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Hagarova I, Zemberyova M, Shearman A, Chroma O:
"Speciation of inorganic Arsenic in waters and its determination by Hydride Generation Atomic Absorption Spectrometry",
In 4AACD: (Sep 2004)

Keywords
Abstract	Hydride generation is a well-known technique for the determination of arsenic at trace levels, which consists of the reaction of some arsenic compounds with sodium tetrahydroborate in acidic medium to produce various arsines. Arsenic can be found in water samples most commonly as arsenite or arsenate. The reaction between NaBH4 and an ion in solution is sensitive to pH and it appears that, for the reaction to proceed rapidly, the target species must not be present in solution as a negatively charged species. In arsenic speciation analyses this means that arsenate must be fully protonated if it is to be converted to arsine. As pKa1 for arsenic acid is 2.3, the reaction must therefore be carried out at very low pH. Arsenious acid, on the other hand, is protonated under most conditions (pKa1 = 9.2) and will react with NaBH4 under conditions which are only mildly acidic [1]. The difference in dissociation constants between arsenious acid and arsenic acid was also used for separation of these two species on an anion exchange cartridge. At neutral pH, arsenious acid is not dissociated and is present as a neutral species As(OH)3. It is not expected to retain on an anion exchange cartridge. In contrast, arsenic acid is dissociated to H2AsO4- and is expected to eluate later due to its retention on the anion exchange cartridge [2].
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