Annex 2 : Arsenic Compounds

This Annex provides further details with respect to some of the more commonly occurring compounds of arsenic, both inorganic and organic forms, and their main uses.

Table A1 summarises the main uses of the more commonly found arsenic compounds.

Table A1 : Common Arsenic Compounds and Their Uses
Compound Chemical Formula Uses
Arsenic acid H3AsO4 Manufacture of arsenates, glass making, wood treating process, defoliant, desiccant for cotton, soil sterilant.
Arsenic disulphide As2S2 Leather industry, depilatory agent, paint pigment, shot manufacture, fireworks, pest control, taxidermy.
Arsenic pentafluoride AsF5 Doping agent in electroconductive polymers
Arsenic pentasulphide As2S5 Paint pigments, light filters, other arsenic compounds
Arsenic pentoxide As2O5 Arsenates, insecticides, dyeing and printing, weed killer, coloured glass, metal adhesives
Arsenic thioarsenate As(AsS4) Scavenger for certain oxidation catalysts and thermal protectant for metal bonded adhesives and coating resins
Arsenic tribromide AsBr3 Analytical chemistry and medicine.
Arsenic trichloride AsCl3 Intermediate for organic arsenicals, ceramics
Arsenic trifluoride AsF3 fluorinating reagent, catalyst, ion implantation source and dopant
Arsenic trioxide As2O3 Pigments, ceramic enamels, aniline colours, decolourising agent in glass, insecticide, rodenticide, herbicide, sheep and cattle dip, hide preservative, preparation of other arsenic compounds.
Arsenic trisulphide As2S3 Pigment, reducing agent, pyrotechnics, glass used for infrared lenses, semiconductors, hide tanning
Arsenic hydride AsH3 Organic synthesis, chemical warfare, doping agent for solid state electronic compounds.
Monomethylarsonic acid (MMA) (CH3)AsO(OH)2 Pesticide industry
Dimethylarsinic acid (DMA) (CH3)2AsO(OH) Pesticide industry

  A more detailed consideration of inorganic and organic arsenic compounds is given below.

Inorganic Arsenic Compounds

Arsenic Hydrides

Arsine (AsH3) is used as a dopant in the semiconductor industry, and is used to produce gallium arsenide (GaAs) which is used in the field of opto-electronic and microwave devices.

Arsine is a colourless, very poisonous gas that exhibits an unpleasant garlic-like odour. It is the only known hydrogen compound of arsenic and is formed when any inorganic arsenic metal is brought into contact with zinc or sulphuric acid. It can be accidentally formed by the reaction of arsenic impurities in commercial acids stored in metal tanks. Arsine is not particularly stable and begins to decompose into its constituent elements below 572° F. In the presence of moisture, light can affect the decomposition.

In general, other than arsine, other arsenic hydrides have little commercial use. Diarsine (As2H), is formed as a by-product from the preparation of arsine (AsH3) by treatment of a magnesium aluminium arsenide alloy with dilute sulphuric acid. Diarsine can also occur by passing arsine at low pressure through an ozoniser-type discharge tube.

Arsenic Halides

Table A2 below lists some known arsenic halides.

All of the arsenic halides are covalent compounds which hydrolyse in water and can be formed by direct combination of the elements. Arsenic trichloride is the most common and commercially significant of all arsenic halides. With a low boiling point, it is easily separated from tin chloride and the chlorides of other metals. It can also be formed by spontaneous combustion of the elements. Arsenic trichloride has been used as a starting material for the production of numerous organoarsenic compounds and for the preparation of chlorine derivatives of the arsines. In addition, it is used as a dopant in the semiconductor industry and in the production of high-purity arsenic metal. Other arsenic halides include arsenic trifluoride, arsenic pentafluoride, arsenic pentachloride, arsenic tribromide, arsenic triiodide, and arsenic diiodide.

Arsenic Oxides and Acids

Arsenic trioxide (As2O3) is the most commercially important arsenic compound. It can occur in two different crystalline forms and one amorphous variety. The octahedral (or cubic) modification, arsenolite, is the most common form and is stable at room temperature. It changes into a monoclonic modification, (consisting of sheets of AsO3 pyramids sharing oxygen) at temperatures above 221° C. This modification is formed when condensation occurs at temperatures above 430° F. Condensation above 250° C will generally form the amorphous, glassy phase which devitrifies into the octahedral modification at room temperature. This octahedral variety is a white solid that sublimes above 135 ° C and melts at 275° C under its own vapour pressure.

