aquametry
TRANSCRIPT
East West University
Aquametry
PHRM 309
Tareq Hasan8/11/2011
Table of Contents
Aquametry............................................................2
Introduction..........................................................2
Importance of Water Determination................2
Methods of Water Determination....................2
Thermal Methods of Water Determination..........2
Loss on Drying..................................................2
Theory..................................................................2
Factors Affecting Weight Loss of Sample in Loss on Drying 3
Modification of Loss on Drying Technique for Specific Water Determination 3
Overcoming the problem of Degradation of Sample due to High Heat 3
Overcoming the problem of Interference by Volatile Materials present in Sample 3
Limitation of Loss on Drying.................................3
Azeotropic Distillation......................................4
Azeotrope / Azeotropic Mixture..........................4
Principle of Azeotropic Distillation Method.........4
Criteria of Organic Solvents in Azeotropic Distillation Method 4
Procedure of Azeotropic Distillation Method (Dean – Stark Trap) 5
Advantage of Azeotropic Distillation Method......6
Limitations of Azeotropic Distillation Method......6
Chemical Method of Water Determination..........6
Karl Fischer Titration.........................................6
Theory..................................................................6
Karl Fischer Reagent.............................................6
Composition of Karl Fischer Reagent...............6
Problem with Karl Fischer Reagent and the Solution 7
Chemistry of Karl Fischer Titration.......................7
Primary Condition of Titration.........................7
Justification for Using Pyridine and Methanol in Karl Fischer Reagent 8
Justification for Using Anhydrous Pyridine......8
Justification for Using Anhydrous Methanol. . .9
Classification of Karl Fischer Titration...............9
Volumetric Karl Fischer Titration..........................9
Purpose of Performing Volumetric Karl Fischer Titration 9
Volumetric Karl Fischer Direct Titration..........9
1 | P a g e
End – Point Detection (Volumetric Karl Fischer Direct Titration) 10
Volumetric Karl Fischer Back Titration...........10
End – Point Detection (Volumetric Karl Fischer Back Titration) 11
Purpose of Performing Volumetric Karl Fischer Back Titration 11
Coulometric Karl Fischer Titration......................11
Purpose of Performing Coulometric Karl Fischer Titration 12
Instruments for Coulometric Karl Fischer Titration 12
Coulometric Karl Fischer Direct Titration......13
End – Point Detection (Coulometric Karl Fischer Direct Titration) 13
Coulometric Karl Fischer Back Titration.........14
End – Point Detection (Coulometric Karl Fischer Back Titration) 14
Comparison between Different Karl Fischer Titration (KFT) Method 15
Advantages of Karl Fischer Titration...............16
Limitations of Karl Fischer Titration................16
2 | P a g e
A q u ame t r y
In t roduct ion
Aquametry is a quantitative method of
determining water content present in a
sample.
Determination of water is one of the most
important and widely practiced analytical
methods in pharmaceutical industry.
Importance of Water Determination
Quantitative Determination of Water in
pharmaceutical products is important
because H2O may be present in the
products as –
Solvent (E.g. H2O in Syrup, Suspension
or Emulsion)
Absorbed water (E.g. Absorbed H2O by
Powder for Suspension)
Water of Crystallization (E.g. Crystals of
salts containing H2O)
Adulterant (E.g. Excess Water in
Digitalis Leaves)
This is important, because –
Physical of a drug or raw material is
modified by the presence of H2O.
Pharmaceutical Procedures of
Granulation, Tablet formation and
coating operations are affected by H2O
Content.
Methods of Water Determination
Methods of Water determination can be of
2 types –
1. Thermal Methods
Loss on Drying
Azeotropic Distillation Method
2. Chemical Methods
Karl Fischer Titration
Thermal Methods of Water Determinat ion
Loss on Drying
Theory
Loss on Drying involves the loss of
weight of a material upon drying.
The lost weight is usually the weight of
water and Volatile materials present in
the sample.
So,
Wt .of H 2O present∈the sample=Lost wt . of Sample=Wt .of SamplebeforeDrying−Wt .of Sample after Drying
Both BP and USP have specified the
temperature and time for drying for a
fixed amount of Material / Sample and
also the amount of lost weight in
percentage.
