorescein produces the red color. E.g., Titration of NaCl with AgNO
3
.
During titration colloidal particles are formed, the surface of colloidal particles becomes
negatively charged due to the adsorption of negatively charged chloride ions.
The primary adsorption layer is negatively charged and when the equivalence point is
reached, the surface of particles becomes neutral because there are no particles of excess chloride
ions.
After the equivalence point, there will be excess silver ions which attract negatively charged
fluorescein indicator ions to form a red color complex.
Precaution: Silver indicator complex is light sensitive and tends to decompose; therefore
light exposure should be avoided.
Ag
+
Na
+
Na
+
Na
+
Na
+
Ag
+
Ag
+
Ag
+
Ag
+
Cl
-
Cl
-
Cl
-
Cl
-
Cl
-
Cl
-
Cl
-
Adsorption Layer
Preparation of 0.1M Silver Nitrate Solution
Dissolve 17.5 g of silver nitrate in water and make up the volume to 1000 mL.
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A Textbook of Pharmaceutical
Chemistry
Standardization of 0.1M Silver Nitrate Solution (as per USP)
Weigh accurately 0.1 g of sodium chloride previously dried at 110C for 2 hours, dissolve in
5 mL of water and add 5 mL acetic acid, 50 mL methanol, 0.5 mL eosin Y as an indicator and
titrate with 0.1M silver nitrate solution.
Endpoint: Yellow color
Factor: 0.005844 g of sodium chloride is equivalent to 1 mL of 0.1 M Silver nitrate.
N =
3
______ g NaCl × 1000
______ mL of AgNO × 58.44
4.3 NON-AQUEOUS TITRATION
As the name indicates these titrations are devoid of water in either titrant or titre.
What is the need for non-aqueous titration?
To determine the sharp endpoint of the titration.
For too weak acids.
For too weak bases.
For poorly water-soluble compounds.
For organic acids and bases that are insoluble in water but are soluble in non-aqueous
solvents.
Solvents used in Non-aqueous Titration
(A) Aprotic Solvents
These solvents do not tend to give or accept proton or hydrogen ions.
E.g., Carbon tetrachloride, hydrocarbons, etc.
(B) Protogenic Solvents
These solvents have a tendency to generate protons.
E.g., H
2
SO
4
, CH
3
COOH, etc.
(C) Protophilic Solvents
These solvents have a high affinity for hydrogen ions or protons.
E.g., Pyridine, ethers, etc.
27
Titrations
(D) Amphiprotic Solvents
These solvents donate as well as accept protons.
E.g., Water.
Leveling Effect
Strongly basic solvent abstract proton from an acid whether it is strong or weak acid. This
effect is called as leveling effect and the strong basic solvent is called as leveling solvent. It does
not differentiate between strong or weak acids.
E.g., Acetic acid in water reacts incompletely and slowly to form hydronium ion (H
3
O
+
) and
behaves as a weak acid but in a base such as ethylene diamine reacts completely and behaves as a
strong acid.
Differentiating Solvent
A weakly basic solvent has a weak tendency to accept a proton. Hence solvent is called a
differentiating solvent. Because it can abstract a proton from strong acids and not from weak
acids.
Preparation of 0.1N/0.1M Perchloric acid (as per USP)
Gradually add 8.5 mL of perchloric acid to 900 mL of glacial acetic acid with vigorously and
continuous stirring. Then add 30 mL of acetic anhydride and make up the volume to 1 litre with
glacial acetic acid and allow to stand for 24 hours before use.
The acetic anhydride reacts with the water (approx. 30%) in perchloric acid and some traces
in glacial acetic acid thereby making the resulting mixture practically anhydrous.
Precautions to be Taken
1. Chances of an explosion, so make addition slowly.
2. Conversion of acetic anhydride to acetic acid requires 40-45 minutes for completion. It is
an exothermic reaction; the solution must be allowed to cool to room temperature before
adding glacial acetic acid to make up the volume.
3. Avoid adding an excess of acetic anhydride especially when primary and secondary
amines are to be assayed, because these may be converted rapidly to their corresponding
acetylated compounds.