Arsenic trioxide slightly dissolves in water to form a weakly acidic solution. It is soluble in acids and bases (amphoteric). It can be made by burning arsenic in air, or by the hydrolysis of an arsenic trihalide. Commercially, it is prepared by roasting arsenopyrite. It is often used as primary analytical standard in oxidimetry since it is readily attainable in a high state of purity and is quantitatively oxidised by many reagents commonly used in volumetric analysis. (e.g. dichromate, nitric acid, hypochlorite, and iron(III)).

Arsenic pentoxide (As2O5) is a ‘white glassy mass’ made up of equal numbers of octahedral and tetrahedral sharing corner oxygens to give cross linked strands. It is an oxidising agent capable of liberating chlorine from hydrogen chloride. The compound deliquesces in air to form arsenic acid. It dissolves water slowly, is thermally unstable, and begins to decompose near the melting point, around 300° C. The vapour is made up of arsenic trioxide and oxygen. The pentoxide can be made by reacting arsenic trioxide with oxygen under pressure, or by dehydration of crystalline arsenic acid at temperatures above 200° C.
 
 

Table A2 : Physical Properties of Arsenic Halides
Arsenic halide Colour and Physical State at 25 oC Mp, (oC) Bp, (oC)
Arsenic trifluoride (AsF3) colourless liquid -6.0 62.8
Arsenic pentafluoride (AsF5) colourless gas -79.8 2.8
Arsenic trichloride (AsCl3) colourless liquid -16.2 130.2
Arsenic tribromide (AsBr3) yellow solid 31.2 221
Arsenic triiodide (AsI3) red solid ca. 400 ca. 400


Arsenic acid is known in the solid state as the hemihydrate H3AsO4.0.5H20 and occurs as rhombic, deliquescent crystals. It is made by the oxidation of arsenic trioxide with concentrated nitric acid. Arsenic acid will lose water upon heating to 120° C and forms pyroarsenic acid. At elevated temperatures, more water is lost and meta-arsenic acid forms. In an acidic solution, arsenic acid and its salts are strong oxidising agents. Arsenic acid is used as a defoliant and as a starting material for important inorganic and organic arsenic compounds. Various salts (arsenates) are derived from arsenic acid, and are described in detail below.

Arsenates

Arsenates are oxidising agents and are reduced with concentrated hydrochloric acid or sulphur dioxide. Of the many salts of arsenic acid, the salts of potassium, sodium, calcium and lead are important commercially. Arsenates of calcium or lead are often used as insecticides. When a solution of ortho-arsenate is treated with silver nitrate in neutral solution, a chocolate brown precipitate of silver ortho-arsenate form. Silver ortho-arsenate can be used as a test to distinguish arsenates from phosphates. With hydrofluoric acid, ortho-arsenate solutions yield hexafluoroarsenates (e.g. potassium hexafluoroarsenate).

Arsenic Sulphides

Table A3 below presents the common physical properties of the common arsenic sulphides. These are described in more detail below.

Table A3 : Physical Properties of common arsenic sulphides

Arsenic Sulphides Molecular Formula Colour and Physical

State at 25° C
Arsenous sulphide As2S3 yellow solid
Arsenic sulphide AsS4 gold or orange solid
Arsenic pentasulphide As4S10 yellow solid
Tetraarsenic trisulphide As4S3 orange yellow
Tetraarsenic pentasulphide As4S5 (not known)

  Arsenic disulphide (‘red glass’) exists in the ruby-red crystals or as an amorphous reddish mass. It occurs naturally as the mineral realger. At 5267° C it changes into a black allotrophic modification and at 307° C the compound melts. Its purity and fineness rather than its chemical composition determine its commercial value. Industrially manufactured red arsenic glass varies in composition. Today, red glasses typically contain around 61 to 64% arsenic and 39 to 36% sulphur. Commercially, the compound is produced by heating a mixture of iron pyrites and arsenopyrites or by heating arsenic trioxide with sulphur. It can also be made by prolonged treatment of arsenous sulphide with boiling aqueous sodium bicarbonate, or by heating a sodium bicarbonate-arsenous sulphide mixture in a sealed tube. Water does not affect it, however it will oxidise in nitric acid and inflame in chlorine. ‘Red glass’ is primarily used as depilatory in the manufacture of fine leather, and also used in pyrotechnics.

