Estimation of Arsenic

As Sulphide

Acidified solutions of arsenites yield with hydrogen sulphide a yellow flocculent precipitate of arsenious sulphide, As₂S₃, insoluble in dilute acids. With very concentrated hydrochloric acid the sulphide is decomposed with liberation of hydrogen sulphide and the volatile arsenic trichloride. Concentrated nitric acid also decomposes it -

3As₂S₃ + 28HNO₃ + 4H₂O = 6H₃AsO₄ + 9H₂SO₄ + 28NO

as also does ammoniacal hydrogen peroxide

As₂S₃ + 14H₂O₂ + 12NH₄OH = 2(NH₄)₃AsO₄ + 3(NH₄)₂SO₄ + 20H₂O

The precipitate dissolves in aqueous alkali hydroxides, carbonates and sulphides with formation of arsenites and thioarsenites, thus:

As₂S₃ + 6KOH = K₃NaO₃ + K₃AsS₃ + 3H₂O
and
As₂S₃ + S(NH₄)₂S = 2(NH₄)₃AsS₃

Little or no precipitation may occur, therefore, in solutions of normal or monohydrogen arsenites, and to ensure complete precipitation the solution must contain sufficient acid to prevent the formation of thio-salts. The solution for estimation should be strongly acidified with hydrochloric acid and hydrogen sulphide be passed in the cold. Excess of the latter is removed by passing carbon dioxide, and the precipitate, after washing with hot water, is dried at 105° C. and weighed as As₂S₃.

If hydrogen sulphide is passed into a cold solution of an arsenate in dilute hydrochloric acid no precipitate is formed, owing to the formation of soluble thioarsenic acids, e.g. H₃NaO₃S, until after a long time, when arsenious sulphide separates. If, however, a large excess of concentrated acid is added to the solution and the gas passed in the cold, the pentasulphide, As₂S₅, is precipitated. The precipitation is quantitative1 if the concentration of the hydrochloric acid is at least 4N and the solution is saturated rapidly with the gas. After keeping the solution for two hours in a stoppered flask, the precipitate should be removed, washed with water and hot alcohol, dried at 105° C. and weighed as As₂S₅.

If hydrogen sulphide is passed into a hot solution of an arsenate in concentrated hydrochloric acid a mixture of the tri- and penta-sulphides is precipitated. To bring about a rapid precipitation in the form of As₂S₃, the arsenate may be reduced by boiling with sulphurous acid (boiling off excess of sulphur dioxide) or by heating with ammonium iodide or potassium thiocyanate. The pentasulphide, like the trisulphide, is soluble in aqueous alkali hydroxides, carbonates and sulphides, forming thioarsenates, while fuming nitric acid or ammoniacal hydrogen peroxide converts it to arsenic acid.

The sensitivity of hydrogen sulphide as a reagent for detecting ter-valent arsenic by the yellow colour produced in the presence of hydrochloric acid has been stated in widely different terms; the smallest amount thus detected is given as 1 part of As₂O₃ in 1,024,000 parts of water.

Arsenious sulphide is completely precipitated from a hot acid solution of an arsenite, and arsenic pentasulphide from one of an arsenate, by the addition of sodium thiosulphate, whilst both arsenites and arsenates are precipitated quantitatively as arsenious sulphide by ammonium thioacetate.

A solution of normal sodium arsenate in aqueous ammonia and methyl or ethyl alcohol yields with lithium salts a pale pink precipitate which is quantitative and may be dried, ignited and weighed. An arsenite does not precipitate lithium. A mixture of the two acids may thus be quantitatively separated, the arsenate first by lithium in the presence of aqueous ammonia and alcohol, and the arsenite in the filtrate by precipitation with magnesium chloride solution.

