LIMIT TEST FOR ARSENIC

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) og the substances being examined.

All reagents used for the test should have a low a content of arsenic as possible.

Apparatus

The apparatus 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 ml ´ 5 mm). The lower part of the test tube is drawn to an internal diameter of 1.0 mm, and 15 mm from its tip is a lateral orifice 2 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 surface at right angles to the axis of the tube. A second glass tube 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 ´ 15 mm).

Method

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 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.

Part II  - TEST FOR ARSENIC IN MINERAL OILS & RM ADDITIVES

APPARATUS:

Kjeldahl flask (500 ml capacity)

REAGENTS:

• Lead Acetate solution: 10 % (m/v)- A 10 % w/v solution of lead acetate in   carbon - dioxide free water.
• Lead Acetate paper strips: Soak filter paper strips of 70 x 50 mm size in lead acetate solution contained in a glass bottle. Before use, take out the strips and dry them in an atmosphere free from hydrogen sulphide.
• Absorbent cotton wool impregnated with lead acetate: Saturate cotton wool with lead acetate solution, drain press and dry under vacuum at room temperature.
• Mercuric Bromide solution: Dissolve 5 g of mercuric bromide in 100 ml of rectified spirit
• Sensitized Mercuric Bromide paper strips: Soak filter paper strips cut to a size of 70 x 50 mm in mercuric bromide solution contained in an amber coloured glass bottle in dark. Before use, remove the strips, press it between sheets of filter paper, and dry it in an atmosphere free from hydrogen sulphide.
• Dil. Sulphuric acid: approximately 5 N     - Dilute 270 ml of sulphuric acid to 1000 ml with water
• Potassium Iodide solution: 15 percent (m/v)
• Stannous chloride solution: Dissolve 40 g of stannous chloride in 100 ml of hydrochloric acid, Discard this solution if an appreciable deposit forms.
• Zinc: granules, of size 0.5 to 1.0 mm
• Standard arsenic stock solution: Weigh to the nearest 0.1 mg, 0.1320g of arsenic trioxide and transfer it to a beaker of suitable capacity (for example 100 ml). Dissolve the arsenic trioxide with 2 ml of sodium hydroxide solution containing 50 g / l. Transfer the solution to a 1000-ml volumetric flask. Wash the beaker several times. Collect the washings in the volumetric flask and make up the volume. One ml of this standard solution contains 100 mg of sample.
• Concentrated Sulphuric acid                     
• Concentrated Nitric acid

PROCEDURE:

A. Preparation of the sample:

Weigh 2.000 g of the sample in a Kjeldahl flask of 500-ml capacity. Add 15 ml of concentrated sulphuric acid followed by 4 ml of concentrated nitric acid. Heat cautiously. Add drop-by-drop more nitric acid, if required, from a pipette to speed up the oxidation of the sample. The total amount of nitric acid shall be noted for use in the control test. When oxidation is complete, the solution is clear and faint yellow; at that stage, add 20 ml of water and again boil to fuming. Ensure removal of all nitric acid.

B. Test for Arsenic:

• Place the dry lead acetate paper in the lower portion of the tube Band cotton wool moistened with lead acetate in it’s upper portion.
• Place the sensitized strip of mercuric bromide paper in tube A and connect the tubes together with a rubber stopper.
• Introduce the solution of sample into bottle C & add 10 ml of dilute sulphuric acid
• Add 0.5 ml of stannous chloride solution, 5 ml of potassium iodide solution and make up the volume with water to about 50 ml .Mix the contents and drop in about 10 g of Zinc.
• Fit in position the rubber stopper carrying the tube B immediately. Place the bottle in a warm place at about 40 deg C. Remove the test strips after 40 minutes with the help of tweezers.
• Conduct the test in exactly similar manner by using 0.004 mg of standard arsenic solution and compare the stain produced with that produced with the arsenic solution.
• The limit prescribed in the specification shall be taken as not having been exceeded if the length of the stain as well as the intensity of it’s colour produced in the test with the material is not greater than those produced with the corresponding standard arsenic solution.

