This is based on the reaction of the arsenic gas with hydrogen ion to a yellow stain of mercuric chloride paper. The test is provided to demonstrate that the content of arsenic does not exceed the limit given the individual monograph in terms of arsenic per gram of the test substance.
Potassium iodide used as a lessening agent. It is also known as the Gutzeit test and a special apparatus is required. The limit for arsenic is indicated in the individual monographs in terms of ppm. All reagent used should be as low as a content of arsenic as possible.
Arsenic which is present as arsenic in the taste is decreased to arsenious acidic by minimizing agents just like potassium iodide, stannous acid, zinc and hydrochloric acid.
The arsenous acid is further reduced to arsine gas by hydrogen and reacts with mercuric chloride paper to give a yellow stain.
H3AsO4 + H2SnO2 -— H3As03 +H2SnO3.
Arsenic acid Arsenous acid.
H3AsO3 + 3H —— AsH3 + 3H2O.
Arsenous acid Arsine
The quality of arsenic present in the same will determine the amount of the yellow stain on the mercuric chloride.
They consist of a 100 ml bottle of conical flask closed with a ground rubber stopper. A glass tube of about 20 cm by 5mm passes through it.
The lower part of the tube is drawn to an internal diameter of 1.0 mm, a lateral orifice of 15 mm from its tips and 3 mm in diameter.
Procedures for limit test for arsenic:
Introduce the test solution prepared as directed in the individual monograph into the conical flask.
Add 5 ml of 1M potassium iodide and 10 g of zinc AsT.
Assemble the apparatus and immerse the flask in a water bath at a temperature of about 40 deg. C to ensure that a homogenous evolution of a gas is maintained. The temperature may be adjusted to obtain a regular but not too violent evolution of the gas.
The reaction may be increased by placing the apparatus on a warm surface, care being taken for the mercuric chloride paper to remain quite dry throughout the test.
After 40 mins any stain produced on the mercuric chloride, the paper is not more intense than that obtained by treating, in the same manner, 1.0 ml arsenic standard solutions (10 ppm As) diluted to 50ml with water.
Between successive tests, the tube must be washed with hydrochloric acid (250 g/l) AsTS, rinsed with water and dried.
Obtain by preparation of a solution by adding 10 ml of stagnated hydrochloric acid and (250 g/l) AsTs and 1ml of dilute arsenic AsTs, to 50 ml of water. The resulting solution, when treated as described in the general test, yield a stain on the mercuric iodide paper as ASR, Known as the Standard Stain.
- Stannous chloride is commonly used for full expansion of arsine.
- Zinc, potassium iodide are used as reducing agents.
- Hydrochloric acid is used as a tool to make the solution of acidic.
- Mercuric iodide papers are used to trap any hydrogen sulphide which may be evolved along the arsine.
Arsenic compounds are very toxic and care should be taken when handling this test. It really important and should be aware of this.