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Arsenic Trihydride (Arsine, Arseniuretted Hydrogen), AsH3

Arsenic Trihydride (Arsine, Arseniuretted Hydrogen), AsH3, was discovered in 1775 by Scheele, who obtained it by the action of aqueous arsenic acid on zinc. In 1798 Proust observed the reaction, which soon became the basis of the most important method of arsenic detection and estimation, namely, the liberation of arsine, admixed with hydrogen, upon the addition of dilute sulphuric or hydrochloric acid to zinc in the presence of arsenious acid. The zinc may be replaced by magnesium, and it is evident that the reaction is between the nascent hydrogen produced and the arsenic compound. The reaction is inhibited by the presence of mercuric chloride. If the zinc is replaced by iron, very little arsine is produced, and it has been stated that, with iron, the gas is not formed at all, especially if commercial sulphuric acid containing nitric acid is employed. Investigations of a large number of cases of poisoning in the steel industry, however, reveal that arsine is generated by the action of 5 per cent, sulphuric acid on steel, the arsenic being derived from impurities in both the acid (especially when made from sulphide ores) and the steel. A sample of air (25 litres) taken at the surface of the acid in a pickling tank contained 0.006 mg. AsH3, while 10 feet away quantity present was negligible. Laboratory experiments show that crude dilute sulphuric acid acts on iron in the presence of reducible arsenic compounds, even at atmospheric temperatures, to produce arsine, and at higher temperatures extremely dilute acid will so react; moreover, the small amounts of nitric acid present in technical sulphuric acid do not prevent the reaction. Thiele found that the yield of arsine is much increased in this reaction by the addition of a little antimony trichloride, and stibine, SbH3, is not formed.

It was soon found that arsenic itself in the presence of zinc and dilute sulphuric or hydrochloric acid gives a mixture of arsine and hydrogen, and that the yield of the former is increased if an alloy of zinc and arsenic, or zinc arsenide, is used, it being possible to obtain a gas containing as much as 99 per cent, arsine. Other metallic arsenides or alloys with arsenic yield arsine on reacting with water or dilute acids; thus the action of water is sufficient in the case of the arsenides of the alkali metals, calcium arsenide, Ca3As2, or aluminium arsenide, Al3As2, almost pure arsine being produced. An alloy of potassium, antimony and arsenic has been used successfully. Less pure products are obtained by warming arsenic alloys with tin or iron and dilute acids.

The reduction of arsenic compounds to arsine by nascent hydrogen may also be effected in alkaline medium; thus in the presence of caustic alkali with zinc, aluminium or sodium amalgam, and also in ammonia or ammonium chloride with zinc.

The combination of arsenic with dry nascent hydrogen was observed by Vournazos, who obtained a mixture of hydrogen and arsine by heating rapidly to 400° C. in a round-bottomed flask a mixture of three parts of powdered arsenic with eight parts of dry sodium formate. The addition of sodium hydroxide or lime to the mixture prevents the formation of sodium oxalate and hence of carbon monoxide. Arsenious oxide, sodium arsenite or arsenic acid may be used in place of arsenic, but the yields are small. The gas is also formed if arsenic vapour is passed over heated sodium formate. Also, if the sulphide or phosphide of arsenic is heated with the formate, hydrides of both components of the arsenic compound are formed; but with metallic arsenides the hydride of the non-volatile component is not formed.