Part III  - TEST FOR ARSENIC IN COLOURS & TBHQ

in arsenic limit test color of mercuric chloride paper changes into

APPARATUS

Distillation Flask

Conical Flask

REAGENTS

Sulphuric Acid (sp.gr. 1.84)

Potassium Permanganate Solution (0.1 N)

Ferrous Sulphate - freshly powdered

Hydrochloric Acid - 38 %

Potassium Bromide Solution - 20 %

Aluminium strips - 8 x 8 x 1 mm

Tin Chloride Solution - 2 % Tin Chloride in 10 % Hydrochloric Acid

Test paper - soak strips of filter paper in saturated ethanolic solution of Mercuric bromide and allow to dry

PROCEDURE

Digestion:

• Weigh 1.0 g of sample for substances with 10 mg / kg limit or 0.5 g for those with 20 mg / kg limit. 
• Place with 5 ml of water, 5 ml of 65 % Nitric Acid and 5 ml of Sulphuric Acid in a digestion flask.
• Warm slightly.  If foaming becomes excessive, add little water.
• Evaporate the mixture.
• Maintain strongly oxidizing conditions in the flasks during digestion by adding cautiously small quantities of Nitric Acid whenever the mixture begins to turn brown or dark.  Continue digestion until organic matter is destroyed and Sulphuric Trioxide fumes are copiously evolved.  The final solution should be colourless or at most slight straw colour.
• To remove Nitrosylsulphuric acid, after partial cooling, transfer the residue to a dish, rinse with 25 ml of water, evaporate and heat again to fuming point.  Add 25 ml of water and evaporate and heat again.

Distillation: 

• Pour the Digest in the distillation flask.
• Rinse the dish twice using each time 2.5-ml of water and add the water to the digest.  Cool the solution. 
• Add some drops of Potassium Permanganate solution until the red colour persists.
• Add 0.250 g of freshly powdered Ferrous Sulphate, 2.5 ml of Hydrochloric Acid and 0.1 ml of Potassium Bromide solution.
• Close the flask.  Place a reagent tube containing 8 ml of water in a beaker with water.
• Heat with microburner.  In the beginning a slow stream of air bubbles appears, followed after 3 - 5 mins. by Hydrochloric Acid gas.  Heat then a little stronger so that the solution boils.  After bulb has become very hot, boil for 40 secs.  Thereupon lower the reagent tube and remove the flames.  The Hydrochloric Acid content of the distillate shall be 9 - 10 %, only than has Arsenic been completely distilled over, moreover this acid concentration is required for the test with Mercury Bromide paper.

Test with Mercury Bromide Paper:

The distillation should be transferred to a conical flask of 25-ml capacity.  The flask should be closed with cotton gauze soaked in 5% Lead Acetate solution and dried and containing a small piece of test paper.

Add three pieces of Aluminium strips, 1 ml of Tin Chloride solution and immediately close the flask with a stopper.

 Allow the flask to stand in a water bath of 25 - 30° C for 50 - 60 mins.

At the same time carry a parallel test using a solution of 2, 4, 6 or 8 mg of Arsenic in 10 ml of 10% Hydrochloric Acid in place of the test sample.

 Compare the colour of the test papers for evaluating Arsenic content of the sample.

Part IV  - TEST FOR ARSENIC IN SALTS

1.    Reagents:

Lead Acetate Cotton

Std. Arsenci Soln.

Stannous Chloride AST

Hydrochloric acid AST

Hydrochloric acid AST Brominated

Zn AST

Sample Soln.

2. Apparatus:

The apparatus 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 ´ 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 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 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 weighed 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 ´ 15 mm ).

3. Procedure:

Into the bottle or conical flask introduce the test solution prepared as directed in the individual monograph, add 5 ml of 1M potassium iodide and 10 g zinc AsT. Immediately assembly 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.

Part V  - TEST FOR ARSENIC IN CITRIC ACID

APPARATUS

Wide Mouth Bottle - 120 ml

Glass tube - Made from ordinary glass tubing & having total length of 200 mm. Internal diameter of exactly 6.5 mm & external diameter of about 8 mm. It is drawn out at one end to a diameter of about 1 ml & a hole not less than 2 mm in diameter is blow in the side of the tube, near the constricted part. The upper end of the tube is cut off square & is either rounded off slightly or ground smooth.

Rubber Bungs - three

One fits exactly into the mouth of the wide-mouthed bottle & has a hole bored centrally to take the tube from its constricted end. Each of the other two rubber bungs (about 25 x 25 mm) has a hole, exactly 6.5 mm in diameter, bored centrally & is fitted with a rubber band or spring clip for holding them tightly together.

Preparation of the Glass Tube - Moisten a small quantity of cotton wool with lead acetate solution & then dry it in a dust-free atmosphere. Lightly pack the glass tube with this cotton wool, so that the upper surface of the cotton wool is not less than 25 mm below the top of the tube. Insert the upper end of the tube into the narrow end of one of the pair of rubber bungs (about 25x 25 mm size) (either to a depth of about 10 mm when the tube has a rounded off end, or so that the ground end of the tube is flush with the larger end of the Bung. Place a piece of mercuric chloride paper flat on the top of the bung. Place the other bung over this with its larger end in contact with the piece of mercuric chloride paper. Fasten the two bungs by means of the rubber band or the sprint clip, in such a manner that the borings of the two meet to form a true tube of 6.5 mm diameter interrupted by a diaphragm of Mercuric chloride paper.