E.g. BP has specified that At least 5%
weight will be lost for 1 gm
Paracetamol dried at 1200C
temperature for 2 hours.
3 | P a g e
Factors Affecting Weight Loss of Sample in Loss on Drying
Loss on Drying is not specific for H2O
determination, since weight loss of
material upon drying depends on the
following factor –
Evaporation of Moisture / H2O
Evaporation of Volatile material
present in the sample
Degradation of the Material /
Sample due to High Heat
Modification of Loss on Drying Technique for Specific Water
Determination
The usual loss on drying technique can
be modified for more specific and
accurate measurement of H2O by
overcoming the factors which affect
Loss of weight in Loss on Drying except
H2O Evaporation.
These are –
1. Degradation of Sample due to High
heat
2. Interference by Volatile Material
Overcoming the problem of Degradation of Sample due to High Heat
Degradation of Sample due to high
heat can be overcome by drying the
sample at a lower temperature and
reduced pressure environment.
Overcoming the problem of Interference by Volatile Materials present in Sample
Interference by volatile material
can be controlled by using water
selective absorbents such as –
Anhydrous Mg perchlorate
(Dehydrite)
CaSO4
Phosphorous pentaoxide (PO5)
Barium oxide
CaCl2
Anhydrous Silica Gel
In this case, an inert gas is used to
carry the formed H2O vapor to the
absorbents.
So,
Wt .of H 2O∈the sample=Wt .of H 2Oabsorbed by Absorbents=Wt .of Absorbents after Drying−Wt .of Absorbents beforedrying
Limitation of Loss on Drying
The Loss on drying process is not
specific for water, because weight loss
of sample occurs also due to –
Degradation of Sample due to High
heat
Interference by Volatile Material
Sample totally degrades due to high
heat involved and cannot be reused. So,
it is not good for expensive materials.
The process is time consuming.
Azeotropic Distillation
Azeotrope / Azeotropic Mixture
4 | P a g e
Azeotrope is a mixture of 2 liquids
which boils at a constant –
Temperature regardless of the
boiling points of the individual
liquids in the mixture.
Ratio of Composition as in their
liquid forms and as in their formed
vapor due to evaporation.
E.g. –
Water (1000C) – Toluene
(110.60C) mixture forms an
Azeotrope with a boiling point
84.10C and the Azeotrope contains
19.6% H2O.
Water (1000C) – Xylene (139.10C)
mixture forms an Azeotrope with a
boiling point 94.50C and the
Azeotrope contains 40% H2O.
Water (1000C) – CCl4 (76.80C)
mixture forms an Azeotrope with a
boiling point 66.80C and the
Azeotrope contains 4.1% H2O.
Principle of Azeotropic Distillation Method
A known weight of sample is placed in
flask with an organic solvent.
The flask containing the sample and the
solvent is attached to a condenser and
the mixture is heated.
Upon heating, the water in the sample
evaporates and moves up into the
condenser where it is cooled and
condensed back into water which is
then deposited into the graduated
tube.
When all of the water in sample is
collected into the tube, the distillation
process is stopped and the volume of
the water is read directly from tube.
Procedure of Azeotropic Distillation Method (Dean – Stark Trap)
5 | P a g e
Criteria of Organic Solvents in Azeotropic Distillation Method
Solvents used in Azeotropic
Distillation Method must have
the following criteria:
Solvent must be water
immiscible and insoluble
Density of the solvent must
be lesser than H2O.
Boiling point of solvent must
be higher than Water.
The solvent must be safe to
use.
Azeotropic Distillation method follows
the addition of a water insoluble and
immiscible organic solvent to the
Sample containing water / moisture and
in this manner to co – distill any water
present.
Upon heating, the water present in
sample will evaporate at first, since
Boiling point of organic solvent is higher
than Water.
It continues until the ratio of
composition for Azeotropic mixture is
achieved.
Figure 1: Dean - Stark Trap
When Azeotropic composition is
achieved, both water and the organic
solvent will start to boil at a constant
temperature and evaporate at the same
Azeotropic composition.
Figure 2: Apparatus used in Azeotropic Distillation
Evaporated solvents will be condensed
in the condenser and go downwards to
deposit in the graduated tube of the
Dean – Stark Trap.
