Environmental Engineering Reference
In-Depth Information
samples, but does not convert all organic aresenics to As(V). Organic
selenium compounds have rarely been observed in water.
Total As /Se: H
2
SO
4
-K
2
S
2
O
8
digestion is effective in converting organic
As to As(V) and organic Se to Se (VI) in potable and surface waters and
in most wastewaters, the contents of As/Se thus measured are termed total
As/Se.
The digestion for As is performed in a specific container called the Berzelius beaker,
which allows for accepting acid digestion, adding liquid sodium borohydride
reagents, purging with N
2
gas and stirring. The hydride (arsine, AsH
3
) generated is
transported to an atomic absorption atomizer for analysis (See Sec. 9.4). With EPA
270.3, Se in the sample is reduced from the oxidation state of þ6toþ4 by the
addition of SnCl
2
. Zn is added to the acidified sample, producing H
2
and converting
Se into the hydride (hydrogen selenide, SeH
2
). This gaseous SeH
2
is swept into an
argon-hydrogen flame of an atomic absorption spectrometer.
Measurement of As species By incorporating proper sample preparation into
an analytical scheme, it is possible to determine arsenite, arsenate, and total arsenic.
Arsenite: Arsenite can be reduced selectively by an aqueous sodium
borohydride (NaBH
4
) solution to arsine (AsH
3
)atpH6.
3BH
4
þ3H
þ
þ4H
3
AsO
3
! 3H
3
BO
4
þ4 AsH
3
"þ3H
2
O
ð7
:
8Þ
Arsenate, methylarsonic acid, and dimethylarsenic acid are not reduced
under these conditions. AsH
3
is swept by a stream of oxygen-free N
2
from
the reduction vessel through a scrubber containing lead acetate
[(Pb(CH
3
COO)
2
] solution into an absorber tube containing silver diethyl-
dithiocarbamate [AgSCSN(C
2
H
5
)
2
] and morpholine dissolved in chloro-
form, forming a red color compound that can be measured colorimetrically
at 520 nm.
Arsenate: The same sample described above is then acidified with hydro-
chloric acid (HCl) and another portion of sodium borohydride solution is
added. The arsine formed corresponds to the amount of arsenate present in
the sample.
Total inorganic arsenic: A portion of subsample is reduced at a pH of about
one followed by the same procedure as described above assuming the
absence of methylarsenic compounds in the sample.
Measurement of Se species The digestion described below has an increasing
order of rigorousness to suit for the analysis of various Se species:
Persulfate (S
2
O
2
8
) digestion: A small amount of ammonium or potassium
persulfate (K
2
S
2
O
8
) is added to the mixture of sample and HCl to remove
interferences from reducing agents and to oxidize relatively labile organic
Search WWH ::
Custom Search