Limit test

Lets know the test for impurities with limitations

1. LIMIT TEST
By,
Revathi Gnanavelou, M.Pharm.,
Assistant Professor
Department of Pharmaceutical Chemistry
SVCP

2. LIMIT TEST
Limit tests are quantitative tests or semi-quantitative tests which are designed to detect
and limit / control small quantities of impurities present in the substance.
All the limit tests that are prescribed in the pharmacopoeias are based on the comparison of
standard turbidity or colour with that of the sample under test.
Usually the limits are prescribed in parts per million (PPM).
For the preparation of standard turbidity or colour the pharmacopoeias prescribe the limit
of particular impurities for particular substances and it varies for different compounds.
The amount of test samples to be taken is mentioned in the individual monograph of the
pharmacopoeias.

3. 3 TYPES
•Testing as prescribed, there is no color, opalescence or precipitate
Negative test indicate the absences of Large impurity
Test In Which There Is No Visible Reaction
•Compare the amount of impurity in the substance with a standard of
known concentration and determine whether impurity is within or the
excess of the limit prescribed`
Comparison Methods
•Amount of impurity present in actually determined and compared with the
numerical limit given in Pharmacopoeia
Quantitative Determination

4. Quantitative Determination


Limits of soluble
matter
Limits of
Insoluble matter
Limits of
moisture and
volatile matter
Limits of residue
on ignition
Limits of non-
volatile matter
Loss on ignition
&
Ash values

5. 1. Limit Test for Chlorides
•A solution of the substance is acidified with nitric acid, diluted to definite volume
and treated with silver nitrate and the opalescence so produced is compared with
that of standard opalescence containing known amount of sodium chloride
solution.
Cl
-
+ AgNO
3 → AgCl + NO
3-
• Presence of nitric acid prevents the precipitation caused by silver carbonate or
silver hydroxide which may result due to alkaline impurities in the solution.

6. PRINCIPLE
•Limit test for chloride is based on the well-known reaction between silver
nitrate and soluble chlorides  forming precipitate of silver chloride which
is insoluble in nitric acid.
•The test solution becomes turbid, the extent of turbidity depending upon
the amount of silver chloride produced which in turn depends upon the
amount of chloride present in the test sample.

7. •The opalescence produced depends upon the amount of chloride present
in the given sample.
•It is compared with the opalescence produced in a standard solution
containing the known quantity of chloride similarly treated if the
opalescence in the sample is less that in standard. Its passes the test.
•If it is more, it fails the limit test which is done in Nessler’s cylinder and
viewed transversely against a black background.

8. EXPERIMENT

9. PROCEDURE
Standard Solution Test Solution
Dissolve chloride/unknown sample
5ml Distilled water
10 ml Dil.Nitric acid
Make up-to 50ml distilled water
1 ml of Silver nitrate solution
COMPARISON

10. 2. Limit Test for Sulphate
PRINCIPLE
•This is based on the reaction between barium chloride and soluble sulphates in
presence of dilute hydrochloric acid.
•An opalescence is produced by the precipitation of barium sulphate and compared
with the opalescence produced in a standard containing a known quantity of
sulphate and similarly treated.
SO
4
−
+ BaCl
2 → BaSO
4 + 2Cl
−

•Test substance passes the limit test, if the opalescence in it is less intense than that
in the standard. If opalescence is found to be more, then it fails the test.

11. •The barium sulphate reagent used contains barium chloride, sulphate free-
alcohol and traces of potassium sulphate
• The potassium sulphate increases the sensitivity of the test by giving
ionic concentrations in the reagent which just exceed the solubility
product of barium sulphate.
•Barium sulphate present in the reagent in a small quantity acts as a
seeding agent for precipitation of barium sulphate if sulphate is present in
the substance under test.
•Presence of alcohol helps to prevent super saturation and a more uniform
turbidity is formed.

12. COMPARISON

13. EXPERIMENT

14. 3) Limit Test for Iron
PRINCIPLE
•This depends upon the reaction of iron with thioglycolic acid in the
presence of citric acid and ammonia when a pale pink to deep reddish
purple colour is produced.
•Citric acid forms a complex with iron and prevents its precipitation by
ammonia.
•The colour produced is due to the formation of a ferrous compound with
thioglycolic acid.

15. •This is stable in excess of air and fades when exposed to air due to oxidation.
•The original state of oxidation is immaterial as thioglycolic acid is a reducing
agent and reduces ferric ion to ferrous.
•Only because of this advantage this test has been selected as the limit test, since
other tests such as the ammonium thiocyanate test give reaction with only one
type of iron like the ferric .
•The thioglycolic acid test is also considered to be more sensitive.
•Ferrous thioglycollate is colourless in acid or neutral solutions.
•The colour develops only in the presence of alkali.