Since, organic solvent is less dense than
water it will deposit above the water
layer and volume of the water can be
measured directly.
Advantage of Azeotropic Distillation Method
6 | P a g e
Azeotropic Distillation method for
water determination is popular because
of being –
Accurate
An Easy Method for Water
Determination
An Efficient method
Economic
This method is advantageous for
moisture determination in bulk
materials such as plant parts, soap
solution etc.
Limitations of Azeotropic Distillation Method
Large amount of samples are needed.
Applicable for sample with large
amount of moisture (E.g. Syrup,
Suspension, Plant Materials). Trace
amount of moisture cannot be
detected.
Sample totally degrades due to high
heat and cannot be reused.
Chemical Method of Water Determinat ion
Karl Fischer Titration
Theory
Karl Fischer Titration method is the
most widely used and accepted method
of determination of water in the field of
pharmaceutical analysis.
The fundamental principle behind it is
based on the Bunsen reaction
between I2 and SO2 in an aqueous
medium.
H 2O+SO2+ I 2↔2HI +SO 3
Karl Fischer Reagent
Karl Fischer Reagent is the titrant used
in the Karl Fisher Reagent to determine
the amount of H2O in the Sample /
Analyte.
Composition of Karl Fischer Reagent
Karl Fischer Reagent is a mixture
containing the following
components –
Table 1: Composition of Karl Fischer
Reagent
Component Amount
Iodine 125 gm
Anhydrous Pyridine 170 ml
Anhydrous Methanol
670 ml
Liquid Sulfur Dioxide 100 ml
Recently, the pyridine has been
replaced due to its objectionable
odor by amine compounds such as
Imidazole.
7 | P a g e
Figure 3: Pyridine & Imidazole
1 ml of Karl Fischer Reagent is
equivalent to 5 mg (or 3 – 6 mg)
water.
Problem with Karl Fischer Reagent and the Solution
Problem: If all of the
components of Karl Fischer
Reagent are premixed way long
before titration, then
components reacts with each
other resulting in numerous
side reactions and as a
consequence, the effectiveness
of the reagent reduces.
Solution: This problem can be
overcome by adding the Liquid
SO2 to the stock of solution
containing I2, Anhydrous
Pyridine and Anhydrous
Methanol promptly before
performing the titration.
Chemistry of Karl Fischer Titration
In presence of water from the sample I2
and SO2 from Karl Fischer Reagent will
be reduced and oxidized respectively in
the following manner –
H 2O+SO2+ I 2↔2HI +SO 3
This reversible and slow reaction
becomes unidirectory and rapid by the
large quantity of pyridine present in the
Karl Fischer Reagent which can form
complexes with I2 and SO2 as C5H5N.I2
and C5H5N.SO2 respectively and the
reaction will occur in the following
manner –
8 | P a g e
A Freshly prepared Reagent has strength
about 80% of theoretical value.
A 1 month old reagent has 50% strength.
A 3 months old reagent has 40%
strength.
Primary Condition of Titration
The primary condition of
performing titration is that all
reactions must be -
Irreversible or Unidirectory
Rapid
The formed Pyridinium Sulfite, an
inner salt reacts with water from
sample giving Pyridinium Salt of
Hydrogen Sulfate. As a result, H2O
does not react with iodine in the main
Bunsen reaction.
Presence of large amount of
Anhydrous Methanol in the Karl
Fischer Reagent prevents the reaction
of water with Pyridinium sulfite by
forming Pyridinium salt of methyl
sulfate.
9 | P a g e
Justification for Using Pyridine and Methanol in Karl Fischer
Reagent
Justification for Using Anhydrous Pyridine
In presence of water from the
sample, I2 and SO2 from Karl Fischer
Reagent will be reduced and oxidized
respectively in the following manner
–
H 2O+SO2+ I 2↔2HI +SO 3
Large quantity of pyridine present in
the Karl Fischer Reagent forms
complexes with I2 and SO2 as C5H5N.I2
and C5H5N.SO2 respectively. As a
result, the reaction becomes –
Irreversible or Unidirectory
Rapid
the reaction will occur in the
following manner –
For this, Anhydrous Pyridine is used in
the Karl Fischer Reagent.
