Potential Metal Interferences on the Determination of Arsenic(III)
The interference of 16 metal species on the determination of As(III) was tested by spiking synthetic samples containing 10 μg/L As(III) and 10 μg/L As(V) with 0.0001 to 1,000 mg/L Al, Cd, Co, Cr(III), Cr(VI), Cu(II), Fe(III), Fe(II), Mn, Ni, Pb, Sb(III), Sb(V), Se(IV), Se(VI), and Zn and determining the As(III) concentration using the HGAAS procedure (without cationexchange treatment). Arsenic(III) recoveries for samples spiked with Al, Co, Cr(III), Fe(II), Mn, Ni, and Zn ranged from 84 to 107 percent over the entire concentration range tested (fig. 5). Low As(III) recoveries occurred when Cd exceeded 25 mg/L, Cr(VI) exceeded 0.03 mg/L, Cu(II) exceeded 2.0 mg/L, or Fe(III) exceeded 1.0 mg/L. Cadmium and Cu(II) are likely reduced by NaBH4 in the FIAS reaction chamber and, consequently, arsine production is limited by the available NaBH4. Chromium(VI) and Fe(III) inhibit arsine production by either competing with As(III) for the available NaBH4 or oxidizing As(III) to As(V). Arsenic(III) is quickly oxidized as Fe(III) undergoes photoreduction (Emett and Khoe, 2001). These synthetic samples were prepared in clear volumetric flasks and exposed to light. Therefore, the low As(III) recovery in the presence of only Fe(III) may be due to photochemical reactions with Fe(III) prior to the As(III) determination. However, when Fe(II) concentrations were twice the concentration of Fe(III), low As(III) recoveries occurred only when Fe(III) exceeded 10 mg/L (fig. 6). Iron(II) relieves the interference from Fe(III), within a limited Fe(III) concentration range, by reacting with free radicals generated as Fe(III) undergoes photoreduction (Emett and Khoe, 2001). Among the hydride-forming metals, Sb(III) at concentrations greater than 0.1 mg/L, Sb(V) greater than 0.4 mg/L, and Se(IV) greater than 0.02 mg/L interfered with As(III) determinations, whereas Pb and Se(VI) did not (fig. 7). Antimony(III), Sb(V), and Se(IV) suppress AsH3(g) production by reacting with the available NaBH4. An additive effect of interfering metals on the determination of As(III) was demonstrated by adding increasing amounts of Fe(III) to a solution containing 0.5 mg/L Cu and 10 μg/L As(III) (fig. 8). Low As(III) recoveries occurred when the molar ratios of metals to As(III) were: Cu(II) greater than 120, Fe(III) greater than 70, Cr(VI) greater than 2, Cd greater than 800, Sb(III) greater than 3, Sb(V) greater than 12, or Se(IV) greater than 1. The sample could not be diluted to an As(III) concentration (As(III) less than 20 μg/L) below which these interferences were absent. Copper(II) and Fe(III) are of primary concern because water generated from acid mine drainage potentially contains high concentrations of Cu(II) and Fe(III).
Cation Exchange Separation of Iron(III) and Copper
Interfering metals can be removed using cation-exchange resin prior to the determination of As(III). The relative selectivity for AG 50W-X8 cation-exchange resin is 2.95 for Cd, 2.9 for Cu(II), and 1.0 for H+ (Bio-Rad, 2003). Therefore, as long as the capacity of the resin (1.7 millieqivalents (meq) per mL of resin) is not exceeded, Cd and Cu(II) are expected to be removed from solution. The relative selectivity for Fe(III) is not published in the AG 50W instruction manual. Laboratory experiments done for this report demonstrated that AG 50W-X8 cationexchange resin in the H+ form removed large amounts of Cu(II) and Fe(III) while maintaining the existing As(III)/As(T) ratio (table 1). When the As(III) concentration is high relative to that of the interfering metal, the sample may be diluted before analysis to a Cd, Cu(II), Cr(VI), Fe(III), Sb(III), or Se(IV) concentration that does not interfere with the As(III) determination. Chromium(VI) and the Se and Sb oxyanions will not be removed by cation exchange.
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