As Magnesium Pyroarsenate

The arsenic must be present as arsenate, so that any arsenite must first be oxidised, for example by means of chlorine or bromine in the presence of aqueous alkali, or bromine water in the presence of hydrochloric acid. The addition of magnesia mixture, i.e. a solution of magnesium and ammonium chlorides, in the presence of aqueous ammonia, then precipitates white crystalline magnesium ammonium arsenate, MgNH₄AsO₄.6H₂O. This, after 12 hours, is washed with ammonia, and is usually heated to about 500° C. until no more ammonia is expelled, then being ignited at 800° to 900° C. and so converted to magnesium pyroarsenate, Mg₂As₂O₇, and weighed. Accurate results have been obtained for potassium dihydrogen arsenate, with which the precipitation is complete in half an hour, by ignition to constant weight at 500° to 600° C. The long standing does not appear to be necessary for complete precipitation.

The magnesium ammonium arsenate may also be weighed as such with accurate results. The precipitated mixture should be filtered after cooling for about 2 hours at 0° to 5° C., washed with alcohol and ether, dried in a vacuum at room temperature, and weighed as MgNH₄AsO₄.6H₂O. This process is suitable for semimicro-determinations.

Iodometrically

A hot solution of arsenious oxide or an arsenite in concentrated hydrochloric or sulphuric acid when treated with potassium iodide gives a red precipitate of arsenic triiodide.

When potassium iodide is added to a strongly acid solution of an arsenate, reduction to arsenite occurs with liberation of iodine:

Na₃AsO₄ + 2HI = Na₃NaO₃ + I₂ + H₂O

This reaction is quantitative if the iodine is removed by titration with thiosulphate. It is necessary to allow the solution to stand for 10 to 15 minutes, preferably in the dark, before completing titration, to ensure completion of the reaction. During titration the high acid concentration may cause decomposition of the thiosulphate; the solution is therefore diluted. The above reaction, however, is reversible, and considerable dilution will cause reversal, leading to low results; satisfactory results are obtained when the solution is about 4N with respect to hydrochloric acid and contains about 1 per cent, of potassium iodide. The titration should be made at the ordinary temperature and starch indicator may or may not be used. The addition of sodium chloride to the arsenate solution has been advocated. When only very small quantities of arsenic are being determined, an atmosphere of carbon dioxide is. essential for accurate results, as iodine is liberated by the action of air on strongly acid iodide solutions.

An alternative method of employing the reaction is to expel the iodine by heating, or by a current of carbon monoxide and dioxide generated by the action of sulphuric acid on oxalic acid, and after adding excess of sodium bicarbonate estimate the equivalent quantity of arsenite remaining in the solution by titration with iodine as described below.

If iodine solution is added to a neutral or alkaline solution of an arsenite, the former is decolorised and oxidation occurs:

Na₃NaO₃ + I₂ + H₂O = Na₃AsO₄ + 2HI

This is the reverse of the reaction discussed above, and for the reaction to run completely in this direction the hydrogen iodide must be neutralised as fast as it is formed. An alkali cannot be added as it would react with the iodine, but the hydrogen iodide may be safely neutralised with sodium bicarbonate:

NaHCO₃ + HI = NaI + H₂O + CO₂

The reaction is then quantitative and important use of it is made in volumetric analysis, not only as a method for estimating arsenic, but a standard solution of pure arsenious oxide containing sodium bicarbonate is used as a standard in iodimetry.

To estimate arsenite and arsenate when present together, the former may first be determined in a portion of the solution by titration with iodine in the presence of sodium bicarbonate. Another portion is acidified strongly with hydrochloric acid, some ferrous sulphate and potassium bromide are added and the whole of the arsenic is distilled off as chloride and collected in water. The reduction may also be accomplished by cuprous chloride. The arsenious acid in the aqueous distillate is determined as above and the arsenic acid found by difference.

Small amounts of arsenic (as little as 0.00002 g.) may be determined by converting to arsine and absorbing the latter in standard iodine, the residual iodine being titrated.

Many modifications of the iodometric method have been applied.

