Limit Test for Arsenic-Metallic Impurities

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Limit test for Arsenic Introduction

What is Arsenic test?

The official compendia lay a great deal of emphasis on the control of physiologically dangerous, cumulative poisonous and harmful impurities, such as lead, arsenic and iron present in a host of pharmaceutical chemicals.

Metallic impurities most of the time incorporates into the final product through a number of means as described below,

(a) Through atmospheric pollution.

(b) Most frequently derived from the raw materials.

(c) By means of raw materials used in the procedure of manufacturing.

(d) Due to solvent action on the metal of the plant in which the substance is prepared.

In short, all suggested tests for metallic impurities in the Pharmacopoeia usually has a fixed limits of tolerance.

For lead, arsenic and iron general quantitative or limit tests are precisely laid down which, with necessary variations and modification are rigidly applicable to pharmaceutical substances.

Limit Test for Arsenic Theory:

The official process is a development of the Gutzeit Test wherein all arsenic present is duly converted into arsine gas (AsH3) by subjecting it to reduction with zinc and hydrochloric acid.

Further, it depends upon the fact that when arsine comes into contact with dry paper permeated with mercuric (Hg2+) chloride it produces a yellow strain, the intensity of which is directly proportional to the quantity of arsenic present.

The various chemical reactions involved may be expressed by the following equations:

Zn + 2HCl (hydrochloride) → ZnCl2 (zinc-chloride) + 2H (Hydrogen).

 2As (arsenic) + 6H (Hydrogen) → 2AsH3 ↑

HgCl2 (mercury chloride) + AsH3 → HgCl2. AsH3 (yellow colour complex).

The details of experimental procedure described in the Pharmacopoeia are actually based upon a paper by Hill and Collins but have been adequately modified from time to time in accordance with the accumulated and acquired experience. It is stated clearly that the expressions described in the Pharmacopoeia for limits of metallic impurity like limit test for arsenic exclusively refer to parts per million.

why lead acetate soaked cotton plug is used in arsenic limit test?

Materials Requirement : Apparatus of Arsenic limit test; Mercury chloride paper, smooth white colour filter paper (having thickness in mm of Four hundred (400) paper = weight in grams per Square meter), soaked in a saturated solution of mercury chloride , pressed to get rid of excess of solution, and dried at about 60°C in the dark.

Lead acetate solution 10.0% w/v solution, of Lead II acetate (PbAc2) in carbon dioxide free water, potassium Iodide (KI) (AsT), 1 gram, Zn (AsT), l0 grams, Dilute Arsenic solution (AST), Standard stains, Test Solutions are prepared according to the Indian Pharmacopoeia 1996.

Arsenic Limit Test Apparatus

A wide-mouthed glass bottle capable of holding about 120 ml is fitted with a rubber bung through which passes a glass tube.

They are prepared from simple glass tubing which has a total length of about two hundred (200) mm and an internal diameter of exactly size about 6.5 mm (external diameter is measured about 8 mm).

It is drawn out at one end to a diameter of about 1 mm and a hole not less than 2 mm in diameter is blown in the side of the tube, near the constricted part.

When the bung is inserted in the bottle containing 70 ml of liquid, the constricted end of the tube is kept above the surface of the liquid, and the hole in the side is below the bottom of the bung.

The upper end of the tube is cut off square, and is either slightly rounded or ground smooth.

The rubber bungs (about 25 mm × 25 mm), each with a hole bored centrally and through exactly 6.5 mm in diameter, are fitted with a rubber band or spring clip for holding them tightly in place.

Procedure of limit test for Arsenic

Fig: Limit test for Arsenic Diagram

The cotton wool is used for packing glass tube, already soaked with lead acetate solution and later it is dried, so that the superior surface of the cotton wool should not be less than 25 mm below the top of the tube.

The upper end of the tube is then inserted into the narrow end of one of the pair of rubber bungs, to a depth of l0 mm (the tube must have a rounded-off end).

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A piece of mercuric chloride (hgcl2) paper is kept on flat on the top of the bung and the other bung placed over it and protected by means of the spring clip in such a way that the holes of the 2 bungs meet to form a true tube of thickness about 6.5 mm diameter placed in between the mercuric chloride paper.

The test solution which is prepared as mentioned in pharmacopeia, is placed in the wide-open mouthed bottle, 1 gram of potassium iodide (KI) (AsT) and 10 grams of Zinc (AsT) are added into it, and then prepared glass tube is kept rapidly in position.

The reaction of chemicals is allowed to proceed for about fourty minutes.

The yellow colour stain that is produced on the mercury chloride paper if it is present is compared by standard stains produced by performing a similar manner with known quantities of dilute arsenic solution (AsT).

The comparison of the stains is made immediately at the completion of the test.

By matching the intensity and depth of colour with standard stains, the proportion of arsenic in the substance may be estimated.

Thus, this stain which is equivalent to one ml standard stain produced by performing on l0 grams of a substance implies that the proportion of Arsenic is 1 part per million.

Cautions during limit test for arsenic

  1. Mercury chloride paper should be protected from sunlight during the test to avoid lighter stain or no stain.
  2. The standard and test stains must be differentiated immediately as they fade out on retaining.
  3. The reaction may be catalysed by the application of heat which speed up the reaction and 40°C is considered to be the most ideal temperature.
  4. The tube should be washed with hydrochloric acid (HCl) rinsed with distilled water, and then dried in between successive tests.

Special Techniques used: The special techniques are generally applicable to a host of pharmaceutical substances prior to the normal test can be performed. A few typical examples would be discussed briefly here, namely

  • Free Acids: They are first converted to their respective sodium salts with Na2CO3 and As3+ oxidized to As5+ by evaporating the solution with Br2.
  • The residue is heated carefully till it get carbonized to destroy organic matter, while it is placed as non-volatile sodium arsenate (Ar-Na).
  • The resulting residue is dissolved in brominated HCl and the test carried out in the normal manner. Examples: Aspirin, Saccharin, Sodium Salicylate, Sodium Amino salicylate.
  • Substances Reacting Vigorously with HCl : The As is readily converted to AsCl3 which being volatile in nature is also carried off along with relatively large volumes of CO2 (generated by the substance and HCl). Examples: Magnesium Carbonate, Light Magnesium Oxide, Calcium Hydroxide, Chalk, KOH, NaOH.
  • Insoluble Substances: The insoluble substances are those entities that do not interfere with the solution of Arsenic and its succeeding reduction to AsH3 (arsine).
  • Such type of substances are placed in water along with stannated-HydroChloride, and the normal test is performed.
  • Examples: Magnesium Trisilicate, Bentonite, Barium Sulphate, Light and Heavy Kaolin.
  • Metals Interfering with Normal Reaction
  • Iron: Iron gets deposited on the surface of Zinc and later depressing the intensity of reaction between Zinc and HCl to produce hydrogen.


The sample is dissolved in water and stannous HCl to allow reaction and conversion of all Arsenic (As) to As3+ and finally to AsCl3. After reaction product being volatile in nature can be collected by distillation from remaining metallic salts and the distillate is inspected in the normal manner.

Example: Ferrous Sulphate.

Antimony: Antimony compounds are also reduced at same time by Zinc/HCl to produce/yield SbH3 (stibine) that reacts with mercury chloride paper to give a stain.

Therefore, the sample is first distilled with HCl to yield a distillate containing all the as AsC3 (volatile), but yields only a fraction of Sb as SbCl3 (non-volatile). A repeated distillation obviously gets rid of even the last traces of Sb.

Examples: Antimony Potassium Tartrate, Antimony Sodium Tartrate.


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