The hydride is also produced by the action of activated hydrogenon arsenic, and in small quantities, with other reduction products, by the action of hydrogen under pressure on heated arsenites and arsenates. Electrolytic methods for the production of arsine have been investigated by a number of workers,3 and in the reduction of arsenites and arsenates the mercury electrode has been recommended. The efficiency of such processes, however, is low. Thus, using solutions of arsenic acid in 2N-sulphuric acid containing up to 180 milligrams of elementary arsenic in 10 c.c., the efficiency under the best conditions, calculated as the percentage of the hydrogen produced at the cathode which was converted to arsine, was found by Lloyd to vary between 1.71 and 14.1 per cent. In these experiments the arsenic acid was admitted to the cathode during periods varying from 20 to 60 minutes and, when measured by a commutator method, the overvoltage of the mercury cathode fell during this addition to a value representing the overvoltage of arsenic. Some arsenic therefore appears to be deposited on the mercury, but the amount is extremely small and if the cathode is put into pure sulphuric acid solution the normal cathodic overpotential of mercury is quickly re-established. With uninterrupted current, the decrease in overvoltage is less and varies directly with increasing concentration of arsenic acid. By using a zinc amalgam cathode, or by adding zinc sulphate, the decrease in overvoltage is diminished and the yield of arsine is greater. The reduction is also facilitated by increasing the current density, the concentration of the acid electrolyte and the time of the electrolysis. Lloyd obtained the high yield of 60 per cent, of arsine from 4N-hydrochloric acid solution by employing alternating current electrolysis with an arsenic cathode, using a source of continuous current and a revolving three-point commutator. The optimum anodic and cathodic current densities were 44 and 525 milliamps. per sq. cm., respectively.

Arsine is formed also during the electrolysis of concentrated aqueous solutions of sodium acetate made acid with acetic acid and using an arsenic cathode. At constant potential difference the yield rises with increasing current density, but never attains a high value, and the electrolytic method is greatly inferior as a mode of preparation of arsine to the usual method of acting upon metallic arsenides with dilute acids.

When fused borosilicate glass is drawn out so as to expose a fresh surface, a garlic-like odour may be observed; this has been ascribed to the formation of arsine by reduction of arsenic present in the glass.

Preparation of Pure Arsine

The gas was obtained in a very pure state by Lebeau and Moissan by the action of water or dilute acid on calcium arsenide. The moisture was completely removed from the gas by cooling to -20° C. and then passing through a series of tubes containing metaphosphoric acid; the gas was then liquefied by means of a mixture of acetone and solid carbon dioxide. Natta and Gasazza used pure zinc and arsenious oxide with aqueous hydrochloric acid, dried the gas with calcium chloride and obtained solidified arsine by surrounding with liquid air. The use of drying agents is liable to cause slight decomposition of the gas; 2 this is true of alkali hydroxides, calcium oxide, concentrated sulphuric acid, calcium chloride, and also of phosphorus pentoxide. The action is least with the last two agents, and when any quantity of the gas is to be prepared, hydrated and weathered calcium chloride and phosphorus pentoxide may be employed.

robertson preparation arsine
Robertson's Apparatus for the preparation of pure Arsine.
Robertson and his co-workers prepared the pure liquid by the action of aqueous sulphuric acid on an alloy of zinc and arsenic. The apparatus used is illustrated in fig., the procedure being as follows. By means of a pump attached at N the apparatus to the right of the tap D was evacuated. The tubes E and O contained phosphorus pentoxide, and the U-tube F calcium chloride. Hydrogen was introduced through C, passing by way of the trap G, containing mercury and aqueous copper sulphate, to the copper sulphate bubbler H, whence it escaped to the flue through S until the generating flask A was deemed to be free from oxygen. The alloy, of approximate composition 53 per cent. Zn, 47 per cent. As, was introduced into A, a 200-ml. flask, which was then sealed with wax and hydrogen again passed for several minutes. Aqueous sulphuric acid (30 per cent.) was run in from B and the flask warmed. After the gas had been evolved for a short time, the tap D was opened and the receiver L was immersed in a mixture of solid carbon dioxide and chloroform contained in a Dewar flask M. When the evolution of gas in A ceased, D was closed and all the uncondensed gas was pumped off at N. The liquid arsine in L was then allowed to evaporate, the gas passing through the mercury and copper sulphate trap K to escape through H until samples taken from N showed complete absorption in copper sulphate solution. A little more gas was sent to waste and the middle fraction from L collected in the gas reservoir P, control samples being taken at R and tested for complete absorption in copper sulphate solution.

