Arsenic oxidation state determining

Analytical. Arsenic oxidation state determinations were per-formed by hydride generation-flame atomic absorption spectroscopy (AAS) at the University of Arizona Analytical Center. The analytical procedures are discussed in Brown, et al. (12). 

Since crushed basalt has been recommended as a major backfill component (1), experiments were completed to evaluate the rate of dissolved oxygen consumption and the redox conditions that develop in basalt-water systems under conditions similar to those expected in the near-field environment of a waste package. Two approaches to this problem were used in this study (l)the As(III)/As(V) redox couple as an indirect method of monitoring Eh and (2) the measurement of dissolved oxygen levels in solutions from hydrothermal experiments as a function of time. The first approach involves oxidation state determinations on trace levels of arsenic in solution (4-5) and provides an estimate of redox conditions over restricted intervals of time, depending on reaction rates and sensitivities of the analyses. The arsenic oxidation state approach also provides data at conditions that are more reducing than in solutions with detectable levels of dissolved oxygen. 

Molybdenum blue method. When arsenic, as arsenate, is treated with ammonium molybdate solution and the resulting heteropolymolybdoarsenate (arseno-molybdate) is reduced with hydrazinium sulphate or with tin(II) chloride, a blue soluble complex molybdenum blue is formed. The constitution is uncertain, but it is evident that the molybdenum is present in a lower oxidation state. The stable blue colour has a maximum absorption at about 840 nm and shows no appreciable change in 24 hours. Various techniques for carrying out the determination are available, but only one can be given here. Phosphate reacts in the same manner as arsenate (and with about the same sensitivity) and must be absent. 

Johnson and Pilson [229] have described a spectrophotometric molybdenum blue method for the determination of phosphate, arsenate, and arsenite in estuary water and sea water. A reducing reagent is used to lower the oxidation state of any arsenic present to +3, which eliminates any absorbance caused by molybdoarsenate, since arsenite will not form the molybdenum complex. This results in an absorbance value for phosphate only. 

With the exception of antimony (V), which requires the presence of iodide for its reduction, all species can be reduced in an acid medium at a pH of 1 -2. However, the reduction of some species, including antimony (III), arsenic (III), and all tin species, will also proceed at higher pH, where arsenic (V) and antimony (V) are not converted to their hydrides. This effect permits the selective determination of the various oxidation states of these elements [714, 716]. In the case of tin, reduction can be achieved at the pH of the Tris-HCl... 

The optimal reaction conditions for the generation of the hydrides can be quite different for the various elements. The type of acid and its concentration in the sample solution often have a marked effect on sensitivity. Additional complications arise because many of the hydrideforming elements exist in two oxidation states which are not equally amenable to borohydride reduction. For example, potassium iodide is often used to pre-reduce AsV and SbV to the 3+ oxidation state for maximum sensitivity, but this can also cause reduction of Se IV to elemental selenium from which no hydride is formed. For this and other reasons Thompson et al. [132] found it necessary to develop a separate procedure for the determination of selenium in soils and sediments although arsenic, antimony and bismuth could be determined simultaneously [133]. A method for simultaneous determination of As III, Sb III and Se IV has been reported in which the problem of reduction of Se IV to Se O by potassium iodide was circumvented by adding the potassium iodide after the addition of sodium borohydride [134], Goulden et al. [123] have reported the simultaneous determination of arsenic, antimony, selenium, tin and bismuth, but it appears that in this case the generation of arsine and stibene occurs from the 5+ oxidation state. 

Imagine that a friend has asked you to review the title and abstract of a paper that is being written with other researchers on chromated copper arsenate (CCA), a compound used to preserve wood. The research team examined the chemical structure of arsenic (As) and chromium (Cr) in CCA to determine if the oxidation state of As and Cr changed over time due to weathering. 

The speciation of trace elements in solid phases determines their mobility and toxicity. Spectroscopic techniques such as XANES and XAFS, can be used to determine directly the oxidation and structural state of elements in coal combustion byproducts. For example, Huggins et al. (2000) used these synchrotron techniques to determine that Cr and As occur predominately in the less toxic oxidation states Cr(IIl) and As(V) in CCBs. In addition, they found As, Cd, Cr, Ni, and Zn were present primarily as oxidized species (i.e., as oxides, sulphates, arsenates, etc.) in unweathered CCBs. 

Ion chromatography can readily determine arsenate, selenite, and selenate species in water in the absence of interferences. However, they will usually be completely obscured by the major anions. Arsenic and selenium are ordinarily at their highest oxidation state in surface waters but that is not necessarily the case in ground waters. 

These workers showed that dissolved arsenic and antimony in natural waters can exist in die trivalent and pentavalent oxidation states, and the biochemical and geochemical reactivities of these elements are dependent upon their chemical forms. They developed a method for the simultaneous determination of arsenic (III)+antimony (III+V)+ antimony (III+V) that uses selective hydride generation, liquid nitrogen cooled trapping, and gas chromatography-photoionisation detection. The detection limit for arsenic is lOpmol L 1 while that for antimony is 3.3pmol L 1 precision (as relative standard deviation) for both elements is better than 3%. 

Coulometric determinations can be carried out in which no physical separation occurs but simply a quantitative change in oxidation state. For example, MacNevin and Baker determined iron and arsenic by anodic oxidation of iron(II) to iron(III) and arsenic(III) to arsenic(V). The reduction of titanium(IV) to titanium(Hl) and the reverse oxidation have been used for the analysis of titanium alloys. Conversely, the output current from a cell made from a silver-gauze cathode and a lead anode with potassium hydroxide electrolyte can be used to measure low concentrations of oxygen in inert gases. ... 

Solid-phase speciation has been measured both by wet chemical extraction and, for arsenic, by instrumental methods principally X-ray absorption near edge structure spectroscopy (XANES) (Brown et al., 1999). La Force et al. (2000) used XANES and selective extractions to determine the likely speciation of arsenic in a wetland affected by mine wastes. They identified seasonal effects with As(El) and As(V) thought to be associated with carbonates in the summer, iron oxides in the autumn and winter, and silicates in the spring. Extended X-ray absorption fine stmcture spectroscopy (EXAES) has been used to determine the oxidation state of arsenic in arsenic-rich Californian mine wastes (Eoster et al., 1998b). Typical concentrations of arsenic in sods and sediments (arsenic <20 mg kg ) are often too low for EXAFS measurements, but as more powerful photon beams become available, the use of such techniques should increase. 

Pre-treatment to destroy organic matter. Organic selenium species are more widespread in the environment than comparable arsenic species. The determination of total selenium by most analytical methods requires samples to be pre-treated to remove organic matter, release selenium, and change its oxidation state. 

Dissolved As speciation is important in determining the extent of reaction with the solid phase and therefore the mobility of As in groundwater. Arsenic is generally present as arsenate [As(V)] or arsenite [As(III)] for Eh conditions prevalent in most groundwaters (Fig. 1). Arsenic metal rarely occurs, and the -3 oxidation state is found only in very reducing environments. Arsenite has been considered to be the more toxic oxidation state (U.S. Enviromnental Protection Agency, 1976) however, more recent studies have shown that most ingested As(V) can be reduced to As(III). 

Yu MQ, Liu GQ and Jin Q (1983) Determination of trace arsenic, antimony, selenium and tellurium in various oxidation states in water by hydride generation atomic absorption spectrometry after enrichment and separation with thiol cotton. Talanta 30 57-62. 

---