Limit Test for Arsenic with Apparatus and Reaction

Arsenic is a heavy metal that is very harmful. Besides, as an impurity, it also creates instability in pharmaceutical preparations.

However, it is impossible to avoid arsenic impurity from being present in the finished products. But, its presence in meager part per million is acceptable

So, it is essential to check the limits of arsenic impurity in the samples. This check for its limits is done by arsenic limit test.

Limit tests are semi-quantitative tests designed to identify small quantities of impurities in a given sample.

PRINCIPLE of Limit test for arsenic

The limit test for arsenic is performed in the Gutzeit test apparatus.

The principle is based upon the fact that when a sample is dissolved in an acid, the arsenic impurity present in it gets converted into arsenic acid depending upon its valency state. This, when further treated with a reducing agent like potassium iodide (KI), stannous chloride (H₂SnO₂), and zinc, the arsenic acid is reduced into arsenious acid (H₃AsO₃).

The nascent hydrogen formed during the reaction further reduces the arsenious acid (H₃AsO₃) to arsine gas (AsH₃).

This arsine gas when passed over the mercury bromide (HgBr₂), produces a stain that ranges in color from yellow to brown. The intensity and length of the color are proportional to the amount of arsenic present in the sample.

A standard stain prepared from a defined quantity of arsenic is used for comparison.  This provides the limit which the sample under the test should not exceed.

The reduction of arsenic to arsine in both standard and test is achieved by the combined action of zinc, stannous chloride (H₂SnO₃), and potassium iodide (KI).

Chemical reaction Involved

Apparatus

1. To perform the limit test for arsenic, A glass vessel consisting of a wide-mouthed glass bottle (120ml capacity) having closure mouth with 2.5cm diameter is used.

2. The bottle is fitted with rubber bung through which a glass tube of 20cm long, 0.8cm external diameter and 0.65cm internal diameter is inserted.

3. The glass tube is constricted towards its lower end to about 2mm diameter and has a hole.

4. The tip of the glass tube is fixed with two rubber bungs with each having a hole at the center.

5. At the upper end of the glass tube, the second bung is placed and fixed upon the first bung. The mercury chloride (HgCl₂) paper is placed in between two rubber bungs like a sand witch.

6. The bungs are kept in contact with each other by a spring clip.

7. The glass tube is tightly packed with cotton wool. This cotton wool is previously moistened with a solution of lead acetate and dried.

8. The upper end of the tube is inserted into the narrow end of the bungs, and two bungs are secured by the spring clip after insertion of the mercury chloride (HgCl₂) paper.

Note

• BP (British Pharmacopeia) suggests the use of mercury chloride (HgCl₂) instead of mercury bromide (HgBr₂)
• An electrolytic apparatus is used to generate hydrogen to convert the arsenic into arsine.
• The reagents which are used for the limit test for arsenic are marked and differentiated by ‘As T,’ which means they are all free from arsenic.

Take two 120ml wide mouth bottles and each as a test and a standard.

Preparation of standard solution:

Weigh accurately 0.132gms of arsenic trioxide (AsO₃) in 100ml solution. From this, 1ml is taken and diluted to 100ml with water. This is taken as a solution to produce a standard stain.

Steps for Experiment

Steps	Test	Standard
1	Take 5ml of the test solution along with 50ml of water and transfer it into the wide-mouthed bottle.	Take 5ml of the arsenic standard solution along with 50ml of water and transfer it into the wide-mouthed bottle.
2	Add 10ml of stannous acid.	Add 10ml of stannous acid.
3	Add 5ml of potassium iodide (KI) followed by 10gms of zinc dust.	Add 5ml potassium iodide (KI) followed by 10gms of zinc dust.
4	Place the cork immediately over the bottle with attachments and place the bottle in a water bath at a suitable temperature of 40 degrees.	Place the cork immediately over the bottle with attachments and place the bottle in a water bath at a suitable temperature of 40 degrees.
5	Allow the reaction to take place for 40 minutes.	Allow the reaction to take place for 40 minutes.
6	Compare the stain on mercury chloride (HgCl 2 )paper with the standard.	Compare the stain on mercury chloride (HgCl 2) paper with the test.

Observation

Compare the depth of color in the test paper with the standard stain

• If the test stain is intense than the standard solution stain, it fails the limit test for arsenic.
• If the test stain is not more intense than the standard stain, it passes the limit test for arsenic.

Also, see the limit test for lead.