16. The reactions are;

17. PROCEDURE

18. EXPERIMENT

19. 4. Limit Test for Heavy Metals
PRINCIPLE
•All Metals like Copper, Bismuth, Lead, Mercury, Arsenic, Antimony, Silver etc
(except alkali metals and alkaline earth metals) are coloured by sulphide ions (H
2S
or Na
2S) under specified conditions.
•The limit test for detecting and limiting the impurity of Heavy metals.
•The HM are precipitated as Metal sulphides by the addition of hydrogen sulphide
or sodium sulphide solution.
•Depends upon the quantity of the metal the colour varies from brown to
black.

20. •There are three methods are prescribed in I.P to determine the presence of heavy metals.
•The sample is dissolved in acid or alkali for making a solution.
•Method A and B are carried out in acid conditions with hydrogen sulphide reagent and
method C involves alkaline medium with the use of sodium sulphide reagent.
•Method A
 A solution of substance is adjusted to a pH 3 to 4 (by adding ammonia (or) acetic
acid) and hydrogen sulphide reagent is mixed with this
 And comparison of black colour produced with a standard colour containing a known
amount of lead.

21. •Method B (For Organic Compounds)
 The substance is ignited well in presence of conc. sulphuric acid and treated with
mixture of nitric and sulphuric acids. The resulting solution is digested with dilute
hydrochloric acid.
 Then extracted with hot water and proceeded as in method A.


•Method C
 The solution of the substance is treated with sodium hydroxide solution and sodium
sulphide reagent.
 Then it is compared with that a standard colour.

Pb
++
+ Na
2S → PbS + 2Na
+
Pb
++
+ H
2S → PbS + 2H
+

22. •Method D
 Is based upon the precipitation of relatively insoluble and
characterisitically coloured sulphides of heavy metals when aqueous solutions are
treated with alkali metal sulphides (NaSH).
 NaSH is generated immediately before use by heating thioacetamide with
sodium hydroxide solution.
 In this test, there is a formation of brown colour because of the
precipitation of metal sulphides at about pH 3.5 in colloidal form which is
stabilised by glycerine.

23. The test is done in two 50 ml Nessler’s cylinder, test is prepared in one and standard solution
is prepared in other cylinder.
Taking the prescribed volume of std lead solution and reagent solution is added in both , they
are diluted to the Mark and mixed after standing for 5 minutes and compared with colour
intensity.

24. 5. Limit Test for Arsenic
PRINCIPLE
•The limit test for arsenic is based on the reduction of the arsenic in the
arsenious state to the arsine gas (AsH
3) with zinc and hydrochloric acid.
•In this test arsenic impurities if at all present is converted in to arsine gas (ASH3)
which when contact with a mercuric chloride paper produces yellow stain.
Zn + 2HCl → ZnCl
2 + 2 [H]+
As
3+
 H
3AsO
4
(Impurity) (Arsenic acid)
•The intensity of the stain is proportional to the amount of arsenic present.
•Apparatus used for arsenic limit test is called Gutzeit apparatus

25. •A drug solution is prepared and placed in wide mouthed bottle, potassium iodide, zinc
dust, hydrochloric acid, stannous chloride are added into it.
•When the sample is dissolved in hydrochloric acid whereby arsenic present as impurity in
the sample gets converted to either arsenic or arsenious acid depending upon on its
valency state.
H
3ASO
4 + 2 [H] → H
3ASO
3 + H
2O
•Then it is further treated with reducing agents like stannous chloride, potassium iodide
etc., arsenic acid is reduced to form arsenious acid (pentavalent arsenic is reduced to the
trivalent state)
•Nascent hydrogen gas is generated by the presence of stannous chloride, hydrochloric
acid (stannated hydrochloride) and potassium iodide on arsenic free granulated zinc.

26. •The arsine gas produced in the bottle escapes through the tube and the
lead acetate impregnated cotton wool kept in the centre of the tube
entraps the hydrogen sulphide if any from the arsine gas.
H
3ASO
3 +6 [H] → ASH
3 + 3H
2O
•When arsine gas escapes through glass tube and reacts with mercuric
chloride paper kept in the clips and produces yellow stain. The reaction is
allowed to proceed for forty minutes maintained at 40
0
C.
2ASH
3 + HgCl
2 → Hg (ASH
3)
2 + 2HCl

27. •The presence of stannous chloride and hydrochloric acid ensures rapid
reaction between acid and potassium iodide and produces nascent
hydrogen gas.
•Depth of the yellow stain depends upon the amount of arsenic present in
the sample, which is less intense in colour when compared with that of
standard stain produced.
•It passes the limit test
•Permitted limit for arsenic is 1 PPM

32. THANK YOU
FOR YOUR
ATTENTION !!.....