Robinson and his co-workers prepared pure arsine in pure hydrogen by slowly dropping (during 2 hours) a solution of arsenious oxide (20 g.) in freshly boiled hydrochloric acid (250 c.c. acid: 50 c.c. water) on pure magnesium turnings (50 g.) in a 750-c.c. round-bottomed flask cooled in water. The reaction products passed through aqueous potash into successive tubes containing potash pellets, fused calcium chloride and phosphorus pentoxide, and thence to a vessel immersed in liquid air. The pure arsine was finally obtained by careful fractionation.

Physical Properties of Arsine

Pure arsine at ordinary temperatures is a colourless gas, with an obnoxious odour. The vapour density was determined by Dumas, who found the value 2.695 (air = 1; theory AsH3 = 2.692).

The viscosity of the gas has been determined to be 0.0001470 at 0° C.; 0.0001552 at 15° C.; and 0.0001997 at 100° C. Rankine has used these values to calculate the " mean collision area," that is the average area presented by the molecule in all possible orientations, which determines the frequency of molecular collisions; the value obtained was 0.985×10-15 sq. cm.

The gas is somewhat soluble in water, 100 volumes of the latter dissolving 20 volumes of arsine. It is not absorbed by aqueous alkalies, or by alcohol or ether; but it is rapidly absorbed by turpentine, and slightly by fixed oils.

The trihydride is slowly decomposed by ultraviolet light, hydrogen and a brownish-black deposit, which is probably arsenic but may contain hydrogen, being formed. The ultraviolet absorption spectrum is a continuous one, showing no diffuse bands such as are observed with phosphine. The limit of the absorption depends on the conditions of the experiment - light intensity, pressure, and length of tube. There appears to be a regular gradation in the nature of the absorption spectra of the gaseous hydrides of the Group V B elements, that of ammonia showing a series of predissociation bands between λ2100 and 1700 A., phosphine showing two or three more diffuse bands between 2320 and 2200 A., while stibine resembles arsine in giving only continuous absorption.

The infra-red spectrum between λ 15,800 and 16,600 A. has been investigated; there is a very weak band at 16,300 A.

The heat of formation of arsine from crystalline arsenic is -36,700 calories.

The magnetic rotatory power of the gas has been examined and the Verdet constant at 0° C. and 760 mm. found to be 68×10-6 minute of arc. The molecular rotation is 44×10-5 radian ( λ = 578 μμ).

The dielectric constants of arsine have been determined at three temperatures and at three frequencies, the following being the mean absolute values at 1 atm. Pressure:

t, ° C-4716100

From these figures the value of B in Debye's equation, ε - 1 = N(A + B/T), may be calculated and is found to be substantially zero, so that as far as its dielectric constant is concerned, arsine resembles the permanent gases. The variation of the dielectric constant with pressure justifies the assumption that ε - 1 is proportional to the density, at least to a first approximation; thus at -47° C., using arbitrary units, the calculated values being derived on the above assumption, Watson gives the following figures:

p (mm.).(ε-1) calc.(ε-1) obs.

The gas may readily be liquefied, the colourless liquid having the following physical properties as determined by Robinson and his co-workers.

Physical Properties of Liquid Arsine.° C.° C.
Density at b.pt1.621
Mol. heat of vaporisation.4340 calories
Coefficient of expansion at×10-5
Mol. vol. at b.pt48.11
Mol. diameter8.69×10-9
Surface tension, σ, at
Ramsay.Shields constant, K2.10
Trouton's constant20.26
Mean mol. parachor104.2

Olszewsky found the boiling point to be -54.8° C., and by cooling the liquid to -118-9° C. he obtained a white crystalline mass which melted at -113.5° C. The solid arsine is quite stable in air at -170° C.

The density of the liquid at various temperatures is as follows:

t, ° C.0-20-40-60
d, g./c.c.1.4451.5011.5621.625

The vapour pressures are given in the following table:

Temperature, °C.Vapour Pressure, mm.
Vapour Pressure, atm.