4. For titration, moisture content should not exceed the limit of 0.05%.
Standardization of 0.1M Perchloric Acid (as per USP)
Weigh accurately about 0.5g of potassium hydrogen phthalate in a 100 mL conical flask.
Add 25 mL of glacial acetic acid and shake until the salt is completely dissolved. Titrate with
0.1N Perchloric acid by using crystal violet as an indicator, also perform the blank determination.
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A Textbook of Pharmaceutical
Chemistry
Endpoint: Blue to blue-green
Factor: 0.020422 g of potassium hydrogen phthalate is equivalent to 1 mL of 0.1M HClO
4
.
N =
4
______ g Potassium Hydrogen Phthalate ×
1000
______ mL of HClO × 204.22
Preparation of Crystal Violet Solution
0.5% w/v crystal violet in glacial acetic acid.
Other indicators used are:
(i) Oracet blue B
(ii) α-Naphtholbenzein
(iii) Quinaldine red
4.4 REDOX TITRATION
As the name indicates these titrations involve an oxidation reduction reaction.
Oxidation
Addition of oxygen atom.
Loss of one or more electrons.
Loss of hydrogen atom.
It gives a positive oxidation state.
Reduction
Loss of oxygen atom.
Gain of one or more electrons.
Addition of hydrogen atom.
It gives a negative oxidation state.
Oxidizing Agent
The compounds which accept one or more electrons are called oxidizing agents.
E.g., KMnO
4
, H
2
O
2
, Ceric sulfate, Potassium dichromate (K
2
Cr
2
O
7
), Iodine, etc.
29
Titrations
Reducing Agent
The compounds which donate one or more electrons are called reducing agents.
E.g., Oxalic acid, arsenious oxide, zinc, etc.
Oxidation Number
The oxidation number is defined as a number assigned to an element in a chemical
combination which represents the number of electrons lost or gained by an atom.
It is generally positive, negative, or zeroes integer.
A positive oxidation state reflects the tendency of an atom to lose electrons.
A negative oxidation state reflects the tendency of an atom to gain electrons.
Redox Titration
Redox titrations involve oxidation and reduction reactions simultaneously.
Redox Reaction
Oxidation Reaction
Na
Na
+
+ e
–
Reduction Reaction
Cl + e
–
Cl
–
Iodometry
In iodometry titration, liberated iodine in a chemical reaction is used as an oxidizing agent
for further titration. It is an indirect method.
Iodimetry
In iodometry titration, external iodine is used as an oxidizing agent for titration. It is a direct
method.
Preparation of 0.1N Iodine Solution
Dissolve about 14 g of iodine in a solution of 36 g of potassium iodide in 100 mL of water
and 3 drops of HCl acid, dilute with water to 1000 mL.
Standardization of 0.1N Iodine Solution
Transfer 25 mL of iodine solution to the flask, dilute with water to 100 mL, and add 1 mL of
1N HCl. Titrate with 0.1N sodium thiosulfate solution until the solution has a pale yellow color.
Add 2 mL of starch indicator and continue the titration until the solution is colorless.
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A Textbook of Pharmaceutical
Chemistry
Preparation of Starch Indicator
Place 1 g of starch powder in 5 mL cold water and add 100 mL hot water to dissolve.
Role of Potassium Iodide (KI)
Iodine is water-insoluble. To make it water-soluble potassium iodide is added in a solution, it
forms a potassium tri-iodide complex which is water-soluble.
I
2
+ H
2
O
Insoluble
I
2
+ KI
3
Potasium Tri-iodide Complex
Water Soluble
KI
Preparation of 0.1N Sodium Thiosulfate (Na
2
S
2
O
3
.5H
2
O)
Dissolve about 26 g of sodium thiosulfate and 200 mg of sodium carbonate in 1000 mL of
recently boiled water.