Classification of Karl Fischer Titration
Karl Fischer Titration can be of 2 type –
1. Volumetric Karl Fischer Titration
2. Coulometric Karl Fischer Titration
Volumetric Karl Fischer Titration
Volumetric Karl Fischer Titration can be
defined as a method of titration where
an exact volume of Karl Fischer Reagent
is consumed during the course of
titration from which equivalent amount
of Water present in the sample can be
detected; i.e. 1 ml of Karl Fischer
Reagent is equivalent to 5 mg (or 3 –
6 mg) water.
End – point in this titration is detected
by color change in the solution.
Volumetric Karl Fischer Titration can be
of 2 type –
1. Volumetric Karl Fischer Direct
Titration
2. Volumetric Karl Fischer Back
Titration
Purpose of Performing Volumetric Karl Fischer Titration
Volumetric Karl Fischer Titration is
performed when the sample is not
colored.
Volumetric Karl Fischer Direct Titration
10 | P a g e
Justification for Using Anhydrous Methanol
Due to the presence of Pyridine in
the Karl Fischer Reagent, an inner
salt Pyridinium sulfite is formed.
Pyridinium sulfite reacts with water
from sample giving pyridinium salt
of hydrogen sulfate. As a result,
H2O does not react with iodine in the
main Bunsen reaction.
Presence of large amount of
Anhydrous Methanol in the Karl
Fischer Reagent prevents the
reaction of water with Pyridinium
sulfite by forming Pyridinium salt of
methyl sulfate.
For this, Anhydrous Methanol is used
in the Karl Fischer Reagent.
In Volumetric Karl Fischer Direct
Titration, sample is dissolved in
excess anhydrous methanol and the
solution is then filtered to remove
impurities.
Then, Karl Fischer Reagent is added
to the solution drop by drop with
the help of a burette; thus, forming
a pale yellow solution.
When all of the H2O present in the
solution will react with Karl Fischer
Reagent, the color of the solution
will suddenly change into Dark
Brown from pale yellow; thus
indicating the End – Point of the
Titration.
End – Point Detection (Volumetric Karl Fischer Direct Titration)
An End – Point in Volumetric
Karl Fischer Direct Titration can
be observed visually based on
the color change from Pale
Yellow to Dark Brown color of
the excess Karl Fischer Reagent.
Figure 4: Diagrammatic Representation of
Detection of End – Point in Volumetric Karl Fischer
Direct Titration
Volumetric Karl Fischer Back Titration
In Volumetric Karl Fischer Back
Titration, at first the sample is
mixed with excess Karl Fischer
Reagent giving the solution a Dark
Brown color of excess Karl Fischer
Reagent, since all of the water in
the sample has already reacted with
the Karl Fischer Reagent.
Then, A Standard Water – in –
Methanol is added drop by drop to
the solution with the help of
burette.
When all of the H2O – in – Methanol
reacts with Excess Karl Fischer
Reagent, the color of the solution
11 | P a g e
End - Point
Addition of Karl Fischer Reagent drop by drop
Colorless Solution of Sample in Anhydrous Methanol
No Color
Formation of Pale Yellow Liquid
Change of Clolor from Pale Yellow
to Dark Brown
will suddenly change into Pale
Yellow from Dark Brown; thus
indicating the End – Point of the
Titration.
End – Point Detection (Volumetric Karl Fischer Back Titration)
An End – Point in Volumetric
Karl Fischer Back Titration can
be observed visually based on
the color change from Dark
Brown of excess Karl Fischer
Reagent into Pale Yellow.
Figure 5: Diagrammatic Representation of
Detection of End – Point in Volumetric Karl Fischer
Back Titration
Purpose of Performing Volumetric Karl Fischer Back Titration
Purpose of Performing
Volumetric Karl Fischer Back
Titration is to determine the
accuracy of the Volumetric Karl
Fischer Direct Titration.
After the reaction in Volumetric
Karl Fischer Direct Titration is
complete, the excess amount of
Karl Fischer Reagent is
determined by titration with a
standard H2O – in – Methanol
Solution.
