Water Sanitation Health

Arsenic in drinking-water

Background document for development of WHO Guidelines for Drinking-water Quality


General description


Arsenic exists in oxidation states of -3, 0, 3 and 5. It is widely distributed throughout the Earth’s crust, most often as arsenic sulfide or as metal arsenates and arsenides. In water, it is most likely to be present as arsenate, with an oxidation state of 5, if the water is oxygenated. However, under reducing conditions (<200 mV), it is more likely to be present as arsenite, with an oxidation state of 3 (IPCS, 2001).

1 Identity


Compound CAS No. Molecular formula
Arsenic 7440-38-2 As
Arsenic trioxide 1327-53-3 As2O3
Arsenic pentoxide 1303-28-2 As2O5
Arsenic sulfide 1303-33-9 As2S3
Dimethylarsinic acid (DMA) 75-60-5 (CH3)2AsO(OH)
Monomethylarsonic acid (MMA) 124-58-3 (CH3)AsO(OH)2
Lead arsenate 10102-48-4 PbHAsO4
Potassium arsenate 7784-41-0 KH2AsO4
Potassium arsenite 10124-50-2 KAsO2HAsO2


2 Physicochemical properties (IARC, 1980; Lide, 1992–1993)


Compound Melting point (°C) Boiling point (°C) Density (g/cm3) Water solubility (g/litre)
As 613 5.727 at 14 °C insoluble
As2O3 312.3 465 3.738 37 at 20 °C
As2O5 315 (decomposes) 4.32 1500 at 16 °C
As2S3 300 707 3.43 5 × 10-4 at 18 °C
(CH3)2AsO(OH) 200 829 at 22 °C
CH3AsO(OH)2
PbHAsO4 720 (decomposes) 5.79 very slightly soluble
KH2AsO4 288 2.867 190 at 6 °C
KAsO2HAsO2 soluble


3 Major uses

Arsenicals are used commercially and industrially as alloying agents in the manufacture of transistors, lasers and semiconductors, as well as in the processing of glass, pigments, textiles, paper, metal adhesives, wood preservatives and ammunition. They are also used in the hide tanning process and, to a limited extent, as pesticides, feed additives and pharmaceuticals.

4 Environmental fate

Arsenic is introduced into water through the dissolution of rocks, minerals and ores, from industrial effluents, including mining wastes, and via atmospheric deposition (IPCS, 1981; Nadakavukaren et al., 1984; Hindmarsh & McCurdy, 1986). In well oxygenated surface waters, arsenic(V) is generally the most common arsenic species present (Irgolic, 1982; Cui & Liu, 1988); under reducing conditions, such as those often found in deep lake sediments or groundwater, the predominant form is arsenic(III) (Lemmo et al., 1983; Welch et al., 1988). An increase in pH may increase the concentration of dissolved arsenic in water (Slooff et al., 1990).

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