Standardization of 0.1M Sodium Thiosulfate Solution
Accurately weigh about 0.21 g of potassium dichromate, previously pulverized and dried at
120C for 4 hours and dissolve in 100 mL of water in a stoppered volumetric flask. Add 3 g of
potassium iodide, 2 g of sodium bicarbonate, and 5 mL of hydrochloric acid into it. Allow the
flask to stand for 10 minutes in a dark place. Then titrate the liberated iodine with sodium
thiosulfate solution until the solution is yellowish-green in color. Add 3 mL of starch indicator
and continue the titration until the blue color disappears. Perform blank determination.
Factor: 0.004904 g of potassium dichromate is equivalent to 1 mL of 0.1 M Na
2
S
2
O
3
.
Formula:
N =
2 2 7
2 2 3
______ g K Cr O
49.04 × ______ mL of Na S O
4.5 PERMANGANOMETRY
In these titrations, potassium permanganate is used for titration, hence it is called
Permanganometry. Potassium permanganate is a powerful widely used oxidizing agent. Due to its
intrinsic intense color, it makes it easy to detect the endpoint of titrations. Therefore it also acts as
a self indicator and does not need an external indicator.
Depending upon the reaction conditions permanganate ion is reduced to manganese in the 2
+
,
3
+
, 4
+
, or 6
+
oxidation states.
31
Titrations
Preparation of 0.1N Potassium Permanganate Solution
Dissolve about 3.3 g potassium permanganate in 1000 mL of water in a flask, and boil the
solution for about 15 minutes. Insert stopper and allow to stand for 2 days and then filter.
Standardization of 0.1N Potassium Permanganate Solution (as per USP)
Accurately weigh about 0.2 g sodium oxalate and dissolve in 250 mL of water. Add 7 mL of
sulfuric acid, heat to about 70°C, and then slowly add permanganate solution from a burette until
pale pink color is produced, which persists for 15 seconds.
Factor: Each 0.0067 g of sodium oxalate is equivalent to 1 mL of 0.1N KMnO
4
.
Formula:
N =
2 2 4
4
______ g Na C O × 1000
______ mL KMnO Solution × 67.00
Precautions to be Taken
Always keep the solution in an amber-colored stoppered glass bottle because it reacts
with rubber and organic substances.
Keep away from light and air because it is oxidized.
4.6 CERIMETRY
Cerimetry is a method of volumetric chemical analysis coined by Ion Atanasiu. It is also
called cerate oximetry. Here, cerium (IV) salt is used as a reagent for titration, hence it is called
Cerimetry.
Cerium sulfate is a strong oxidizing agent and also acts as a self indicator.
Advantages of Cerimetry
1. Cerium sulfate solutions are stable for a long time.
2. It does not need protection from light.
3. It is useful in the determination of reducing agents in a highly acidic medium.
4. Due to the reasonable color of the solution, the endpoint detection is easy.
Standardization of 0.1M Cerium Sulfate
Accurately weigh 0.2 g of sodium oxalate primary standard and dissolve in 75 mL of water.
Add with stirring 2 mL sulfuric acid previously mixed with 5 mL water. Mix well, add 10 mL
HCl and heat at 70-75C and titrate with 0.1N ceric sulfate.
Endpoint: Permanent pale yellow.
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A Textbook of Pharmaceutical
Chemistry
Each 0.0067 g of sodium oxalate
˜
1 mL of 0.1N Ce(SO
4
)
2
N =
2 2 4
4 2
______ g Na C O × 1000
______ mL Ce(So ) × 67.00
Role of Osmium Tetraoxide
To enhance the rate of a chemical reaction between arsenic trioxide and cerium sulfate, in
which it acts as a catalyst.
Preparation of Osmic acid Solution (Very Toxic in Nature)
Dissolve 0.1 g of osmium tetraoxide in 40 mL of 0.05 M sulfuric acid.
4.7 COMPLEXOMETRIC TITRATION
The name itself indicates the formation of complex i.e. metal complex or chelate in the
reaction.
Complexometric titrations are useful for the determination of a mixture of metal ions in a
solution. It is also known as chelatometry because of the formation of chelates. Ethylene diamine
tetra acetic acid (EDTA) is a very important and widely used chelating reagent for complex
formation.