The Actual Amount of Karl
Fischer Reagent reacting with
desired amount of Water is
calculated by subtracting the
volume consumed in the Back
Titration from the volume
added in the Direct Titration.
Actual Amount of Karl Fischer Reagent
ReactingwithH 2O=Volume of Karl Fischer Reagent
Added∈the Direct Titration−¿
Volumeof Karl Fischer Reagent
Consumed∈theBack Titration
Coulometric Karl Fischer Titration
Coulometric Karl Fischer Titration can
be defined as a method of titration
where an exact volume of Karl Fischer
12 | P a g e
End - point
Addition of Standard Water - in - Methanol
Addition of Karl Fischer Reagent in Excess Amount
Solid Sample No Color
Dark Brown Color of Excess Karl
Fischer Reagent
Reaction of Water with Excess Karl
Fischer Reagent
Sudden Color Change into Pale Yellow from Dark
Brown
Reagent is consumed during the course
of titration from which equivalent
amount of Water present in the sample
can be detected; i.e. 1 ml of Karl
Fischer Reagent is equivalent to 5 mg
(or 3 – 6 mg) water.
End – point in this titration is detected
by sudden change in the electricity
current flow.
Coulometric Karl Fischer Titration can
be of 2 type –
1. Coulometric Karl Fischer Direct
Titration
2. Coulometric Karl Fischer Back
Titration
Purpose of Performing Coulometric Karl Fischer Titration
Coulometric Karl Fischer Titration is
performed when the sample is
colored.
As a result the End – Point cannot
be detected by color change.
Instruments for Coulometric Karl Fischer Titration
The Coulometric Karl Fischer
Titration vessel is fitted with 1.5 –
2.9 V Dry cell across a variable
resistance of about 2000 which is
in series with two platinum
electrodes and a microammeter,
mechanical stirrer and a burette.
Figure 6: Instruments for Coulometric Karl Fischer Reaction
13 | P a g e
Coulometric Karl Fischer Direct Titration
In Coulometric Karl Fischer Direct
Titration, at first sample is dissolved
in excess anhydrous methanol and
the colored solution is then filtered
to remove impurities.
Then, Karl Fischer Reagent is added
to the solution drop by drop with
the help of a burette.
As a result the following reaction
occurs, assuming that pyridine is
present in the Karl Fischer Reagent
making the reaction rapid and
unidirectory.
H 2O+SO2+ I 2→2HI +SO 3
Under these conditions, a constant
low voltage of current flows within
the circuit observed through the
microammeter due to the presence
of only Iodide ion (I–) from HI in the
reaction medium.
2HI→2H+¿+2 I−¿¿¿
When all of the H2O water reacts
with reagent, free I2 enters the
system and reversible Iodine /
Iodide couple / (I2/I–) completes the
circuit resulting in a sudden
increase in the flow of current,
which indicates the End – point of
the Titration.
End – Point Detection (Coulometric Karl Fischer Direct Titration)
Figure 7: Diagrammatic Representation of End -
point Detection in Coulometric Karl Fischer Direct
Titration
An End – Point in Coulometric
Karl Fischer Direct Titration can
be observed visually on the
Microammeter when the flow
of current suddenly increases to
maximum from very low
current flow.
14 | P a g e
End - Point / Kick-off point
Addition of Karl Fischer Reagent drop by drop
Colored Solution of Sample in Anhydrous Methanol
Colored Solution
Flow of low voltage of current within the
circuit observed through the microammeter due to the presence of only
Iodide Ion
Maximum Increase in flow of Current from
Very Low Current due to the presence of I2/I-
Couple
This point is also called Kick –
off Point.
Coulometric Karl Fischer Back Titration
In Coulometric Karl Fischer Back
Titration, at first the sample is
mixed with excess Karl Fischer
Reagent. As a result –
All of the water in the sample
reacts with the Karl Fischer
Reagent.
The reaction medium contains
I2/I– Couple due to the
presence of free excess Iodine
and the Iodide ion as a result of
reaction between iodine and
water.
The Microammeter shows
Maximum Flow of Current.
Then, A Standard Water – in –
Methanol is added drop by drop to
the solution with the help of
burette.
