Fundamentals of Sample Preparation for Environmental Analysis - Fundamentals of Environmental Sampling and Analysis - page 165

HClO 4 with a functioning spray-nozzle wash-down system. Perchloric acid
salts could be explosive!
Wear goggles all the time during digestion and use rubber gloves when
pouring or handling acids. Keep workbench and hood dry. Wipe any liquid
that could be concentrated acid. Flush the hood system for at least 10
minutes at the end of acid digestion.
Always include a preparation blank that has all the reagents except sample
itself (see section 5.4.2). Any trace level of metals from the blank should be
subtracted from sample results.
7.2.3 Alkaline Digestion and Other Extraction
Methods
This section briefly introduces specific digestion/extraction procedures that are required
for four metals and metalloids (Cr, Hg, As, Se). For these elements, speciations are
more important than the total metal concentration because of the distinctly different
environmental effects that exist among various species. In other words, a separate
sample digestion/extraction is needed for these several elements.
1. Chromium (Cr)
The toxicity of Cr and its mobility in aquatic and terrestrial environments depend
on its oxidation state. Two most stable oxidation sates of chromium in the
environment are hexavalent, Cr(VI), and trivalent, Cr(III). In natural waters,
trivalent chromium exists as Cr 3þ , Cr(OH) 2þ , Cr(OH) 2 ,andCr(OH) 4 .Inthe
hexavalent form, chromium exists as CrO 2 4 and Cr 2 O 2 7 . Chromium(III) is
expected to form strong complexes with amines, and can be adsorbed by clay
minerals. Chromium may exist in water supplies in both the hexavalent and the
trivalent state although the trivalent form rarely occurs in potable water.
Chromium is considered nonessential for plants, but an essential trace element
for animals. Hexavalent compounds have been shown to be carcinogenic by
inhalation and are corrosive to tissue.
The EPA method (3061A) calls for an alkaline digestion procedure for Cr(VI),
which is distinctly different from the acid digestion described above for other
metals. With this method, an alkaline solution containing 0.28 M Na 2 CO 3 /0.5 M
NaOH is mixed with the sample and heated at 90-95 o C for 60 min. This treatment
extracts/dissolves the Cr(VI) from soluble (e.g., K 2 Cr 2 O 7 ), adsorbed, and
precipitated forms (e.g., PbCrO 4 ) of Cr compounds in soils, sludges, sediments,
and similar waste materials. The pH of the digestate must be carefully adjusted
during the digestion to maintain the integrity of the Cr species (i.e., avoid reduction
of Cr(VI) or the oxidation of Cr(III)).
If samples contain soluble Cr(III), its oxidation must be suppressed by the
addition of Mg 2þ in a phosphate buffer system. The concentration of soluble Cr(III)
can also be measured by extracting the sample with DI water and analyzing the