This document describes a test to detect the presence of arsenic in medicinal substances. The test works by reducing any arsenic in the sample to arsine gas using zinc, acid, and other reducing agents. The arsine gas is carried through a glass tube to mercuric chloride paper, where it will produce a yellow stain if arsenic is present. The intensity of the stain is compared to standard stains produced using known quantities of arsenic to determine if the level of arsenic passes the limit test. The test is run for 40 minutes and any yellow stain less intense than the standard passes, indicating the level of arsenic is below the limit.

1. LIMIT TEST FOR ARSENIC

5. PRINCIPLE
• Arsenic is a well-known undesirable and harmful impurity which is
present in medicinal substances.
• Pharmacopoeial method is based on the Gutzeit test
• Arsenic in the arsenious state can be easily reduced to arsine gas (AsH3).
When this gas is passed over mercuric bromide paper, it produces a
stain, which ranges in colour from yellow to brown, the intensity and
length of which are proportional to the amount of arsenic.
• B.P Suggests the use of mercuric chloride paper instead of mercuric
bromide paper.

8. • Reduction of the arsenic to arsine, both in the standard and the sample may
be achieved by the combined actions of zinc, acid, stannous chloride,
potassium iodide.
• The arsine is carried over by hydrogen to the mercuric bromide or mercuric
chloride paper which are supported in test apparatus.
• When the sample is dissolved in acid, the arsenic present in the sample gets
converted to arsenic acid which gets reduced, by reducing agents (like
potassium iodide, stannous acid etc.) to arsenious acid. The nascent
hydrogen formed during the reaction, further reduces arsenious acid to
arsine(gas), which reacts with mercuric chloride paper, giving a yellow stain.
• The depth of yellow stain on mercuric chloride paper will depend upon the
quantity of arsenic present in the sample.

10. APPARATUS

13. APPARATUS
 It is having a wide mouthed glass bottle of 120 ml capacity having mouth of
about 2.5cm in diameter. This bottle is fitted with a rubber bung through
which passes a glass tube, 20cm long having external diameter of about
0.8cm and internal diameter of 0.65cm.
 The tube is constricted at its lower end extremity to about 1mm diameter
and there is blown a hole, not less than 2 mm in diameter, in the side of
the tube near the constricted part.
 The upper end of the glass tube has been fitted with two rubber bungs,
each having hole bored centrally and exactly 6.5 mm in diameter.
 One of the bungs has been fitted to the upper end of the tube, while the
second bung has to be fitted upon the first bung in such a way that
mercuric chloride paper gets exactly sandwiched between the center
perforation of two.

14. APPARATUS
 The bungs are kept in close contact by using rubber band or spring clip in
such a manner that gas evolved from bottle must have to pass through
the 0.65mm internal circle of mercuric chloride paper.

15. PROCEDURE
 The glass tube is first of lightly packed with cotton wool, which previously
moistened with solution of lead acetate and dried (because H2S will be
trapped by lead acetate otherwise it would stain the mercuric chloride
paper).
 The upper end of the tube has been inserted into the narrow end of one
of the bungs and the two bungs secured by using the rubber band after
keeping the mercuric chloride paper in between them.
 The solution to be examined and prepared as specified usually 5ml from
1g of substance is kept in wide mouthed bottle.
 To this 1g of KI AsT, 5ml of stannous chloride acid solution and 10g of zinc
AsT are added.

16. PROCEDURE
 The glass tube is kept in position quickly. The action is allowed to
continue for 40 minutes. A yellow stain which gets produced on the HgCl2
paper, if arsenic is present has been compared by daylight with standard
stains which are produced by operating in a similar manner with known
quantitites of dilute arsenic solution AsT.
STANDARD STAINS:
 Standard stains are produced by using dilute arsenic solution. Arsenic
solution strong AsT is having 0.132g of As2O3 per 100ml solution.
 One ml of above solution is diluted with water to make 100ml of dilute
arsenic solution, one ml of which would be having 0.00001g of As.
 If the sample under investigation shows a stain of lesser intensity than
that of the standard then it passes the test.