The Ramsay-Shields and Trouton constants indicate that arsine is a normal liquid and differs from phosphine and to a greater extent from ammonia in not being associated.

The surface tension varies with temperature as follows:

t, ° C-60-50-40-30-20
σ, dynes/cm.22.2020.4018.6016.8115.08

The liquid is a very poor conductor of electricity.

The structure of solid arsine has been investigated by X-ray methods at -170° C. It crystallises in the cubic system and is isomorphous with phosphine. Its elementary cell contains four molecules, with a dimension of 6.40 ± 0.02 A., a volume of 2.62×10-26 c.c. and density (calculated) of 1.96. The position of the As atom corresponds to a face- centred cube.

The dielectric constants and molecular rotations of solid and liquid arsine have been determined from the temperature of liquid hydrogen to the boiling point over the frequency range 0.5 to 50 kilocycles. The molecule rotates freely down to 30.1° Abs.

Chemical Properties of Arsine

Arsine in the pure state is fairly stable, but in accordance with its endothermic nature it undergoes gradual decomposition into arsenic and hydrogen even when kept in a sealed tube in the dark. Under the latter conditions black particles of arsenic become visible after about 8 days. The decomposition is accelerated by exposure to light, by passing the gas through glass wool or cotton wool, by the presence of alcohol, and especially by gently warming; a sublimate of arsenic is rapidly obtainable at 230° C., the formation of this sublimate being made use of in Marsh's test. At 300° C. the amount of decomposition reaches 95 per cent, after 3 days and is practically complete (99.93 per cent.) after 7 days. The decomposition is catalysed by the film of arsenic which forms on the walls of the containing vessel, and until the walls are uniformly covered a velocity constant for the reaction is unobtainable. In the presence of a gas which may react with the arsenic film, such as hydrogen sulphide or oxygen, the decomposition is sensibly retarded. The gas may be decomposed explosively by detonation with mercury fulminate.

In contact with air or oxygen the gas may be ignited either by a flame or by the electric spark. It burns with a bluish-white flame and is oxidised according to the equation

2AsH3 + 3O2 = As2O3 + 3H2O

This quantitative relationship was observed by Dumas and Soubeiran. With excess of oxygen the hydride explodes violently, but if the supply of oxygen is insufficient the hydrogen is first oxidised and the arsenic liberated, and this takes place also in the spontaneous oxidation of arsine by oxygen at ordinary temperatures. Exposure of the mixed gases to β- or γ-rays results in the formation of arsenious acid:

2AsH3 + 3O2 = 2H3NaO3

The aqueous solution in contact with air gradually deposits the solid hydride, but if the water is free from dissolved oxygen the solution appears to be stable. Arsine may be completely oxidised by prolonged shaking with hydrogen peroxide solution; arsenic is first deposited and is then gradually oxidised to arsenious or arsenic acid.

The gas reacts vigorously with the halogens. When mixed with chlorine a flame is produced and arsenic and hydrogen chloride are formed; with excess of chlorine arsenic trichloride is produced, and in the presence of water arsenious and arsenic acids result. Bromine reacts similarly, the oxidation in the presence of water to arsenic acid being quantitative. With liquid chlorine, arsine reacts at temperatures as low as -140° C., forming reddish products, apparently containing the unstable hydrochlorides AsH2Cl and AsHCl2. With iodine, arsine reacts slowly in the cold but more rapidly on heating to yield arsenic iodide and hydrogen iodide; 2 with iodine and water the oxidation appears to proceed in two stages -

AsH3 + 3I2 + 3H2O = H3NaO3 + 6HI

and then if the solution is rendered alkaline with potassium hydrogen carbonate the oxidation to arsenate follows:

H3NaO3 + I2 + H2O = H3AsO4 + 2HI

An alcoholic solution of iodine is decolorised by arsine, some arsenious acid being formed in solution, and after passing the gas for some time a black precipitate appears. Admixture of arsine with hydrogen chloride results in the formation of a brown cloud of arsenic; aqueous hydrochloric acid and arsine yield arsenic trichloride. Arsine reacts quantitatively with iodine monochloride in aqueous solution with liberation of iodine, thus:

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

The reaction may be applied to the volumetric determination of arsine. The oxidation of arsine may be accomplished by means of the halogen oxyacids and their salts, although not so readily as with the halogens themselves. Hypochlorites and hypobromites cause complete oxidation to arsenic acid, but side reactions are liable to occur, especially if the gas is present in excess. Chloric acid slowly oxidises arsine to arsenious acid; a trace of silver nitrate catalyses the reaction. Chlorates are quite inactive. More complete oxidation results with solutions of bromic acid and bromates, iodic acid and iodates, especially in the presence of catalysts. The reactions are of the type represented by the equation

5 AsH3 + 8HBrO3 = 5H3AsO4 + 4Br2 + 4H2O

Perchlorates even in the presence of a catalyst have only slight action. Periodates act like iodates, but much more slowly, the reaction being

AsH3 + 4HIO4 = H3AsO4 + 4HIO3

When sulphur is heated with arsine, hydrogen sulphide is formed and a sublimate first of arsenic and then of arsenic sulphide is produced. The reaction proceeds slowly at 100° C. and at lower temperatures in direct sunlight. Hydrogen sulphide does not react with arsine in the absence of air at the ordinary temperature even in direct sunlight, but on admission of air a deposit of arsenious sulphide is rapidly formed whether the reactants are in the gaseous condition or in aqueous solution. If the mixture of gases is heated, separation of arsenious sulphide commences at about 230° C., but the reaction is incomplete even at higher temperatures. It has already been stated that arsine is itself acted upon by air or oxygen with formation of the solid hydride or arsenic, according to conditions, and also that the gas itself commences to decompose at 230° C. In the above reactions, therefore, the formation of arsenious sulphide appears to be a secondary reaction following the liberation of arsenic. Arsine may be entirely removed from hydrogen sulphide by passing the impure gas over " liver of sulphur " (potassium polysulphides) heated at 350° to 360° C. The absorption of the arsine may be represented thus:

2AsH3 + 3K2S3 = 2K3AsS3 + 3H2S

Concentrated sulphuric acid is coloured brown when arsine is passed into it, brown flakes of arsenic, which may contain the solid hydride, separate, and the liquid is found to contain hydrogen sulphide and arsenious sulphide. If the sulphuric acid is heated to 160° to 180° C., the passing in of arsine may result in the formation of an arsenical mirror. In spite of this reaction, according to Lyttkens and Lenz hydrogen containing arsine as an impurity may be dried by concentrated sulphuric acid without any loss of arsenic. Dilute sulphuric acid, even when hot, has little action on the gas. Sulphur trioxide reacts with formation of sulphur dioxide, arsenic and arsenious oxide; while sulphur dioxide also reacts, forming arsenic and arsenious sulphide.

Nitric acid, nitrous acid and nitrogen peroxide decompose the gas with liberation of arsenic, which then undergoes oxidation, as also does the hydrogen. Fuming nitric acid produces explosion and flame. Potassium nitrite in alkaline solution, aqueous ammonium nitrate and concentrated aqueous ammonia in the presence of air also decompose the gas. Phosphorus, when vaporised in arsine, reacts to form arsenic phosphide and phosphine. Phosphorus trichloride also produces arsenic phosphide together with hydrogen chloride, while phosphorus pentachloride first yields the trichloride and the solid hydride. Phosphorus pentoxide has little action and may be used for drying the gas. Hypophosphorous acid is without action on arsine. Arsenic trichloride causes deposition of arsenic with liberation of hydrogen chloride. If arsine is passed into a solution of arsenious oxide in hydrochloric or sulphuric acid, arsenic and water are produced.

