32
River and Stream Sediments
Breder [40] used different mixtures of nitric, hydrochloric and hydrofluoric acids
in his determinations of total metals in river sediments. In the absence of hydrofluoric
acid, 1 g of River Arno sediment yielded 600 mg of silicaceous residue when digested
for 3 h with 4.5 ml nitric acid and 1.5 ml hydrochloric acid at 150°C under pressure.
The inclusion of 2 ml of hydrofluoric acid reduced the residue to 200 mg but further
increases in the hydrofluoric acid addition up to 6 ml did not further reduce the
weight of residue. He also observed that practically the same metal analyses were
obtained with and without hydrofluoric acid (Table 2.16).
Breder [40] also points out that the use of hydrofluoric acid can lead to analytical
errors. Arsenic and selenium fluorides, for example, are very volatile. Also, the
compensation system for unspecified light losses in the atomic absorption spectrometer can fail to operate at high fluoride concentrations. Breder [40] claims that Agemian and Chau [34] did not appreciate this and consequently obtained high results in
nitric-perchloric-hydrofluoric acid digestions (Table 2.16).
Table 2.17 shows results obtained in 3 h pressure digestions of 0.2-1 g of a River
Rhine reference sediment (ICEI) with 4-S ml of various concentrated acids, viz. 3 : 1
nitric acid-hydrochloric acid in PTFE vessels or 5 : 1 nitric-perchloric acid or 3 : 1
hydrochloric-nitric acid (aqua regia) or nitric acid alone.
Only pressurized digestion with nitric acid provided good precision. The mean
values for cobalt and nickel agree well with the reference values. The mercury data are
somewhat lower. The data for other elements tend to be somewhat higher than the
ICEI mean values. Only a small part of the silver compounds are brought into solution
by nitric acid. An addition of hydrochloric acid, however, achieves dissolution and
stabilization by the formation of silver chlorocomplexes. Distinctly lower concentrations were found for silver, cobalt, nickel, lead, and zinc after the open digestion with
aqua regia for 1 h in a 100-ml quartz Erlenmeyer flask, covered with quartz watch
glasses. After 3 h heating in 2S0-ml quartz flasks with reflux condenser, the concentrations of metals approach those of the pressurized digestion except for nickel. The
precision of the analytical data from the digestion experiments with aqua regia and
also with nitric-perchloric acid (5: 1) is only satisfactory iflarge amounts of sediment
and acids are used. Nitric-perchloric acid incompletely digests silver and nickel.
Microwave oxygen plasma digestion gives excellent precision but cannot be applied in
the case of mercury, selenium and arsenic.
Table 2.1S shows the excellent agreement for total element contents in various
reference sediments obtained by pressurized digestion with 3 : 1 nitric-hydrochloric
acid.
Table 2.16
Trace metal concentrations in a River Arno sediment after extraction with inverse aqua
regia with and without hydrofluoric acid addition (mg kg-I) (from [40])
Hg
Cd
Pb
eu
Ni
Mn
Zn
4.5ml HN0 3
0.99
1.01
64.2
64.6
63.0
573
259
1.5 m! HC!
4.5 m! HN0 3
1.5m! HC!
0.91
1.03
60.7
59.5
60.0
558
264
2.0ml HF
River and Stream Sediments
Breder [40] used different mixtures of nitric, hydrochloric and hydrofluoric acids
in his determinations of total metals in river sediments. In the absence of hydrofluoric
acid, 1 g of River Arno sediment yielded 600 mg of silicaceous residue when digested
for 3 h with 4.5 ml nitric acid and 1.5 ml hydrochloric acid at 150°C under pressure.
The inclusion of 2 ml of hydrofluoric acid reduced the residue to 200 mg but further
increases in the hydrofluoric acid addition up to 6 ml did not further reduce the
weight of residue. He also observed that practically the same metal analyses were
obtained with and without hydrofluoric acid (Table 2.16).
Breder [40] also points out that the use of hydrofluoric acid can lead to analytical
errors. Arsenic and selenium fluorides, for example, are very volatile. Also, the
compensation system for unspecified light losses in the atomic absorption spectrometer can fail to operate at high fluoride concentrations. Breder [40] claims that Agemian and Chau [34] did not appreciate this and consequently obtained high results in
nitric-perchloric-hydrofluoric acid digestions (Table 2.16).
Table 2.17 shows results obtained in 3 h pressure digestions of 0.2-1 g of a River
Rhine reference sediment (ICEI) with 4-S ml of various concentrated acids, viz. 3 : 1
nitric acid-hydrochloric acid in PTFE vessels or 5 : 1 nitric-perchloric acid or 3 : 1
hydrochloric-nitric acid (aqua regia) or nitric acid alone.
Only pressurized digestion with nitric acid provided good precision. The mean
values for cobalt and nickel agree well with the reference values. The mercury data are
somewhat lower. The data for other elements tend to be somewhat higher than the
ICEI mean values. Only a small part of the silver compounds are brought into solution
by nitric acid. An addition of hydrochloric acid, however, achieves dissolution and
stabilization by the formation of silver chlorocomplexes. Distinctly lower concentrations were found for silver, cobalt, nickel, lead, and zinc after the open digestion with
aqua regia for 1 h in a 100-ml quartz Erlenmeyer flask, covered with quartz watch
glasses. After 3 h heating in 2S0-ml quartz flasks with reflux condenser, the concentrations of metals approach those of the pressurized digestion except for nickel. The
precision of the analytical data from the digestion experiments with aqua regia and
also with nitric-perchloric acid (5: 1) is only satisfactory iflarge amounts of sediment
and acids are used. Nitric-perchloric acid incompletely digests silver and nickel.
Microwave oxygen plasma digestion gives excellent precision but cannot be applied in
the case of mercury, selenium and arsenic.
Table 2.1S shows the excellent agreement for total element contents in various
reference sediments obtained by pressurized digestion with 3 : 1 nitric-hydrochloric
acid.
Table 2.16
Trace metal concentrations in a River Arno sediment after extraction with inverse aqua
regia with and without hydrofluoric acid addition (mg kg-I) (from [40])
Hg
Cd
Pb
eu
Ni
Mn
Zn
4.5ml HN0 3
0.99
1.01
64.2
64.6
63.0
573
259
1.5 m! HC!
4.5 m! HN0 3
1.5m! HC!
0.91
1.03
60.7
59.5
60.0
558
264
2.0ml HF