2. Outline
Introduction to limit test
Explain the types of limit test
General impurity limit test
Specific impuriity limit test
Describe the limit test for ash values
Describe the limit test for moisture or water content
Describe the procedure for heavy metal limit test
Describe the limit test for acid radicals
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3. Purity
The drug should be physically & chemically pure.
Purity is the state of a chemical compound when no impurity
can be detected or complete freedom from extraneous matter.
The standardization of ‘pharmaceutical chemicals’ and the
dosage forms prepared therefrom plays a vital role.
So that the patient gets the ‘drug’ within the permissible
limits of potency and tolerance.
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4. Drugs should:
Attain the highest attainable standards of purity
Elicit the desired biological response
Biological response of pharmaceutical substances holds equal
importance
Like that of chemical purity
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5. Impurities:
Impurities
Are unwanted chemicals present in the API
Can never be eliminated
Have no therapeutic value
Are potentially harmful
The total impurities should be in certain minimal range
i.e. there should be acceptance criteria for specified
impurities.
Therefore they need to be controlled.
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6. Cont…
The purity of pharmaceuticals can be ascertained through
purity test.
The tests for purity involve:
Tests for the presence of impurity----Identification test
Fix the limits of tolerance for these impurities---Quantification test
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7. The method should be:
Specific- selective reaction with the trace impurity
Sensitive-yield reproducible results
The methods:
General limit test
Specific limit test
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8. 1. General Impurity limit tests
For non-specific impurities
Clarity of solution
Measurement is made by direct comparison with a
reference solution
Solutions are clear if their clarity is the same as that of
water or the solvent used
e.g. Solutions for injections should be reasonably free
from particulate matter.
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9. General limit tests…
Color of solution
A solution may be described as colorless if it has the appearance as:
Water or
The solvent employed in the preparation of the solution being examined
Insoluble matter
The limit test is used to control small amounts of solvent-insoluble
contamination.
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10. Moisture, Volatile matter and residual solvents
A number of pharmaceutical substances usually absorb moisture on
storage thereby causing deterioration.
Such deterioration of hygroscopic substances is limited by a requirement
for the loss in weight (loss on drying).
When the substance is subjected to drying under specified conditions
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General limit tests…
11. Non-volatile matter
These limits are applied to substances which are readily
volatile
To control impurities such as dust in volatile substances
Eg. halothane, anaesthetic ether and hydrogen peroxide
solution.
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General limit tests…
12. Residue on ignition
Limits the amount of residue remained after the substance is
ignited.
Applicable to the ff two categories of pharmaceutical
substances:
Those which are completely volatile when ignited e.g., Hg.
Those which undergo total decomposition thereby leaving a
residue with a definite composition
e.g. Calamine — a basic zinc carbonate that gives rise to
ZnO as the residue.
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General limit tests…
13. Loss on ignition
Generally applied to relatively stable pharmaceutical substances that
are likely to contain thermolabile impurities.
E.g. From official compendia
Pharmaceutical Ignition Ignition Prescribed
Substance Temp. (°C) Time Limits(%)
Magnesium Trisilicate 900 TCW 17.0-34.0
Magnesium Sulphate 450-500 TCW 31.0-34.0
TCW-To Constant Weight
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General limit tests…
14. Ash values
Inorganic residue remaining after incineration.
Show the amount of inorganic substances present in crude drugs.
The ash values are applied :
To ensure the absence of an extraneous mineral matter
To detect adulteration
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General limit tests…
15. 2. Limit tests for specific impurities
limitation is placed for physiologically harmful impurities.
Limit Tests for Metallic Impurities
E.g.The control of toxic elements (As, Sb, Pb, Cd, Hg)
Specific tests for acid radical impurities
The most common acid radical impurities are chloride & sulphate
Also applied for a number of other acid radical contaminants
Cyanide, Oxalate, Phosphate and Silicate
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16. Limits on ash values
The ash values usually represent the inorganic residue present
in official herbal drugs and pharmaceutical substances.
These values are categorized into four heads:
(a) Ash Value (Total Ash)
(b) Acid-Insoluble Ash
(c) Sulphated Ash
(d) Water-Soluble Ash
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Limit tests for specific impurities…
17. i. Ash Value (Total Ash)
Ash value normally designates the presence of inorganic salts
E.g., calcium oxalate found naturally in the drug
Inorganic matter derived from external sources
ii. Acid-Insoluble Ash
Designed to measure the amount of ash insoluble to
diluted hydrochloric acid.
By treating the ‘total ash’ with acid
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Limit tests for specific impurities…
18. iii. Sulphated Ash
It is determined by a double ignition with conc. H2SO4,
metals thus remain as sulphides that are stable to heat.
The estimation of ‘sulphated ash’ is broadly employed in
the case of :
e.g. Activated Charcoal, NMT : 5.00%
Griseofulvin, NMT : 0.10%
Ascorbic acid, NMT: 0.1%
Cephalexin NMT : 0.20%
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Limit tests for specific impurities…
19. iv. Water-Soluble Ash
Water-soluble ash is specifically useful in detecting such
samples which have been extracted with water.
A typical example of an official drug is that of ‘Ginger’, the
water-soluble ash of which is found to be not more than
6.0%.
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Limit tests for specific impurities…
20. iv. Limits on moisture content
Moisture plays remarkable negative role in pharmaceutical
product, particularly for solid dosage forms.
Both physical and chemical stability of some drugs are affected
by moisture.
Moisture is absorbed on the surface of solid drugs and increases
the rate of decomposition.
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Limit tests for specific impurities…
21. Presence of moisture possesses a critical challenge on drug
stability.
Moisture accelerates:
The hydrolysis of drug
Facilitates reaction with other excipients
Affecting stability and shelf life of the final product.
Therefore, it is important to specify limits of moisture
content.
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Limit tests for specific impurities…
22. Cont…
Methods of moisture content determination
Loss on drying method
Karl- Fischer titration method
Other methods
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23. A. Loss on drying method
It is the loss of weight in % w/w resulting from water &
volatile matters that are lost under specified conditions.
Reflects the net weight of a pharmaceutical substance
being dried at a specified temp either at an atmospheric
or under reduced pressure.
Advantages
Easy to use, Relatively rapid
Many samples can be analyzed simultaneously
Disadvantages-Destructive, Time consuming
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24. B .Karl Fischer Method
The Water Determination Test (KF Method) is designed to
determine water content in solid substances
Utilizing the quantitative reaction of water with KF reagent.
KF reagent which is composed of iodine, sulphur dioxide,
pyridine and methanol.
Water present in the analyte reacts with the KF reagent as
shown below:
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25. One molecule of iodine disappears against each molecule of
water present in the given sample.
There are two determination methods:
Volumetric titration method
Coulometric titration method
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Karl Fischer Method …
26. The end point in a Karl Fisher titration can be located in several ways.
1.Visual detection
Before the end point the solution is yellow
After the end point the first excess of the reagent imparts the
characteristic brown I2 color
2.Coulometric detection
current shall tend to flow till free iodine exists, to remove hydrogen
and ultimately depolarize the cathode.
A situation will soon arise when practically all the traces of iodine
have reacted completely thereby setting the current to almost
zero or very close to zero or attain the end-point.
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Karl Fischer Method …