On mixing liquid boron chloride with liquid arsine in an atmosphere of hydrogen at -80° C. white prismatic crystals of the additive compound, boron arsenotrichloride, BCl3.AsH3, are obtained. The hydrogen may be passed first through the arsine and then through the boron chloride, the entrained arsine being sufficient for the reaction. The product dissociates at -40° C., or if it is kept in a sealed tube at room temperature it decomposes into boron chloride, arsenic and hydrogen. With water it forms boric and hydrochloric acids, with liberation of arsine. A similar product, boron arsenotribromide, BBr3.AsH3, is obtained as a white amorphous substance when boron bromide is slowly dropped into liquid arsine at -80° to -100° C., a stream of dry hydrogen being passed through the apparatus, from which all oxygen and moisture has previously been removed. The arsenobromide decomposes on heating, but by careful sublimation in a sealed tube it may be obtained in a crystalline form. At 0° C. slow decomposition into boron bromide and arsine is apparent, and if the latter is removed as it is formed by passing an indifferent gas through the apparatus, the decomposition is accelerated and some arsenic is deposited. It is completely decomposed into boron bromide, hydrogen and arsenic if kept in the dark for some weeks in a sealed vessel at ordinary temperature. It is readily oxidised in air or oxygen, and under certain conditions it is spontaneously inflammable. There is no action in oxygen below -40° C., but above this temperature, in a limited supply of oxygen, the products are boric oxide, hydrogen bromide, arsenic tribromide and arsenic. It decomposes in contact with water, boric acid, hydrobromic acid, arsine and some free arsenic being formed. Concentrated nitric acid causes oxidation with almost explosive violence; concentrated sulphuric acid does not appear to react. Ammonia reacts at 10° C. to form the compound 2BBr3.9NH3. The arsenobromide is insoluble in carbon disulphide, but dissolves in boron tribromide.

When arsine is passed over a heated metal, such as the alkali and alkaline earth metals, zinc or tin, the decomposition of the gas is accelerated and the arsenide of the metal is formed. If platinum is used, the removal of arsenic from the gas is complete. The action of sodium or potassium on arsine in liquid ammonia yields the dihydrogen arsenide (MH2As). Heated alkali hydroxides in the solid form quickly decompose the gas, forming arsenites, and at higher temperatures arsenates and arsenides of the metals. The aqueous and alcoholic solutions have no appreciable action. When the gas is passed over heated calcium oxide the amount of decomposition is not more than that due to the action of heat alone. Heated barium oxide, however, is converted into a dark brown mixture of barium arsenite and arsenate, hydrogen being liberated. The gas is absorbed by soda-lime.

The common salts of the alkali and alkaline earth metals have little, if any, action on arsine, but aqueous solutions of alkali persulphates, chromates, dichromates and neutral ferricyanides absorb the gas to a slight extent. Potassium permanganate in neutral or acid solution, and ferricyanides in alkaline solution, oxidise the gas slowly and incompletely. The main reaction with potassium permanganate may be represented thus:

2KMnO4 + AsH3 = Mn2O3 + K2HAsO4 + H2O

Salts of the heavy metals, both in the solid condition and in aqueous solution, generally react with formation of the metallic arsenide. Thus, dry copper chloride or sulphate yields the arsenide, Cu3As2, and the mineral acid. In aqueous solutions of these salts, and in a solution of cuprous chloride in hydrochloric acid, absorption of arsine is only partial. No precipitation occurs with ferric salts; stannous and stannic salts are decomposed, a yellowish-brown precipitate being formed with stannic chloride. Zinc salts are only slowly decomposed. A concentrated neutral solution (80 per cent.) of cadmium acetate is able to absorb 40 times its own volume of the gas; the absorption is rather slow. Salts of gold, platinum and rhodium give precipitates of the metals. By heating pure tungsten hexachloride in a current of arsine, the temperature being maintained at 150° to 200° C. for a time and then gradually raised to 350° C., a black crystalline mass of tungsten di-arsenide is obtained; by heating the hexachloride with liquid arsine in a sealed tube at 60° to 75° C. bluish-black hygroscopic crystals of tungsten chlorarsenide, W2AsCl9, are formed.

