Title: Limit Test for Arsenic
- Objective: The limit test for arsenic is provided to demonstrate that the content of arsenic does not exceed the limit given in the individual monograph in terms of micrograms of arsenic per gram of the test substance.
- Principle: Limit test of Arsenic is based on the reaction of arsenic gas with hydrogen ion to form yellow stain on mercuric chloride paper in presence of reducing agents like potassium iodide. It is also called as Gutzeit test and requires special apparatus.
3. Procedure:
- Definitions:
- Matched Tubes: Tubes that are matched as closely as possible in internal diameter and in all other respects.
- Preparation of Standard Solutions:
Note: All solutions should be prepared in Distilled/Purified Water.
- Arsenic Standard Solution (10 ppm As): Dissolve 0.330 g of Arsenic trioxide in 5 ml of 2M Sodium Hydroxide and dilute to 250.0 ml with Purified water. Dilute 1 volume of this solution to 100 volumes with water immediately before use.
- Mercury (II) Chloride Paper: In a rectangular dish place a 5% w/v solution of Mercury(II) Chloride in absolute ethanol and immerse in it pieces of white filter paper weighing 60 g m–2 to 120 g m–2 (Whatman No. 1 is suitable), each measuring 20 cm × 15 mm and folded in two. Decant the excess liquid and allow the papers to dry, protected from light, by suspending them over a non-metallic thread. Cut away the folded edges to a width of 1 cm and similarly remove the outer edges. Cut the remaining strips into 15-mm squares or discs of 15 mm diameter. Store in a glass-stoppered container wrapped with black paper.
- Lead Acetate Cotton: Immerse absorbent cotton in a mixture of 10 volumes of Lead Acetate solution and 1 volume of 2M Acetic Acid. Drain off the excess of liquid by placing it on several layers of filter paper without squeezing the cotton. Allow to dry at room temperature. Store in an airtight container.
- Potassium Iodide Solution (6% w/v) (1M): Dissolve 16.6 g of potassium iodide and dilute up to 100 ml with Purified water.
- 2M Sodium Hydroxide: Dissolve 8.0 g of Sodium Hydroxide in sufficient water and dilute to 100 ml with water.
- 5% w/v solution of Mercury (II) Chloride: Dissolve 5.0 g of Mercury (II) Chloride in sufficient water and dilute to 100 ml with water.
- Lead Acetate solution (10%w/v): Dissolve 10.0 g of Lead Acetate in sufficient water and dilute to 100 ml with water.
- 2M Acetic Acid: Add 11.4 ml of glacial acetic acid carefully with equal volume of water and dilute to 100 ml with water.
- Apparatus:
The apparatus consists of 100 ml conical flask or bottle closed with ground-glass stopper through which passes a glass tube about 200 mm long and of internal diameter 5 mm. The lower part of the tube is drawn to an internal diameter of 1.0 mm, and 15 mm from its tip is a lateral orifice 2 mm to 3 mm in diameter. When the tube is in position in the stopper, the lateral orifice should be at least 3 mm below the lower surface of the stopper. The upper end of the tube has a perfectly flat, ground surface at right angles to the axis of the tube. A second glass tube of the same internal diameter and 30 mm long, with a similar flat ground surface, is placed in contact with the first, and is held in position by two spiral springs. Into the lower tube insert 50 mg to 60 mg of lead Acetate cotton, loosely packed, or a small plug of cotton and a rolled piece of Lead Acetate paper weighing 50 mg to 60 mg. Between the flat surfaces of the tubes place a disc or a small square of Mercuric Chloride paper large enough to cover the orifice of the tube (15 mm × 15 mm).
- Precautions:
Rate of evolution of the gas varies somewhat with different batches of granulated zinc Ast, the temperature may be adjusted to obtain a regular, but not too violent, evolution of gas.
The reaction may be accelerated by placing the apparatus on a warm surface, care being taken to ensure that the Mercuric Chloride paper remains quite dry throughout the test.
Between successive tests, the tube must be washed with dilute hydrochloric acid solution Ast, rinsed with water, and dried.
- Method:
To carry out the limit test for Arsenic, a solution is prepared from the test substance by a procedure specified in the monograph. This procedure assures that the solution in every case contains the whole of the arsenic (if any) present in the substance.
The standard stain against which the comparison is made contains 10 μg of As.
A stain equivalent to the 1 ml standard stain produced by operating on 10 g of a substance indicates that the amount of arsenic is 1 μg/g.
In the statements of Arsenic limits, the permitted amount of arsenic is expressed as As.
In the conical flask or bottle introduce the test solution prepared as directed in individual monograph. Add 5 ml of 1 M potassium Iodide solution and 10 g Zinc. Immediately assemble the two parts of the apparatus and immerse the flask in a bath of water at a temperature such that a uniform evolution of gas is maintained.
Limit: After 40 minutes any stain produced on the Mercuric Chloride paper is not more intense than that obtained by treating in a same manner 1.0 ml of Arsenic Standard Solution (10 ppm) diluted to 50 ml with water.