With Potassium Bromate

When a hydrochloric acid solution of an arsenite reacts with potassium bromate, the arsenic is completely oxidised to arsenic acid and the end of the reaction is indicated by the liberation of bromine:

3Na₂HNaO₃ + KBrO₃ + 6HCl = 6NaCl + KBr + 3H₃AsO₄
KBrO₃ + 5KBr + 6HCl = 6KCl + 3H₂O + 3Br₂

If methyl orange is present, an acid reaction is indicated as long as arsenite is present, but the appearance of free bromine renders it colourless. Instead of methyl orange, colloidal red selenium may be used as a reversible reduction-oxidation indicator. This method may be used for determining arsenic in the presence of antimony and tin.

Other Oxidation Methods

Satisfactory titrimetric methods are based on the oxidation of arsenious acid to arsenic acid by means of potassium iodate, potassium dichromate, potassium permanganate and ceric sulphate. The last two methods are of importance.

Potassium permanganate oxidises arsenious acid rapidly and quantitatively in the presence of a trace of potassium iodide, which acts catalytically. The reaction takes place in the presence of mineral acid and the use of an indicator or of sodium bicarbonate is unnecessary. The titration may be carried out at any temperature up to 95° C. A slight correction is necessary for the oxidation of the iodide; other iodides are less effective than the potassium salt. This method is comparable in accuracy with the iodometric method.

Ceric sulphate similarly causes quantitative oxidation. The reaction should take place in the presence of 4N hydrochloric acid, with bivalent manganese present as catalyst and iodine monochloride as indicator. The iodine of the latter is first liberated and then oxidised

I₂ + 2HCl + O = 2ICl + H₂O

and the end-point is determined by disappearance of the iodine. The end-point may also be determined electrometrically.

Some of the volumetric methods described above may also be adapted to the electrometric determination of arsenic. Such methods have been described for titration of arsenites with eerie sulphate, iodine in the presence of sodium bicarbonate, "chloramine" (p-toluene-sulphone chloramide), alkaline potassium ferricyanide solution, potassium bromate or potassium iodate in the presence of hydrochloric acid, silver nitrate (by applying a secondary titration with 0.1N alkali to maintain the desired low H⁺-ion concentration), and with titanium trichloride; and for titration of arsenates with sodium iodide in the presence of sulphuric acid at 95° C., with mercurous nitrate (in 24 per cent, aqueous ethyl alcohol solution), and with silver nitrate.

The Marsh Test

The necessity for determining with accuracy very small quantities of arsenic has led to the perfecting of several methods which require definitely standardised conditions. Compounds of arsenic may be reduced in acid solution by means of nascent hydrogen to form arsine, which by thermal decomposition yields free arsenic. If the hydrogen, with the entrained arsine, is passed through a heated glass or silica tube, the arsenic condenses on the walls of the tube beyond the heated place as a brownish-black mirror. The arsenic may be determined by comparison of the deposit with a series of standard deposits obtained in the same way with known amounts of arsenic. The appearance of the deposit varies with the rate of formation and comparison is satisfactory only when conditions are definite and uniform. The reagents used must be free from arsenic, and this applies also to the materials of the vessels used, for which silica is preferable to glass. Zinc and sulphuric acid are usually employed to generate the hydrogen, but the reaction is more rapid with copper- coated zinc, or a copper or platinum salt may be added as an accelerator. Hydrochloric acid should not be used, as the reaction

Zn + 2HCl = ZnCl₂ + H₂

is reversible and deposition of a zinc mirror may occur. Aluminium may be used with hydrochloric acid. Nitric acid should not be present in the test solution, as nitric oxide is liberated and may cause an explosion. Many modifications of the method, both as regards reagents and apparatus, have been recommended.

Aumonier's Electrolytic Marsh Apparatus

The Electrolytic Method is one of the most satisfactory adaptations of the Marsh process. Instead of generating arsine by the action of zinc on acid, it is produced by cathodic reduction. The amount of nascent hydrogen produced during electrolysis is connected with the phenomenon of cathodie overvoltage, which is influenced by the surface condition of the metallic cathode and depends on the current density. The metal best suited to give constancy of surface and high cathodic overvoltage is mercury, and this metal is employed in the apparatus designed by Aumonier for use in the Government laboratories in London. The apparatus, shown in fig., is suitable for determining quantities of arsenic equivalent to 0.001 to 0.010 mg. As₂O₃, which covers the limits most often to be determined in food examination.

