2. Introduction
A chemical test is a qualitative or quantitative
procedure designed to prove the existence of, or
to quantify, a chemical compound or chemical
group with the aid of a specific reagent.
A presumptive test is specifically used in
medical and forensic science.
3.
Pharmaceutical analysis is the quantitative
measurement of the active ingredient and related
compounds in the pharmaceutical product 1].
These determinations require the highest
accuracy, precision, and reliability because of the
intended use of the data as in:
1. Manufacturing control (identify drug in
formulation),
4. 2. Stability evaluation (determine the impurity
and degradation products in the stability study),
and shelf-life prediction.
3. Determination of drugs and their metabolites in
biological samples, generally plasma or urine, is
important in elucidation of drug metabolism
pathways as well as comparing bioavailability
of different formulas.
5.
There are several methods used in chemical
analysis starting from simple manual
method to complicated and sophisticated
ones of these are: titration,
spectrophotometric ,HPLC with multi
detectors ,……….etc
6.
Titration
Titration involves the addition of a reactant
to a solution being analyzed until some
equivalence point is reached. Most familiar
one is the acid-base titration involving a
color changing indicator. There are many
other types of titrations, for example
potentiometric titrations.
7.
1.
Applications :
Provide standard pharmacopeial methods for
the assay of unformulated drugs and
excipents and some formulated drugs e.g.
those lack strong chromophore
2.
Used for standardization of raw materials and
intermediates used in drug synthesis.
3.
Certain specialist titration such as Karl
Fischer
8.
1.
Advantages:
Capable of higher degree of precision and
accuracy.
2.
The method are generally robust
3.
Analysis can be automated
4.
Cheap to do and not require specialized
apparatus
9.
Limitations:
1.
Non selective
2.
Time consuming if not automated and
require greater level of operator skill
3.
Require large amount of sample
4.
Reaction of standard solution should be rapid
and complete 1]
10. Procedure:
A typical titration begins with a beaker or flask
containing a precise volume of the titrand and a
small amount of indicator placed underneath a
calibrated burette containing the titrant. Small
volumes of the titrant are then added to the
titrand and indicator until the indicator changes,
reflecting arrival at the endpoint of the titration.
11.
Depending on the endpoint desired, single
drops or less than a single drop of the titrant
can make the difference between a
permanent and temporary change in the
indicator. When the end point of the
reaction is reached, the volume of reactant
consumed is measured and used to calculate
the concentration of analyte
12. Preparation techniques
Typical titrations require titrant and analyte to
be in a liquid (solution) form. Though solids
are usually dissolved into an aqueous
solution, other solvents such as glacial acetic
acid or ethanol are also used. [2]
Concentrated analytes are often diluted to
improve accuracy.
13.
Many non-acid-base titrations require a
constant pH throughout the reaction. Therefore
a buffer solution may be added to the titration
chamber to maintain the pH.[3]
In instances where two reactants in a sample
may react with the titrant and only one is the
desired analyte, a separate masking solution
may be added to the reaction chamber which
masks the unwanted ion.[4]
14.
Some redox reactions may require heating
the sample solution and titrating while the
solution is still hot to increase the reaction
rate. For instance, the oxidation of some
oxalate solutions requires heating to 60 °C
(140 °F) to maintain a reasonable rate of
reaction.[5]
15. Titration curves
A titration curve is a curve in the plane whose xcoordinate is the volume of titrant added since
the beginning of the titration, and whose ycoordinate is the concentration of the analyte at
the corresponding stage of the titration (in an
acid-base titration, the y-coordinate is usually
the pH of the solution).[6]
16.
In an acid-base titration, the titration curve
reflects the strength of the corresponding acid
and base. For a strong acid and a strong base,
the curve will be relatively smooth and very
steep near the equivalence point. Because of
this, a small change in titrant volume near the
equivalence point results in a large pH change
and many indicators would be appropriate (e.g.
phenolphthalein or bromothymol blue).
18.
If one reagent is a weak acid or base and the
other is a strong acid or base, the titration curve
is irregular and the pH shifts less with small
additions of titrant near the equivalence point.
For example, the titration curve for the titration
between oxalic acid (a weak acid) and Na OH
(a strong base), the equivalence point occurs
between pH 8-10,
19.
Indicating the solution is basic at the
equivalence point and an indicator such as
phenolphthalein would be appropriate.
Titration curves corresponding to weak bases
and strong acids are similarly behaved, with the
solution being acidic at the equivalence point
and indicators such as methyl orange and
bromothymol blue being most appropriate.
21.
Titrations between a weak acid and a weak
base have titration curves which are highly
irregular. Because of this, no definite
indicator may be appropriate and a pH meter
is often used to monitor the reaction.[7]
The type of function that can be used to
describe the curve is called a sigmoid
function.
23. Endpoint
and equivalence point
Although equivalence point and endpoint are
used interchangeably, they are different terms.
Equivalence point is the theoretical completion
of the reaction: the volume of added titrant at
which the number of moles of titrant is equal to
the number of moles of analyte, or some
multiple thereof (as in polyprotic acids).
24.
Endpoint is what is actually measured, a
physical change in the solution as determined
by an indicator or an instrument mentioned
above.[8]
There is a slight difference between the
endpoint and the equivalence point of the
titration. This error is referred to as an
indicator error, and it is indeterminate.[9]
25. Types of titrations:
There are many types of titrations with
different procedures and goals. The most
common types of quantitative titration are
acid base titrations and redox titrations.