Principle
During a complexometric titration, two complexes are formed i.e., metal-EDTA complex
and metal-indicator complex. The metal-indicator complex must be less stable than the metal-
EDTA complex.
HO N
OH
O
N
OH
O
OH
O
O
EDTA
Ligands
It is an ion that binds to the metal by co-ordinate bonding.
Classification of Ligands
Ligands are classified based on their binding ability to metal ions.
33
Titrations
(i) Monodentate Ligand
These agents bind to metal ions by only a single site; therefore, they are called monodentate
or unidentate ligands. Monodentate ligand-formed complexes are less stable than polydentate
ligands.
E.g., Bromide ion, fluoride ion, cyanide ion, etc.
(ii) Bidentate ligand
These are Lewis bases and donate two pairs of electrons to a metal atom and form a complex.
E.g., Ethylene diamine, oxalate ion, etc.
(iii) Multidentate ligand (Polydentate ligand)
These ligands can bind the metal ion at more than two sites. Multidentate ligand-formed
complexes are more stable than any other ligands.
E.g., EDTA, etc.
EDTA (Ethylene Diamine Tetra Acetic Acid)
EDTA has four carboxyl groups and two amine groups that can act as electron-pair donors or
Lewis bases. The ability of EDTA to potentially donate its six lone pairs of electrons for the
formation of coordinate bonds to metal cations makes EDTA a hexadentate ligand. However, in
practice, EDTA is usually partially ionized and thus forms fewer than six co-ordinate bonds with
metal cations.
Werner’s Co-ordination Number (WCN)
It is the number of small groups which can be attached to the central atom in a complex. This
is in no way related to the valency of ions but related to the steric factor or the space available for
the attachment of atoms.
Element of the second period can accommodate fourth groups, third period – six groups, and
fourth period – eight groups.
It is required to know the maximum number of groups that can be accommodated around a
central atom.
Masking agent (Mask means ‘to Cover/to Hide’)
A masking agent is a reagent used in chemical analysis which reacts with chemical species
that may interfere in the analysis.
E.g., Cyanide ion, triethanolamine, iodide ion, etc.
Cyanide ion masks Cd, Zn, Hg(II), Cu, Co, Ni, Ag, and the platinum metals, but not the
alkaline earth, manganese and lead.
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A Textbook of Pharmaceutical
Chemistry
Triethanolamine can mask aluminum (Al), Titanium (IV), iron (III).
Iodide ions can mask mercury.
Ammonium fluoride can mask aluminum (Al), iron (III), titanium (IV), and tin (II).
Need of Masking Agents
EDTA and its salt form complexes with several metal ions. During the estimation of specific
ions, other ionic impurities are also estimated. This gives false results or a deviation in the results.
Sometimes when two or more ions are to be estimated in a mixture, each ion has to be
selectively titrated.
For the above-given reasons, masking agents are required.
Selectivity can be achieved by any one of the methods given below.
1. Addition of precipitants: In this method, selected precipitants are added and the
collected precipitate is estimated separately.
Interfering Ions Precipitants
Copper, Cobalt, Lead Sodium sulfide, Thioacetamide
Lead, Barium Sulfate
Lead, Calcium Oxalate
Lead, Calcium, Manganese Fluoride
Zinc, Copper Ferrocyanide
2. Formation of a complex with interfering ions: Here, masking agents form the complex
with interfering ions or species which is more stable complex than EDTA complex, and
thus elimination of impurities and estimation of selective ion is made possible.
Interfering Ions Masking Agents
Mercury Potassium iodide
Aluminum, Titanium Tiron
Iron, Aluminum Ammonium fluoride
3. Selective titration by varying pH: By changing the pH of the solution, interference of
unwanted impurities can be bypassed. E.g., EDTA complex with earth metal not stable
below pH 7.
Demasking Agent
Demasking is the process in which the masked substance regains its ability to enter into a
particular reaction.
E.g., Chloral hydrate, acetic acid, formaldehyde, etc.
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