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Limit Test for Arsenic in Pharmaceuticals

Apparatus and Method of testing for Arsenic in pharmaceuticals.
The limit for arsenic is indicated in the individual monographs in terms of ppm, i.e., the parts of arsenic, As, per million parts (by weight) of the substance under examination. All reagents used for the test should have as low a content of arsenic as possible.
Arsenic TestThe apparatus (Fig.) consists of a 100-ml bottle or conical flask closed with a rubber or ground glass stopper through which passes a glass tube (about 20 cm x 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 to 3mm 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 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 surface, is placed in contact with the first and is held in position by two spiral springs or clips. Into the lower tube insert 50 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 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 x 15 mm).


Into the bottle or conical flask introduce the test solution prepared as directed in the individual monograph, add 5 ml of 1 M potassium iodide and 10 g of zinc AsT. Immediately assemble the apparatus and immerse the flask in a water-bath at a temperature such that a uniform evolution of gas is maintained. After 40 minutes any stain produced on the mercuric chloride paper is not more intense than that obtained by treating in the same manner 1.0 ml of arsenic standard solution (10 ppm As) diluted to 50 ml with water.

Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
.moc.enilediugamrahp@ofni :liamENeed Help: Ask Question

4 comments: Post Yours! Read Comment Policy ▼

  1. can you provide any idea of basic chemistry involved in limit tests like arsenic , heavy metals etc.

    1. In essence, the arsenic contained in the sample is converted to arsene gas. This gas reacts with mercuric chloride and produce a stain in the mercuric chloride paper. The intensity of the colour produced by the test should be less than that of the colour produced by the standard.

  2. role of lead acetate of cotton plug in limit test of arsenic


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