The glass vessel A, maintained at an even temperature by a jacket of running water, contains a platinum foil anode B loosely fitting outside the cathode cell C, which is a porous pot, the upper and lower parts of which are made impervious by means of wax, the annulus D being left porous. The rubber stopper E carries a delivery funnel fused to a capillary stem through which dilute sulphuric acid (1 vol. pure conc. H₂SO₄:7 vols. H₂O) is passed at a definite rate, and which is protected from dust by a loose cover. The current is conducted from the mercury cathode by a platinum wire through a tube open at both ends, F. The voltage applied is 4 to 5 volts. The hydrogen and arsine escape through the delivery tube G, passing through a drying tube H containing neutral calcium chloride, and into an electric furnace K consisting of a silica tube, 4 cm. long and 0.5 cm. diam., wound with nichrome wire. The mirrors are formed in tapering tubes drawn out from combustion tubing and containing two gauge marks between which the deposit forms. The deposit tubes are sealed when full of hydrogen, and compared with standards prepared under the same conditions.

If larger quantities of arsenic are present, arsenic may be deposited on the cathode, but at high electric potentials and low temperature, arsine is evolved quantitatively and may be absorbed in a solution of iodine in potassium iodide and the excess of iodine titrated. Suitable forms of apparatus have been described.

Several other types of electrolytic apparatus have been successfully employed, the most satisfactory using either the mercury cathode or a cathode of arsenic-free lead.

The Gutzeit Test

Gutzeit Test

This test is as accurate as the Marsh test and the apparatus necessary is comparatively simple. It consists in allowing the arsine to react with strips or discs of dry filter paper impregnated with silver nitrate or, in the more recent modifications of the method, mercuric chloride or bromide. A yellow to brown or black stain is produced, which is compared with a set of standard strips prepared under similar conditions. The chief difficulty encountered is to obtain a reliable and permanent set of standards; especially is this the case with silver nitrate, the stains of which do not keep. The most satisfactory method5 of preparing such stains is to soak the filter paper in gum tragacanth, dry it, soak it in silver nitrate solution, again dry it, expose to arsine under the requisite conditions and fix the stain by repeated soaking in very dilute ammonia and coating with paraffin. By the use of 66 per cent, silver nitrate solution, 0.1×10^-6 g. of As may be detected.

The use of mercuric chloride is much more convenient and almost as accurate, and this compound is now generally employed. The test may be conducted in the simple apparatus shown in fig; the same precautions as to the purity of all reagents are as necessary as in the Marsh test. The hydrogen is best generated from pure stick zinc and 20 per cent, sulphuric acid or from granular arsenic-free aluminium and 5 per cent, hydrochloric acid, a little stannous chloride being added to ensure a uniform rate of evolution of the gas. The tube A contains glass wool moistened with lead acetate solution to remove traces of hydrogen sulphide which may be formed; if filter paper or cotton wool is employed, the acid vapours cause the cellulose to retain arsenic. The tube B contains a dry strip of mercuric chloride paper prepared by allowing strips of thick drawing paper to remain for an hour in alcoholic mercuric chloride solution and allowing them to dry in the air. The strip is exposed to the arsine-bearing gas for a definite time, sufficient for a maximum depth of colour to be obtained.

Many modifications of the apparatus and method have been recommended. Discs of paper may be fixed across the mouth of the tube B in various ways and the stain, thus localised, is uniform in colour and sharp in outline and therefore more readily compared with standards. Mercuric bromide papers are satisfactory in use, but should be freshly prepared; the stains may be developed in aqueous potassium or cadmium iodide and compared with a colour standard.

Many efforts have been made to render the stains permanent. The length of an arsenic stain is affected by changes in temperature, and by immersing the whole apparatus in a constant temperature bath, say at 30° C., stains of uniform length and intensity are obtainable for any definite quantity of arsenic. The lengths vary from one set of tests to another, but the ratios between the lengths for different weights of arsenic remain constant, and a set of tables can therefore be prepared from which the values from any series of tests may be derived. The use of painted colour strips as permanent standards has been suggested.