When all of the H2O – in – Methanol
reacts with Excess Karl Fischer
Reagent, the excess Iodine is
reduced into Iodide ion.
As a result, there will be a sudden
drop of flow of current from the
maximum; thus indicating the End –
Point of the Titration.
End – Point Detection (Coulometric Karl Fischer Back Titration)
Figure 8: Diagrammatic Representation of End –
Point Detection in Coulometric Karl Fischer Back
Titration
An End – Point in Coulometric
Karl Fischer Back Titration can
15 | P a g e
End - point / Dead Stop Point
Addition of Standard Water - in - Methanol
Presence of I2/I– Couple due to free excess Iodine and the Iodide ion in Reaction Medium
Addition of Karl Fischer Reagent in Excess Amount
Solid Sample No Color
Reaction of Water and Karl Fischer Reagent
Maximum Flow of Current
Reaction of Water with Excess Karl Fischer
Reagent
Sudden drop of Flow of Current
be observed visually on the
Microammeter when the flow
of current suddenly drops down
from maximum current flow.
This point is also called Dead
Stop Point.
Comparison between Different Karl Fischer Titration (KFT) Method
Table 2: Comparison between Different Karl Fischer Titration (KFT) Method
Types of KFT
Volumetric KFT Coulometric KFT
General
End point Detection is depended on Color Change
End point Detection is depended on Change of flow of current
Performed only for colorless samples Performed for both colored and colorless samples
Process is manual Instruments used is simple
Process is automated Instruments used is complex
Direct Titration
Initial Solution of Sample and Anhydrous Methanol is colorless
Addition of Karl Fischer Reagent Drop by Drop forms a pale yellow liquid in the solution
At End – point, Pale Yellow Color suddenly changes into Dark Brown Color of Karl Fischer Reagent.
End – point appears when all of the Water reacts with Karl Fischer Reagent to show the Dark Brown Color of the Reagent.
End – point has no other synonym.
Initial Solution of Sample and Anhydrous Methanol is colored
Addition of Karl Fischer Reagent Drop by Drop results in a constant flow of current at low voltage
At End – point, Flow of current suddenly changes into maximum flow of current.
End – point appears when all of the Water reacts with Karl Fischer reagent, so that excess Free Iodine and Iodide Ion forms (I2/I–) Couple.
End – point is called Kick – off Point.
Back Titration
Use of Methanol to dissolve the sample is not necessary
Titrant is the Standard Water – in – methanol
Excess Karl Fischer Reagent is used to dissolve the sample
Solution forms a dark brown color liquid at initial stage, because all of the H2O reacts with the Karl Fischer Reagent.
At End – point, Dark Brown Color of Karl Fischer Reagent changes into Pale Yellow Color suddenly.
End – point has no other synonym.
Use of Methanol to dissolve the sample is not necessary
Titrant is the Standard Water – in – methanol.
Excess Karl Fischer Reagent is used to dissolve the sample
Solution shows a max flow of current at initial stage, because excess Free Iodine and Iodide Ion forms (I2/I–) Couple and completes the circuit
At End – point, Flow of current suddenly changes from maximum flow of current into minimum.
End – point is called Dead Stop Point.
16 | P a g e
Advantages of Karl Fischer Titration
Karl Fischer Titration Method is very
popular for a large practical advantage
which it holds over other moisture
determination technique,
These includes –
Easy Sample Preparation
High Accuracy and Precision
Independence of presence any volatile
materials
Nearly Unlimited Measuring Range
Selectivity for Water
Short Analysis Duration
Small Sample Quantities required
Suitability for analyzing –
Solids
Liquids
Gases
Suitability for Automation
Limitations of Karl Fischer Titration
1. Presence of Compound which reacts with
Iodine or Iodide will interfere in the
process. E.g. –
Ascorbic Acid will be oxidized by Iodine
present in the reagent.
Quinone will be reduced by Iodine
formed during the reaction.
2. Carbonyl Compounds can react with
Methanol to form Acetals of Ketals and the
liberation of water.
3. The optimal pH range for the Karl Fischer
Reaction is 5 – 8. So, highly acidic / basic
samples need to be buffered to bring the
overall pH into this range.
17 | P a g e