12
River and Stream Sediments
film on the glassy carbon electrode. Subsequently, the metal ions are reduced and
amalgamated in the mercury during the electrolysis step (plating). When the plating is
terminated, the metals are stripped from the mercury fllm back into the solution by
chemical oxidation. During this step, the potential of the carbon electrode (against
SCE) vs time is recorded. The metals are identified by their stripping potentials and
are quantified by measuring the stripping time for each metal.
In this procedure, approximately 0.5 g of dried (105 0c) sediment was decomposed
by boiling for 2 h in a mixture of 3 ml of concentrated hydrochloric acid and 1 ml of
concentrated nitric acid. After the dissolution, the sample was diluted with 10 ml of
redistilled water, filtered (Whatman No. 41) and further diluted with redistilled water
to a total volume of 100 ml. To an aliquot (25 ml) were added 1 ml of 10 % (w Iv)
ascorbic acid solution (antioxidant and reductant for iron(III)) and 1 g of sodium
chloride (to ensure reproducible ionic activity) together with mercury solution
(usually 100 ~ of 1000 ppm Hg solution, but depending on metal concentration) and
an internal standard (cadmium) to correct for variations in oxidation rate.
For mercury plating, ten precoating-stripping cycles were employed. Simultaneously, the sample was deaerated by purging with helium. After precoating and
deaeration of the sample, an appropriate plating time and plating potential were
applied (1-32 min and - 0.95 V vs SCE, respectively).
Copper was quantified by using at least two standard additions. This was done in a
cyclic mode and standards were added immediately after the stripping curve had been
recorded. The plating time was 4 min at - 0.95 V vs SCE; 2.5 /lg of copper was added
at each standard addition and the natural lead content of the sample was used as the
internal standard.
Copper determinations carried out by this procedure were compared to expected
values for a series of reference sediments. The values found were all within the
certified limits. For example, NBS 5RM 1645 reference sediment (reference value 18
± 3 mg kg-I) gave a value of 16.8 mg kg- I by this procedure.
2.1.7
Gallium
Xiao-quan et al. [12] used graphite furnace atomic absorption spectrometry with a
nickel matrix modifier to determine /lg kg-I levels of gallium in perchloric acid digests
of sediments.
2.7.8
Lead
The potentiometric stripping analysis procedure [11] described under copper has also
been applied to the determination of lead in river sediments in amounts down to
7 mg kg- I with a precision of 2.5-4.1 %. Lead determinations carried out by this
procedure were compared to expected values for a series of reference sediments. All
values are within certified limits, for example NBS SRM 1645 sediment (Reference
value 714 ± 2.8 mg kg-I lead gave a value of 722 ± 18 mg kg- I by this procedure.
The graphite furnace atomic absorption spectrometric method described earlier [8]
under cadmium has also been applied to the determination oflead down to 1 mg kg- I
River and Stream Sediments
film on the glassy carbon electrode. Subsequently, the metal ions are reduced and
amalgamated in the mercury during the electrolysis step (plating). When the plating is
terminated, the metals are stripped from the mercury fllm back into the solution by
chemical oxidation. During this step, the potential of the carbon electrode (against
SCE) vs time is recorded. The metals are identified by their stripping potentials and
are quantified by measuring the stripping time for each metal.
In this procedure, approximately 0.5 g of dried (105 0c) sediment was decomposed
by boiling for 2 h in a mixture of 3 ml of concentrated hydrochloric acid and 1 ml of
concentrated nitric acid. After the dissolution, the sample was diluted with 10 ml of
redistilled water, filtered (Whatman No. 41) and further diluted with redistilled water
to a total volume of 100 ml. To an aliquot (25 ml) were added 1 ml of 10 % (w Iv)
ascorbic acid solution (antioxidant and reductant for iron(III)) and 1 g of sodium
chloride (to ensure reproducible ionic activity) together with mercury solution
(usually 100 ~ of 1000 ppm Hg solution, but depending on metal concentration) and
an internal standard (cadmium) to correct for variations in oxidation rate.
For mercury plating, ten precoating-stripping cycles were employed. Simultaneously, the sample was deaerated by purging with helium. After precoating and
deaeration of the sample, an appropriate plating time and plating potential were
applied (1-32 min and - 0.95 V vs SCE, respectively).
Copper was quantified by using at least two standard additions. This was done in a
cyclic mode and standards were added immediately after the stripping curve had been
recorded. The plating time was 4 min at - 0.95 V vs SCE; 2.5 /lg of copper was added
at each standard addition and the natural lead content of the sample was used as the
internal standard.
Copper determinations carried out by this procedure were compared to expected
values for a series of reference sediments. The values found were all within the
certified limits. For example, NBS 5RM 1645 reference sediment (reference value 18
± 3 mg kg-I) gave a value of 16.8 mg kg- I by this procedure.
2.1.7
Gallium
Xiao-quan et al. [12] used graphite furnace atomic absorption spectrometry with a
nickel matrix modifier to determine /lg kg-I levels of gallium in perchloric acid digests
of sediments.
2.7.8
Lead
The potentiometric stripping analysis procedure [11] described under copper has also
been applied to the determination of lead in river sediments in amounts down to
7 mg kg- I with a precision of 2.5-4.1 %. Lead determinations carried out by this
procedure were compared to expected values for a series of reference sediments. All
values are within certified limits, for example NBS SRM 1645 sediment (Reference
value 714 ± 2.8 mg kg-I lead gave a value of 722 ± 18 mg kg- I by this procedure.
The graphite furnace atomic absorption spectrometric method described earlier [8]
under cadmium has also been applied to the determination oflead down to 1 mg kg- I