The document describes procedures for limit tests to detect arsenic and lead impurities. For arsenic, the test uses Gutzeit apparatus to convert any arsenic present into arsine gas, which is detected by the formation of a yellow stain on mercuric chloride paper. For lead, the test uses dithizone to form a violet-colored lead dithizonate complex if lead is present, and the color is compared to a standard. Both tests involve extraction and color comparison to determine if the level of impurity exceeds the specified limit.

1. Limit test for arsenic and Lead
MrAshishR. Chaudhari
ASST. PROFESSOR, M.PHARM (QA)
P.R.PATIL, INSTITUTE OF PHARMACY, TALEGAON
(SP), DIST- WARDHA
Presented by :

2. • The original Gutzeit test was
quantitative one and the liberated
arsine gas is detected by covering the
mouth of the flask with filter paper,
on which small amount of
nitrate crystals were placed.
silver
The
liberated arsine turns silver nitrate
complex black.
Limit test for arsenic

3. • In the modified test, filter paper
dipped in mercuric chloride solution
is used, which yields a yellow colored
complex. It produces a stain in the
filter paper.

4. Principle
• The arsenic impurity present in the sample is
first converted into arsenious acid by the
action of reducing agents like potassium
iodide, Zinc, HCl and stannous chloride.

5. • The arsenious acid is then reduced to arsine
by the action of nascent hydrogen which is
produced by the reaction between Zinc and
HCl.
• The liberated arsine gas reacts with mercuric
chloride paper and forms yellow stain

6. Gutzeit apparatus

7. Procedure
• The required amount of sample solution is placed in
the wide mouthed bottle.
• To this add 1 g of potassium iodide and 10 g of
granulated zinc AST.
• Immediately close the bottle with stopper assembly.
• The acid present in the test solution and zinc reacts,
produces nascent hydrogen which reacts with
arsenious acid present in the sample and liberates
arsine gas which passes through the tube and reacts
with mercuric chloride paper and produces yellow
stain.

8. • The reaction is maintained for 40 minutes to
ensure complete conversion of arsenious acid
to arsine gas.
• Some heat pressure are produced during the
reaction due to the increased amount of gas in
the inside of the apparatus.
• The rate of reaction is increased by placing the
total assembly on a warm surface.

9. Standard of standard stain
• Standard stain is prepared by using dilute
arsenic solution [ 0.132 g of arsenic in 100 ml]
• I ml is diluted to 100 ml .
• The comparison of the stain is done after the
completion of the test immediately, so the
test and standard should be carried out
simultaneously.

10. Precaution
• If the stain present in the filter paper becomes
dark, the test should be repeated by using pure
reagents.
• The most suitable temperature for carry out this
test is 40C.
• Cotton wool dipped in lead acetate solution is
used to trap any hydrogen sulphide gas liberated
with arsine gas.
• Care must be taken that the filter paper remains
quite dry during the reaction.

11. • During the succeeding tests the tube must be
washed with HCl AsT rinsed with water and
dried.
• All the reagents used for this test should be
free from arsenic and mentioned as AsT.

12. LIMIT TEST FOR LEAD
The Lead Limit Test is designed to determine the allowable limit of heavy
metal i.e. Lead contained in a sample.
• Lead is a most undesirable impurity in medical compounds and comes
through use of sulphuric acid, lead lined apparatus and glass bottles
used for storage of chemicals.
• Common sources of
lead impurities are:
Equipment used for
manufacturing.
Storage container.
From packaging material

13. • Principle
• Limit test of lead is based on the reaction of lead and diphenyl
thiocarbazone (dithizone) in alkaline solution to form lead dithizonate
complex which is violet in color.
• Lead present as an impurity in the sample reacts with dithizone to form
lead thizonate which is responsible for the appearance of violet color in
the solution. This reaction will only occur if the given sample contains
lead as an impurity even in trace amount.

14. • In this test, Nessler cylinders are not used, instead it is performed by
extraction with the
help of separating funnel.
• The original color of dithizone in chloroform is green while the lead
dithizonate complex is violet in color. The intensity of the violet color of
the complex depending upon the quantity of lead present in the solution
is compared with that of the standard color produced by treating
standard solution containing definite amount of lead in the similar
manner. Intensity of this violet color is compared in both the tests and
standard solution in a chloroform solvent medium.

16. PROCEDURE
TEST SAMPLE STANDARD SAMPLE REASON
Take a sample and transferred in
separating funnel
Take standard lead solution and
transferred in separating funnel.
To check impurity of lead present in
sample or not
Add 6ml Ammonium citrate Add 6ml Ammonium citrate To maintain optimum pH
Add 2ml potassium cyanide and
2ml hydroxylamine hydrochloride
Add 2ml potassium cyanide and
2ml hydroxylamine hydrochloride
Cyanide for complex with all
interfering metal in solution
Add 2 drop phenol red Add 2 drop phenol drop Act as indicator to develop color
Make solution alkaline by adding
ammonia
Make solution alkaline by adding
ammonia
It provide basic condition to
develop red color
Extract with 5ml dithizone until it
become green
Extract with 5ml dithizone until it
become green
At least 2-3 extraction done and
mix in the end in separating funnel
Combined dithizone extracts are
shaken for 30 mins with 30 ml of
nitric acid and the chloroform layer
is discarded
Combined dithizone extracts are
shaken for 30 mins with 30 ml of
nitric acid and the chloroform layer
is discarded
Acidic layer is separated from
chloroform . Nitric acid make lead
free from chloroform layer

17. To the acid solution add 5ml
standard dithizone
To the acid solution add 5ml
standard dithizone
Violet color produced due to lead-
dithizonate
Add 4ml ammonium cyanide Add 4ml ammonium cyanide Maintain optimum pH
Shake for 30 minutes and observe
the color
Shake for 30 minutes and observe
the color
Compare colour

18. REAGENT PREPARATIONS:
1. Preparation of standard lead solution (1 ppm Pb): Dissolve 0.4 g of lead nitrate in
water containing 2 ml of dilute nitric acid and add sufficient water to produce
250.0 ml. This gives standard lead solution (1% Pb).
2. Standard lead solution (1 ppm Pb) is prepared by diluting 1 volume of standard lead
solution (1%
Pb) to 1000 volumes with water.
3. Preparation of dithizone extraction solution: Dissolve 30 mg of dithizone in 1000 ml of
chloroform and add 5 ml of ethanol (95%). The solution is stored in refrigerator. Before
use, the solution is shaken with about half of its volume of 1% v/v nitric acid solution
and the acid is discarded.
4. Preparation of dithizone standard solution: Dissolve 10 mg of dithizone in 1000 ml of
chloroform.

19. CONCLUSION
• If color produced in the sample solution is less than the standard
solution, the sample will pass the limit test of lead and vice versa..