The sensitivity of the Gutzeit test varies with the conditions. Variation in the humidity of the gas should be avoided. Under ordinary laboratory conditions 10^-6 g. of As may be detected, but amounts of 1, 2 and 3×10^-6 g. are not easily differentiated. If the quantity of arsenic in the aliquot test portion is as much as 0.04 mg. the comparison of the stains is not satisfactory. It has been observed that arsenic stains which are invisible to the naked eye become strikingly visible under ultraviolet rays.

Fleitmann's Test

Fleitmann's Test, which consists in treating the substance with sodium hydroxide and aluminium foil and testing the gases evolved for arsine, is confined to the qualitative detection of arsenic. The liberated gas was formerly allowed to come into contact with silver nitrate paper, but as aluminium foil almost always contains sufficient silicon to cause blackening of the paper by silicane, mercuric chloride papers, similar to those already described above, and which are unaffected by the silicon hydride, should be used.

Golorimetric Methods

Deniges' Molybdenum Blue Test. A sensitive method for the colorimetric estimation of arsenates (or phosphates) consists in the formation of blue-coloured compounds of composition (4MoO₃.MoO₂).XO₄H₃, where X = As or P. The reagents are (a) an acid solution of ammonium molybdate, and (b) a reducing agent. Deniges made the former by mixing a 10 per cent, solution of ammonium molybdate with an equal volume of concentrated sulphuric acid, and this, after dilution with three volumes of water, was reduced by means of copper turnings; the decanted solution, containing quadrivalent molybdenum, could be kept for one week. The following reagents are more satisfactory: (a) 10N-sulphuric acid containing 2.5 g. ammonium molybdate per 100 c.c., (b) a solution of 25 g. of pure stannous chloride crystals in 1 litre of 10 volume-per cent, hydrochloric acid. In making the test, 100 c.c. of the arsenate solution are treated with 4 c.c. of (a) and 6 drops of (b). A blue colour develops immediately. Hydrazine sulphate may also be used as the reducing agent. The test is particularly applicable for the detection of arsenate or phosphate in minute quantity, the sensitivity being about 1 in 1,000,000. The intensity of the colour is proportional to the amount of arsenic (or phosphorus) present and to apply the test quantitatively the blue colour is compared after 10 minutes, when it reaches a maximum, with standards generated under the same conditions. When the quantity of arsenic is very small, the blue colour is liable to fade or to be masked by the brown reduction products of molybdenum; the blue compound should therefore be extracted with methyl or amyl alcohol and compared with standards similarly obtained.

If arsenate and phosphate are present together, the total arsenic and phosphorus is first determined, the arsenic is then removed with hydrogen sulphide and the phosphorus determined alone.

This method for determining arsenic is particularly useful in biological and toxicological studies. The material under test is oxidised with a mixture of sulphuric and nitric acids and perhydrol, the arsenic is precipitated as sulphide, which is then oxidised and the arsenic determined colorimetrically after addition of sodium molybdate and stannous chloride. The formation of the molybdenum blue compound is also applied to the micro-determination of arsenic in soil extracts.

A reagent, prepared by mixing equal volumes of a 1 per cent, solution of potassium molybdate and a 2 per cent, solution of cocaine with two volumes of N-hydrochloric acid, exhibits turbidity with minute traces of arsenates. The presence of neutral salts up to N concentration does not affect the test, but phosphoric acid must be absent. As little as 1×10^-6 mg. As may be determined nephelo-metrically by this means.

Other methods for the colorimetric determination of arsenic have been described. Thus, for example, sodium sulphide is added to an acid solution of the sample and the precipitated arsenious sulphide washed and dissolved in 2 per cent, aqueous ammonia; on adding aqueous silver nitrate a brown colour forms, which may be compared with standards prepared under the same conditions. This test may be applied in the presence of organic substances to the determination of arsenic in concentrations of not less than 0.0001 per cent.

