9.LIMIT TESTS Revised.ppt

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
2

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.
3

 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
4

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.
5

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
6

The method should be:
 Specific- selective reaction with the trace impurity
 Sensitive-yield reproducible results
 The methods:
General limit test
Specific limit test
7

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.
8

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.
9

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
10

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.
11

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.
12

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
13

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
14

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
15

 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
16
Limit tests for specific impurities…

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
17

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%
18

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%.
19

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.
20

 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.
21

Cont…
 Methods of moisture content determination
Loss on drying method
Karl- Fischer titration method
Other methods
22

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
23

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:
24
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2
5
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3
3
5
5
3
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5
5
5
5
2
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2
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)
(
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(
2
3
OCH
SO
O
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N
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OH
CH
SO
N
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C
SO
N
H
C
I
H
N
H
C
N
H
C
SO
I
O
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
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

 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
25
Karl Fischer Method …

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.
26
Karl Fischer Method …

9.LIMIT TESTS Revised.ppt

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