The action of arsine on silver and mercury salts has attracted much attention owing to the important application to analytical methods for arsenic. The action of arsine on a dilute aqueous solution of silver nitrate has long been known to yield metallic silver, arsenious acid and nitric acid. With more concentrated solutions the introduction of a few bubbles of arsine produces a deep lemon-yellow coloration, the liquid also acquiring an acid reaction. The coloration disappears after one or two days, silver is precipitated and the colourless solution contains arsenious and arsenic acids. If a rapid stream of arsine be passed into a concentrated solution of silver nitrate at 0° C. the whole liquid solidifies to a yellow crystalline mass which rapidly blackens with separation of silver. Lassaigne represented the reaction with the dilute solution by the equation

AsH3 + 6ArNO3 + 3H2O = H3NaO3 + 6Ag + 6HNO3

but although the absorption of arsine is complete, the theoretical amount of silver is not at first precipitated unless the solution is alkaline. In neutral solution the reaction

AsH3 + 3ArNO3 = Ag3As + 3HNO3

also occurs, followed by:

Ag3As + 3ArNO3 + 3H2O = H3NaO3 + 6Ag + 3HNO3

The yellow substance produced with more concentrated solutions is silver nitrato-arsenide, Ag3As.3ArNO3, which is formed thus:

AsH3 + 6ArNO3 = Ag3As.3AgNO3 + 3HNO3

This compound does not separate from dilute solutions because it is decomposed by water:

Ag3As.3ArNO3 + 3H2O = H3NaO3 + 6Ag + 3HNO3

In alkaline or ammoniacal solutions, salts of arsenic acid are also formed, probably owing to both of the following reactions:

H3NaO3 + 2AgNO3 + H2O = 2Ag + H3AsO4 + 2HNO3
AsH3 + 8AgNO3 + 4H2O = 8Ag + H3AsO4 + 8HNO3

Fused silver nitrate is coloured first yellow and then black by arsine, the reaction being similar to that in solution.

When arsine is passed into an aqueous solution of mercuric chloride, a yellow to brown precipitate results. This was described by Rose as a chlorarsenide of mercury, AsHg3Cl3. A similar precipitate is obtained from an alcoholic solution, and this was investigated by Partheil and Amort, who passed in arsine largely diluted with hydrogen. The first product was shown to be yellow monochloromercurarsine, AsH2.HgCl, followed by an orange di- and a brown tri-chloromercur- arsine, AsH(HgCl)2 and As(HgCl)3; finally black mercury arsenide, Hg3As2, was formed. The first two, in the presence of excess of mercuric chloride solution, yield arsenious acid, hydrochloric acid and mercurous chloride, while the third yields arsenic and mercurous chloride. With water, the trichloromercurarsine decomposed to form arsenious and hydrochloric acids and mercury. Franceschi also obtained the dichloromercurarsine by the action of arsine on an ether solution of mercuric chloride. The dry salts, mercurous and mercuric chlorides, are both attacked by arsine and coloured yellow to brown; mercuric bromide reacts similarly.

When arsine is passed into a solution of potassium mercuric iodide containing excess of potassium iodide, a brown crystalline precipitate of triiodomercurarsine, As(HgI)3, is obtained. The gas also precipitates from aqueous or alcoholic solutions of mercuric cyanide unstable reddish-brown substances which gradually undergo decomposition, especially in daylight, with liberation of mercury.

According to Lebeau, arsine does not enter into the composition of compounds analogous to the metal-ammines, and the latter in ammoniacal solution react with arsine to produce arsenides of the metals.

The activity of palladium as a catalyst in the determination of hydrogen by combustion is unaffected by the presence of traces of arsine.

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