A drop reaction for tervalent arsenic consists in treating a drop of the sample on filter paper with hydrochloric acid and a 0.5 per cent, aqueous solution of kairin (N-ethyl-8-hydroxytetrahydroquinoline hydrochloride); on adding a drop of aqueous ferric chloride and warming the test paper, a reddish-brown colour appears. The test is sensitive to 6×10^-10 g. Mercury, lead and copper interfere.

Many of the processes already described have been adapted for microchemical methods, especially in connection with organic materials. The organic matter is usually destroyed by digesting with sulphuric acid and hydrogen peroxide or nitric acid, and the arsenic may then be determined iodometrically, as magnesium pyroarsenate or, after removing arsenic as chloride and precipitating as sulphide, the latter may be titrated in alkaline solution with 0.0lN-potassium permanganate or determined colorimetrically.

Applied as a microchemical test, Bettendorff's test is ten times as sensitive as the Marsh test.

A biological test for arsenic employs Penicillium brevieaule (grown on sterilised bread at 37° C.), by which means 0.001 mg. As₂O₃ may be detected, a garlic odour being developed in about 24 hours.

Arsenic may be detected spectroscopically, either an intermittent arc (broken 5 to 10 times per second) or a flame arc being suitable. The electrodes should be metallic, the metal chosen having bands distinct from those of arsenic, and the solid or liquid to be examined should be placed in a small depression in the face of one of the electrodes. In a solid, 2×10^-8 g. As can be detected; in a liquid the limit is 0.01 per cent. As from 2 c.c. The method has been applied to the quantitative determination of arsenic in lead alloys, an error of 10 per cent, being sufficiently accurate for ordinary production control.

To estimate the number of particles of arsenical dust in a sample of air, counts of the number of particles in a dust sample are made before and after heating at 250° C.; at this temperature arsenious oxide is removed but other constituents are unaltered.

In cases of arsenical poisoning, the expired air contains traces of arsenic which may be detected by absorption with bromine and ammonia and treatment with a solution of Penicillium brevicaule.

To remove arsine from hydrogen gas, the mixture should be washed with 6 per cent, potassium permanganate and then passed up a tower containing absorption charcoal. It is probable that the latter absorbs elementary arsenic rather than undecomposed arsine, and it may readily be recovered by digesting with normal sodium hydroxide and normal sulphuric acid. Charcoal is not a satisfactory absorbent for arsine in illuminating gas, however, only a small fraction of that present being removed; the best absorbent in this case is arsenic-free bromine, from which the arsenic is recovered by evaporation on a water bath and determined in the residue.

In determining the constituents of a gaseous mixture containing arsine, Wilmet separated them by using the following absorbents in the order given: for hydrogen sulphide, a neutral solution of zinc acetate; for carbon dioxide, potassium hydroxide solution; for arsine, a neutral or slightly acid solution of cadmium acetate; for phosphine, 30 per cent, selenium dioxide solution; for acetylene, an alkaline solution of potassium mercuri-iodide. An 80 per cent, cadmium acetate solution will absorb 40 times its volume of arsine, but the absorption is somewhat slow.

Kubina recommends the following method for estimating arsine volumetrically. The absorbing medium is an acid solution of iodine monochloride, the iodine being liberated according to the equation

AsH₃ + 8ICl + 4H₂O = H₃AsO₄ + 4I₂ + 8HCl

and titrated with a solution of potassium iodate in the presence of a cyanide, with a drop of carbon tetrachloride as indicator, the reaction being:

HIO₃ + 2I₂ + 5HCN = 5ICN + 3H₂O

To determine arsenic in a volatile liquid, this may be poured into a Marsh apparatus and burned at the jet, partly as vapour and partly as reduction products, the products of combustion being aspirated through a solution of sodium hydroxide. This is then oxidised with hydrogen peroxide, acidified with sulphuric acid, and examined for arsenic in the usual Marsh or Gutzeit apparatus.

The application of the foregoing analytical methods to the detection, separation and estimation of arsenic in a great variety of materials is the subject of an extensive literature, and a list of references to some useful papers of recent date is appended.