Arsenic (III) sulphide is known as orpiment and occurs as a yellow mineral. It is made by precipitation of trivalent arsenic compounds with hydrogen sulphide. The colloidal solution of the arsenic trisulphide can be flocculated with hydrochloric acid, in which it is insoluble. It readily dissolves in basic reagents. Orpiment contains unchanged arsenic trioxide and is poisonous. It was used in the past for cosmetic purposes, but currently it is used in the semiconductor industry, in the production of infrared-permeable windows, and as a pigment.

Arsenic (V) sulphide (also referred to as arsenic pentasulphide) is made by fusing stoichiometric quantities of arsenic and sulphur powder or by precipitation from highly acidic arsenate (V) solution with H2S. Arsenic (V) sulphide will decompose into arsenic (III) sulphide and sulphur. The compound is stable in air up to temperatures of 95° C, but begins to dissociate into arsenous sulphide and sulphur at higher temperatures. It can be hydrolysed by boiling with water resulting in arsenous acid and sulphur.
 
 

Organic Arsenic Compounds

Arsenic combines easily with carbon to form a wide variety of organic compounds with one or more As-C bonds. There are many known organoarsenic compounds. Table A4 below presents a number of examples.

Organic arsenic compounds, once used in agricultural pesticides, have now largely been replaced by metal-free compounds. Organic arsenic compounds can be grouped together into aliphatic organoarsenic compounds and aromatic organoarsenic compounds. Both of these groups are described in detail below.

Aliphatic Organoarsenic Compounds

The use of aliphatic organoarsenic compounds is largely restricted in the UK as a result of increased awareness of their detrimental effects on the environment. However, they are still used as herbicides and fungicides in Eastern Asia. The main aliphatic organoarsenic compounds are described below;

Aromatic Organoarsenic Compounds

The primary use of arsonic acids was in their supplementary processing to arsenobenzenes and "arsenic oxides" by reduction with SO2, phosphorus trichloride, sodium dithionite, phosphorous acid, or tin (II) chloride. Reduction with zinc dust and hydrochloric acid yields the arsines, which are re-oxidised in air (e.g. phenylarsine is rapidly oxidised in air to form the arseno compound C6H5As).

The aromatic arsonic acids are dibasic. Aqueous solutions of the monosodium salts are neutral to mildly acidic, whereas those of the disodium salts are slightly alkaline (pH of 8-9). Magnesium and calcium salts are typically soluble in cold water, but upon heating, they precipitate to practically insoluble deposits. Because magnesium and calcium salts are soluble in cold water, they can be used to separate arsonic salts from cold solutions. Arsonic acids generally crystallise well, and their stability depends upon the substituents on the benzene ring. Some form azo dyes that contain both arsonic acid and sulphonic acid groups, and are used in the analysis of metals.

Aromatic Arsenobenzenes

Aromatic arseno compounds have amino or hydroxyl groups and are soluble in acids and alkalis. Aromatic arseno compounds will become soluble in water with the addition of a formaldehyde sulphoxylate or formaldehyde hydrogen sulphite into the amino group.

Organic Oxoarsenic Compounds

Organic oxoarsenic compounds are the anhydrides of the arsonous acids. They are extremely poisonous amphoteric substances barely soluble in water. When dissolved in acids and alkalis, they form salts and can be precipitated from those solutions by carbon dioxide or ammonium chloride.

The reduction of organoarsenic compounds can be controlled by using an appropriate reducing agent so that reaction terminates at the preferred intermediate stage. However, this does not occur with oxidation. In the most commonly used method for the production of organic oxoarsenic compounds from arsonic acids, the acid is directly reduced to the anhydride of the arsonous acid with SO2.