26. 1. Acid base titrations
Acid-base titrations depend on the neutralization
between an acid and a base when mixed in solution.
In addition to the sample, an appropriate indicator
is added to the titration chamber, reflecting the pH
range of the equivalence point. The acid-base
indicator indicates the endpoint of the titration by
changing color.
27. The following table show some
indicators used with their pH ranges
and colors of each :
Indicator
Color on acidic side
Range of pH color change
Color on basic side
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
4.4–6.3
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
28.
The endpoint and the equivalence point are
not exactly the same because the equivalence
point is determined by the stoichiometry of
the reaction while the endpoint is just the
color change from the indicator. Thus, a
careful selection of the indicator will reduce
the indicator error.
29.
For example, if the equivalence point is at a
pH of 8.4, then the Phenolphthalein indicator
would be used instead of Alizarin Yellow
because phenolphthalein would reduce the
indicator error
When more precise results are required, or
when the reagents are a weak acid and a weak
base, a pH meter or a conductance meter are
used.
30. 2. Redox titration
Redox titrations are based on a reduction-oxidation
reaction between an oxidizing agent and a reducing
agent. A potentiometer or a redox indicator is usually
used to determine the endpoint of the titration, as
when one of the constituents is the oxidizing agent
potassium dichromate, the color change of the
solution from orange to green is not exact, therefore
an indicator such as sodium diphenylamine is used.
31.
Some redox titrations do not require an
indicator, due to the intense color of the
constituents. For example, in permanganometry
a slight faint persisting pink color signals the
endpoint of the titration because of the color of
the excess oxidizing agent potassium
permanganate.[10]
32. 3.Gas phase titration
Gas phase titrations are titrations done in the gas
phase, specifically as methods for determining
reactive species by reaction with an excess of
some other gas, acting as the titrant. In one
common the gas phase titration, gaseous ozone is
titrated with nitrogen oxide according to the
reaction:
O3 + NO → O2 + NO2.[11][12]
33.
After the reaction is complete, the
remaining titrant and product are
quantified (e.g., by FT-IR); this is used to
determine the amount of analyte in the
original sample.
34. 4. Complexometric titration
Complexometric titrations rely on the formation
of a complex between the analyte and the titrant.
In general, they require specialized indicators
that form weak complexes with the analyte.
Common examples are Eriochrome Black T for
the titration of calcium and magnesium ions, and
the chelating agent EDTA used to titrate metal
ions in solution.[13]
35. 5. Back titration
Back titration is a titration done in reverse;
instead of titrating the original sample, a known
excess of standard reagent is added to the
solution, and the excess is titrated. A back
titration is useful if the endpoint of the reverse
titration is easier to identify than the endpoint of
the normal titration, as with precipitation
reactions.
36.
Back titrations are also useful if the reaction
between the analyte and the titrant is very
slow, or when the analyte is in a non-soluble
solid.[16]
37. Measuring the endpoint of a titration
There
are different methods to determine the
endpoint include:[17]
1.
Indicator: A substance that changes color in
response to a chemical change. An acid-base
indicator (e.g., phenolphthalein) changes color
depending on the pH. Redox indicators are also
used. A drop of indicator solution is added to
the titration at the beginning; the endpoint has
been reached when the color changes.
38. 2. Potentiometer: An instrument that measures
the electrode potential of the solution. These are
used for redox titrations; the potential of the
working electrode will suddenly change as the
endpoint is reached.
The pH meter is a potentiometer with an
electrode whose potential depends on the
amount of H+ ion present in the solution. (This
is an example of an ion-selective electrode.)
39. 3. Conductivity: A measurement of ions in a
solution. Ion concentration can change
significantly in a titration, which changes the
conductivity. (For instance, during an acid-base
titration, the H+ and OH- ions react to form
neutral H2O.) As total conductance depends on
all ions present in the solution and not all ions
contribute equally (due to mobility and ionic
strength).
40. 4. Color change: In some reactions, the solution
changes color without any added indicator. This is
often seen in redox titrations when the different
oxidation states of the product and reactant produce
different colors.
5. Spectroscopy: Used to measure the absorption of
light by the solution during titration if the spectrum of
the reactant, titrant or product is known. The
concentration of the material can be determined by
Beer's Law.
41. 6. Precipitation: If a reaction produces a solid, a
precipitate will form during the titration. A
classic example is the reaction between Ag+ and
Cl- to form the insoluble salt AgCl. Cloudy
precipitates usually make it difficult to
determine the endpoint precisely. To
compensate, precipitation titrations often have to
be done as "back" titrations .
42. 7. Amperometry: Measures the current
produced by the titration reaction as a result of
the oxidation or reduction of the analyte. The
endpoint is detected as a change in the current.
This method is most useful when the excess
titrant can be reduced, as in the titration of
halides with Ag+.
43. 8. Isothermal titration calorimeter :
An instrument that measures the heat produced
or consumed by the reaction to determine the
endpoint. Used in biochemical titrations, such
as the determination of how substrates bind to
enzymes.
44. 9. Thermometric titrimetry:
Differentiated from calorimetric titrimetry
because the heat of the reaction (as indicated
by temperature rise or fall) is not used to
determine the amount of analyte in the
sample solution. Instead, the endpoint is
determined by the rate of temperature
change.
