1. ANALYTICAL CHEMISTRY II
Areej Abdelmonim Mohamed
BSc Pharm U of K 2014
MSc Pharm Chemistry U of K 2020
Miscellaneous titration methods
2. Diazotization titrations
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• This type of titration is very useful for the analysis of sulphonamide
antibiotics and aminobenzoic acid-derived local anaesthetics.
• Titration is carried out with acidified sodium nitrite, causing the
primary aromatic amine function to be converted to a diazonium salt
for sulfacetamide.
• A small amount of iodide is included in the titration mixture. At the
end-point the first drop of excess nitrous acid converts iodide to iodine
and this is detected using starch indicator.
3. Diazotization titrations
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• Titration with nitrous acid is used in pharmacopoeial assays of:
benzocaine, dapsone, primaquine, procainamide, procaine,
sulfacetamide, sulfadoxine, sulfamethizole, sulfapyridine and
sulfathiazole.
4. Karl Fischer titration
(coulometric end-point detection)
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• The Karl Fischer titration is a type of
coulometric titration.
• Coulometry measures the amount of charge
that has to be passed through a solution of
analyte in order to reduce or oxidize it.
• According to Faraday’s law; the amount of
charge required can be equated to the
number of moles of analyte present in
solution.
5. Karl Fischer titration
(coulometric end-point detection)
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• Karl Fischer volumetric titration can be used to determine water
content.
• The Karl Fischer reagent consists of a mixture of anhydrous methanol,
an anhydrous base (pyridine, imidazole or diethanolamine), iodine
and sulphur dioxide. It is important for the reliability of the titration
that it remains buffered within the optimal pH range of 4 –7.
• In the case of the Karl Fischer titration, the end-point detection is
based on the following reaction:
I2 + 2 e - → 2 I-
6. Karl Fischer titration
(coulometric end-point detection)
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• A pair of platinum electrodes that provide variable potential, so that a
constant current is supplied to the titration cell, detect the end-point.
• When excess iodine is produced at the end-point, the resistance of
the cell falls. Up to the end-point, the reaction at the cathode of the
electrode pair is:
7. Karl Fischer titration
(coulometric end-point detection)
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H2O + I2 + SO2 → SO3 + HI
SO3 + C5H5N → C5H5N+.SO2
-
C5H5N+.SO2
- + CH3OH → C5H5N+H.CH3OSO2O-
8. Karl Fischer titration
(coulometric end-point detection)
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• At the end-point it is the excess iodine that is reduced, which requires
less voltage than the reaction shown above; this causes the steep
drop in potential at the end-point.
• The reactions that occurs in the presence of water with pyridine as
the basic component; involves the reduction of iodine to iodide by
sulphur dioxide, which itself is oxidized to sulphur trioxide.
9. Karl Fischer titration
(coulometric end-point detection)
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• The substance being examined is introduced through an inlet tube
and the sample is stirred during the titration with a magnetic stirrer.
The potential is adjusted so that a current of 10 mA passes between
the platinum electrodes; at the end of the reaction a steep fall in
potential indicates the presence of excess iodine in solution.
• Coulometric titration is an alternative to volumetric titration. In this
case, the instrument used generates iodine from iodide at a platinum
anode; any water present in the sample solution immediately
converts the iodine back to iodide until the end-point is reached.
10. Karl Fischer titration
(coulometric end-point detection)
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• The end-point is detected using a pair of platinum electrodes, in
addition to the anode that is used to generate the iodine.
• The amount of water present is determined from the number of
coulombs required to generate iodine (2 coulombs are equivalent to 1
mole of water) up to the end-point.
11. Kjeldahl’s Method
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• The Kjeldahl method was developed over 100 years ago for
determining the nitrogen contents in organic and inorganic
substances.
• Description of the method:
• The method consists of heating a substance with sulfuric acid, which
decomposes the organic substance by oxidation.
12. Kjeldahl’s Method
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• As the organic material is oxidized, the carbon it contains is converted
to carbon dioxide and the hydrogen is converted into water. While the
nitrogen, from the amine groups found in the peptide bonds of the
polypeptide chains, is converted to ammonium ion, which dissolves in
the oxidizing solution, and can later be converted to ammonia gas.
• In this step potassium sulfate is added to increase the boiling point of
the medium (169°C to 189°C) and a catalyst, which helps the
conversion of the amine nitrogen to ammonium ions.
13. Kjeldahl’s Method
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• Chemical decomposition of the sample is complete when the
medium has become clear and colorless (initially very dark).
• The solution is then distilled with sodium hydroxide which converts
the ammonium salt to ammonia.
• Finally The amount of ammonia present (the amount of nitrogen
present in the sample) is determined by titration.
14. Kjeldahl’s Method
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• Because the Kjeldahl method does not measure the protein content
directly a conversion factor (F) is needed to convert the measured
nitrogen concentration to a protein concentration.
• The Kjeldahl method consists of three steps, which have to be
carefully carried out in sequence:
Digestion , Distillation & Titration
15. Kjeldahl’s Method
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1. Digestion:
This is the most time-consuming step in the analysis.
The purpose of this step is to break down the bonds that hold the
polypeptides together, and convert them to simpler chemicals such as
water, carbon dioxide and ammonia.
Ammonia gas is not liberated in an acid solution because the ammonia
is in the form of the ammonium ion (NH4
+) which binds to the sulfate
ion (SO4
2-) and thus remains in solution.
Organic N + H2SO4 → (NH4) 2SO4 + H2O + CO2
16. Kjeldahl’s Method
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• The sample to be analyzed is weighed into a digestion flask and then
digested by heating it in the presence of:
• Sulfuric acid (an oxidizing agent which digests the sample).
• Potassium sulfate (K2SO4) (to speed up the reaction by raising the
boiling point of the digesting acid)
• Catalyst: such as copper, selenium, titanium, or mercury (to speed up
the reaction).
17. Kjeldahl’s Method
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2. Distillation:
The purpose of the distillation step, is to separate the ammonia (that
is, the nitrogen) from the digestion mixture.
This is done by raising the pH of the mixture using (NaOH ); which
changing the ammonium (NH4
+) ions (which are dissolved in the liquid)
to ammonia gas (NH3).
(NH4)2SO4 + 2NaOH → 2NH3 + Na2SO4 + 2H2O
18. Kjeldahl’s Method
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3. Titration:
The amount of nitrogen in a sample can be calculated from the
quantified amount of ammonia ion in the receiving solution (Boric
acid). The boric acid captures the ammonia gas, forming an
ammonium-borate complex.
NH3 + H3BO3 → NH4 H2BO3 + H2O (color change occurs
(Red color) (Green color complex )
19. Kjeldahl’s Method
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Ammonium-borate complex titrates with HCl and using Tashiro’s
indicator to quantify the amount of ammonia in the receiving solution.
NH4 H2BO3 + HCl → NH4Cl + H3BO3
(Green color complex ) (Red color)
(color change reverse )
