RESULTS AND DISCUSSION
Selective Reduction of Arsenic(III)
During the As(III) determination, As(V)
can be reduced to AsH3(g) by the NaBH4
reductant causing an overestimation of As(III)
concentration. At a constant carrier
concentration and FIAS flow rate, increasing
the NaBH4 concentration increases the
sensitivity of the As(III) determination (fig.
1); however, when the NaBH4 concentration
exceeds 0.0625 percent, As(V) is reduced,
causing an overestimation of As(III)
concentration (fig. 2). To ensure selective
reduction of As(III), the NaBH4 concentration
is maintained at 0.03 percent for the As(III)
determination for analyses described in this
report.
Potential Metal Interferences on the
Determination of Arsenic(T)
Sixteen metal species including 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 were evaluated as possible interferents
on the determination of As(T). The
interference of each metal was evaluated by
spiking synthetic samples containing 6 μg/L
As(III) and 6 μg/L As(V) with 0.002 to 1,000
mg/L of the potential interfering metal and
then determining the As(T) concentration
using the HGAAS procedure (without cation
exchange treatment). Arsenic(T) recoveries
ranged from 92 to 102 percent for individual
solutions spiked with Al, Cd, Co, Cr(VI),
Cr(III), Cu(II), Fe(II), Fe(III), Mn, Ni, or Zn
(fig. 3). For solutions spiked with a
combination of metals including Al, Cd, Co,
Cr(III), Cr(VI), Cu(II), Fe(II), Fe(III), Mn, Ni,
and Zn, each up to a concentration of 500
mg/L, As(T) recoveries ranged from 97 to
102 percent. Dissolved Cu(II) was reduced to
Cu metal during the pre-reduction step. The
precipitated metal is visible at Cu
concentrations greater than 100 mg/L and the
liquid phase was decanted and analyzed with
little change in As(T) concentration.
Antimony(III) and Sb(V) were the only
hydride-forming species found to interfere
with the formation of AsH3(g) (fig. 4).
Antimony(III) and Sb(V) substantially
suppressed AsH3(g) formation by reacting
with the NaBH4 when the Sb(III)/As(T) molar
ratio exceeded 4 or the Sb(V)/As(T) molar
ratio exceeded 2. In most waters,
concentrations of Sb(III) and Sb(V) are much
lower than As(T) concentrations and are
likely to be diluted (As(T) less than 10 μg/L)
to a concentration that will not interfere with
the As(T) determination. Alkali and alkaline
earth elements do not interfere with the As(T)
determination (Smith, 1975).