Table A1 summarises the main uses of the more commonly found arsenic compounds.
Table A1 : Common Arsenic Compounds and Their 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|
Inorganic Arsenic Compounds
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.
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)).
(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.
|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|
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).
Table A3 below presents the common physical properties of the common arsenic sulphides. These are described in more detail below.
: Physical Properties of common arsenic sulphides
|Arsenic Sulphides||Molecular Formula||Colour
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 (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.
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;
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 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.