Instead of his method of attaching the Mercuric chloride paper, any other method may be used, provided that

(a) whole of the evolved gas passes through the paper

(b) the portion of the paper in contact with the gas is a circle of 6.5 mm diameter &

(c) The paper is protected from sunlight during the test.

REAGENTS

The following reagents are required. The reagents with the exception of Strong solution of Arsenic & Diluted solution of arsenic shall be free from traces of Arsenic

Lead Acetate solution - 10.0 percent m/v in Water, recently boiled & cooled

Mercuric Chloride Paper - smooth white filter paper, not less than 25 mm in width, soaked in a saturated solution of Mercuric Chloride in water, pressed to remove superfluous solution, & dried at 60 deg C in the dark. The grade of the filter paper shall be such that the mass in gm/m² shall be between 65 & 120; the thickness in mm of 400 papers shall be approximately equal to the mass in g/m². Mercuric chloride paper should be stored in a stoppered bottle in the dark. Papers exposed to sunlight or to vapour of ammonia should not be used as they give lighter stains or no stains at all when used.

Bromine Solution - Add to a small quantity of water, 30 g of Potassium bromide. Dissolve it in the water & mix with it 30 g of Bromine. Dilute with water to produce 100 ml & mix.

Concentrated Hydrochloric Acid - Steam pressure Sp gr. 1.16 & complying with the following test. Dilute 10 ml of concentrated HCL with sufficient water to produce 50 ml, add 5 ml of Ammonium thiocyanate solution 10% m/v in water & stir immediately. No colour shall be produced.

Brominated HCl - Mix together 1 ml of Bromine solution & 100 ml of concentrated HC.

Stannous Chloride Solution - Dilute 60 ml of conc. HCl with 20 ml of Water, add to it 20g of Tin, heat gently until gas ceases to evolve & add sufficient water to produce 100 ml allowing the undissolved tin to remain in the solution. Decant the clear solution, add an equal volume of conc. HCl, and boil down the volume & filter through a filter paper.

Potassium Iodide - Crystals or powder.

Zinc - Granulated & complying with the following test.  Take 50 ml of water, 10 ml of Stannated Hcl & 0.1 ml of dilute solution of Arsenic in the wide mouthed bottle. Add 1 g of KI & 10 g of Zn. Quickly place the prepared glass tube in position. Allow the reaction to continue for one hour. A faint but distinct yellow stain shall be produced on the Mercuric chloride paper.

Stannated HCl - Mix together 1 ml of Stannous Chloride Solution & 100 ml of Conc. HCl.

Strong solution of Arsenic - Dissolve 0.132 g of Arsenic trioxide in 50 ml of conc HCL, add sufficient water to produce 100 ml & mix.

Dilute solution of Arsenic - Freshly prepared. Dilute 1 ml of strong solution of Arsenic with water sufficient to produce 100 ml. This solution contains 0.01 mg of Arsenic (or 0.0132 mg of As₂0₃) per ml.

PROCEDURE

Preparation of the Solution - Weight accurately 10 g of the material, dissolve it in 50 ml of water, & add to it 10 ml of Brominated Hcl. Allow the solution to stand for five minutes & remove the excess of Bromine with a few drops of Stannous chloride solution.

Place the whole of the prepared solution in the wide-mouthed bottle & add 1g of Potassium iodide & 10g of Zinc. Quickly place the prepared glass tube in position. Allow the reaction to continue for 40 minutes. Remove the piece of Mercuric chloride paper at the end of this period. Compare the yellow stain, produced on the Mercuric chloride paper if arsenic is presenting the material, by daylight with the standard stain prepared as described.

The limit shall be taken as not having been exceeded if the stain produced by operating the material is not deeper than the standard stain.

Comparison of stains - the comparison of the stains is made with freshly prepared stain immediately at the completion of the test.

A blank determination shall be carried out under the same conditions, on the same reagents & in the test & by the same person but without using the material. The blank shall not produce any visible stain on the Mercuric chloride paper.

Preparation of Standard stain - Mix together 50 ml of water, 10 ml of stannated Hcl & 1ml of dilute solution of Arsenic. Treat the resulting solution as described to prepare the Standard stain.