Detection of Arsenic

Metallic arsenic burns giving a garlic odour, which is associated with the yellow allotrope, and fumes of arsenious oxide. The garlic odour is noticed when any arsenic compound, mixed with sodium carbonate, is heated on charcoal. The oxygen compounds may readily be reduced to arsenic by heating in a reducing flame, and the vapours may be condensed on a cold surface and dissolved in a hypochlorite solution. Similarly, arsenious oxide or sulphide is reduced by fusion with sodium carbonate and potassium cyanide, cyanate or thiocyanate being formed. Arsenites behave similarly providing the metal yields an arsenide easily decomposed on heating. In a suitable vessel a mirror of arsenic is obtained from 0.01 mg. of arsenious acid.

When the trisulphide is ignited with sodium carbonate and nitrate, sulphate and arsenate are formed:

5As₂S₃ + 16Na₂CO₃ + 28NaNO₃ = 10Na₃AsO₄ + 15Na₂SO₄ + 16CO₂ + 14N₂

When the sulphide is heated in a stream of chlorine, or when heated with ammonium chloride and ammonium nitrate (5:1) in a current of air, arsenic trichloride is volatilised.

Copper sulphate gives no precipitate with solutions of arsenious or arsenic acid, but if a little alkali hydroxide is added, the former gives a grass-green precipitate of cupric monohydrogen orthoarsenite, CuHNaO₃, Scheele's green, while the latter gives a pale blue precipitate of the corresponding arsenate, CuHAsO₄. The copper arsenite is soluble in excess of alkali and the solution on boiling precipitates cuprous oxide; the arsenate is not dissolved and is not reduced. This is a sensitive reaction for distinguishing ter- and quinque-valent arsenic, but the presence of certain organic compounds may cause reduction of the arsenate under similar conditions.

Silver nitrate with neutral solutions of arsenites gives a pale yellow precipitate of the silver salt, Ag₃AsO₃, which is soluble in both aqueous ammonia and in nitric acid. From acid solutions the precipitate is a hydrogen arsenite, and in aqueous solutions of the mono- and di-hydrogen arsenites the precipitation is incomplete; the solution should therefore be neutralised with ammonia. From neutral solutions of arsenates, silver nitrate gives a chocolate-coloured precipitate of silver arsenate, Ag₃AsO₄, soluble in acids and ammonia. This test is very sensitive, a brown coloration being obtained with a solution containing as little arsenic as 1 part in 150,000, whilst a distinct precipitate is obtained with 1 part in 60,000, and even in the presence of ammonium nitrate, as the test is usually applied in routine analysis, it is possible to detect 1 part in 15,000.

On adding concentrated hydrochloric acid to a few drops of an arsenite solution and then a little stannous chloride solution in hydrochloric acid, the solution quickly becomes brown and then black owing to the deposition of metallic arsenic:

2AsCl₃ + 3SnCl₂ = 3SnCl₄ + 2As

The reaction proceeds more readily on heating. In the absence of hydrochloric acid there is no reaction, as stannous chloride does not reduce arsenious acid. The test is capable of detecting 0.01 mg. of As₂O₃, but the sensitivity may be considerably increased by the presence of a trace of mercuric chloride (0.00001M), and the rate of deepening of the colour is then a function of the concentration of the latter. At such dilution the mercuric chloride does not produce turbidity with the stannous chloride.

To detect an arsenite and an arsenate in the presence of each other, the neutral solution should be treated with ammonia and magnesia mixture. The precipitate, treated with Bettendorff's reagent, gives a black precipitate of arsenic, indicating arsenate; the filtrate, acidified with hydrochloric acid and treated with hydrogen sulphide, yields a precipitate of arsenious sulphide, indicating arsenite.

Ammonium molybdate, when added in considerable excess to a boiling solution of an arsenate in nitric acid, gives a yellow crystalline precipitate of ammonium 12-molybdo-arsenate, (NH₄)₃H₄[As(Mo₂O₇)₆].4H₂O. Like the corresponding molybdo-phosphate, the precipitate is readily soluble in ammonia or aqueous alkali. The arsenate may be detected in the presence of phosphate by boiling the yellow precipitate with aqueous ammonium acetate until clear; a white precipitate or turbidity on cooling shows the presence of arsenate, and the filtrate may be tested for phosphate.

If a drop of 0.02N aqueous sodium sulphide is added to a drop of a solution containing arsenate or phosphate on a filter paper and a drop of aqueous ammonium molybdate acidified with sulphuric acid also added, a blue colour develops; ferro- and ferri-cyanides and thiocyanate should be absent. The formation of "molybdenum blue" is used in the colorimetric estimation of arsenic.

Sodium hypophosphite, Thiele's or Bougault's reagent, affords a more sensitive test than Bettendorff's reagent. It is prepared by dissolving 20 g. of sodium hypophosphite in 20 c.c. of water and adding 200 c.c. of hydrochloric acid; sodium chloride is removed by filtration and the filtrate kept until a second crop of crystals separates, when it is again filtered and is ready for use. When added to a solution containing arsenic, a brown coloration or precipitate results due to separation of arsenic. This test is recommended by the Dutch and German Pharmacopoeias. In the presence of ferric iron the colour of the latter interferes, and this should therefore first be reduced by means of iron powder or sodium sulphide and the test made on the filtrate. Preparations containing starch or sugar may also give a dark colour with the reagents, and should therefore be ignited with sodium nitrate before the test is made. The reagent cannot be used in the presence of metals whose salts it reduces, such as silver, mercury, gold, palladium and platinum, also selenium and tellurium.

Calcium hypophosphite instead of the sodium salt may be used in the presence of hydrochloric acid, and the reaction may be applied for the gravimetric estimation of the element, the arsenic in the residues being re-dissolved and determined as magnesium pyroarsenate, or by titration.

When a strip of polished copper foil is placed in a solution of arsenious acid a grey film is formed on the copper owing to deposition of arsenic and formation of copper arsenide, Cu₅As₂. The deposition occurs in the cold with concentrated solutions, but only on warming with dilute solutions. It is possible to detect by this means 1 part of As₂O₃ in 250,000 parts of water, but at great dilutions the time required for the deposition may be from half to one hour. Arsenic acid is similarly reduced by copper, but only on warming. If much arsenic is precipitated the deposit may not adhere to the copper. Oxidising agents must be absent.

Since other metals, namely, mercury, antimony and bismuth, are liable to be deposited on the copper, it is necessary to test for arsenic by gently heating the strip in a narrow tube, when a characteristic sublimate of octahedral crystals of arsenious oxide is formed, readily distinguished from mercury, which yields mirror-like globules, or antimony, which gives a sublimate, partly needle-shaped and largely amorphous, only on prolonged heating; bismuth gives no sublimate. This test is of great importance in toxicology.

Other important reactions of arsenic employed for the detection of the element are also adapted for its determination either when it occurs in quantity or in very small amounts. These reactions will therefore be dealt with from the point of view of their quantitative, as well as their qualitative, application.