Metals
19
There was one unsatisfactory recovery with perchloric acid, this being that of
chromium. The low boiling point of chromyl chloride (Cr0 2 CI 2 ), 116°C, compared
with about 200°C for perchloric acid, probably results in volatilisation losses. With
nitric acid or aqua regia, these losses do not occur because the boiling points of nitric
and hydrochloric acids are lower.
Aqua regia and nitric acid are weaker extracting agents than perchloric acid. Aqua
regia (Table 2.10) is a stronger oxidising and extracting agent than nitric acid as a
result of the presence of free or nascent chlorine. Nitric acid, aqua regia and perchloric acid have their strongest leaching effect when they are boiling. Perchloric acid,
especially, is a strong leaching, dehydrating and oxidising agent only when it is hot
and concentrated.
Cold extraction methods are the weakest as they do not attack the silicate lattice
appreciably. With such methods it is usually desirable to extract the non-residual
metal from the sediments. The three methods of this type studied are compared in
Table 2.11, together with a total extraction method for manganese, iron and aluminium, for thirteen sediments of different types. The extraction efficiency of the three
methods is in the increasing order 0.05 N ethylenediaminetetraacetic acid, 1 N hydroxylammonium chloride plus 25 % acetic acid, and 0.5 N hydrochloric acid. This trend
correlates with the decreasing pHs of 4.8, 1.5 and 0.3, respectively, for the above
reagents. From their chemical properties, it would be expected that these methods
would extract the adsorbed, precipitated and complexed metals.
The results given in Table 2.11 indicate that the cold-extraction methods extract
only a very small fraction of the total aluminium from the sediments (compare
methods a, b, and c wi!h method d). On the other hand, a considerable amount of the
manganese is extracted. The fraction of total iron extracted by use of these methods is
intermediate between that of manganese and aluminium. Both iron and aluminium
are mainly found in the residual phase. About 14 ± 10, 17 ± 10 and 32 ± 6 % of the
total iron was extracted by methods a, band c, respectively (Table 2.11). For aluminium, about 2 ± 1.5, 4 ± 4 and 10 ± 4 % of the total was extracted, respectively, for
methods a, band c. This shows that these methods do not affect the crystalline
structure appreciably. To substantiate further the above contention, the amount of
silicon extracted by the total metal and 0.5 N hydrochloric acid extraction methods
was measured. Table 2.11 gives the total amount of silicon in each of the samples. It
was found that the mean amount of silicon extracted by use of the 0.5 N hydrochloric
acid method was about 1 % of the total.
Table 2.12 shows the results obtained for the same samples, with the three partial
extraction methods, for the seven trace elements studied. The order of efficiency of
extraction of the methods is again increasing from 0.05 N ethylenediamine tetraacetic
acid to 0.5 N hydrochloric acid, except for copper, where 1 N hydroxylammonium
chloride plus 25 % acetic acid gives the lowest extraction. Hydrochloric acid (pH 1.0)
liberates copper complexed with humic compounds isolated from soils [43]. However,
1 N hydroxylammonium chloride plus 25 % acetic acid mixture is not a strong enough
complexing agent to compete in complex equilibria, nor acidic enough to cause
dissociation of the natural complexes. The result of this effect is seen with copper
(which correlates highly with organic matter), where ethylenediaminetetraacetic acid,
which is the weakest extracting agent, shows higher values for copper than the
19
There was one unsatisfactory recovery with perchloric acid, this being that of
chromium. The low boiling point of chromyl chloride (Cr0 2 CI 2 ), 116°C, compared
with about 200°C for perchloric acid, probably results in volatilisation losses. With
nitric acid or aqua regia, these losses do not occur because the boiling points of nitric
and hydrochloric acids are lower.
Aqua regia and nitric acid are weaker extracting agents than perchloric acid. Aqua
regia (Table 2.10) is a stronger oxidising and extracting agent than nitric acid as a
result of the presence of free or nascent chlorine. Nitric acid, aqua regia and perchloric acid have their strongest leaching effect when they are boiling. Perchloric acid,
especially, is a strong leaching, dehydrating and oxidising agent only when it is hot
and concentrated.
Cold extraction methods are the weakest as they do not attack the silicate lattice
appreciably. With such methods it is usually desirable to extract the non-residual
metal from the sediments. The three methods of this type studied are compared in
Table 2.11, together with a total extraction method for manganese, iron and aluminium, for thirteen sediments of different types. The extraction efficiency of the three
methods is in the increasing order 0.05 N ethylenediaminetetraacetic acid, 1 N hydroxylammonium chloride plus 25 % acetic acid, and 0.5 N hydrochloric acid. This trend
correlates with the decreasing pHs of 4.8, 1.5 and 0.3, respectively, for the above
reagents. From their chemical properties, it would be expected that these methods
would extract the adsorbed, precipitated and complexed metals.
The results given in Table 2.11 indicate that the cold-extraction methods extract
only a very small fraction of the total aluminium from the sediments (compare
methods a, b, and c wi!h method d). On the other hand, a considerable amount of the
manganese is extracted. The fraction of total iron extracted by use of these methods is
intermediate between that of manganese and aluminium. Both iron and aluminium
are mainly found in the residual phase. About 14 ± 10, 17 ± 10 and 32 ± 6 % of the
total iron was extracted by methods a, band c, respectively (Table 2.11). For aluminium, about 2 ± 1.5, 4 ± 4 and 10 ± 4 % of the total was extracted, respectively, for
methods a, band c. This shows that these methods do not affect the crystalline
structure appreciably. To substantiate further the above contention, the amount of
silicon extracted by the total metal and 0.5 N hydrochloric acid extraction methods
was measured. Table 2.11 gives the total amount of silicon in each of the samples. It
was found that the mean amount of silicon extracted by use of the 0.5 N hydrochloric
acid method was about 1 % of the total.
Table 2.12 shows the results obtained for the same samples, with the three partial
extraction methods, for the seven trace elements studied. The order of efficiency of
extraction of the methods is again increasing from 0.05 N ethylenediamine tetraacetic
acid to 0.5 N hydrochloric acid, except for copper, where 1 N hydroxylammonium
chloride plus 25 % acetic acid gives the lowest extraction. Hydrochloric acid (pH 1.0)
liberates copper complexed with humic compounds isolated from soils [43]. However,
1 N hydroxylammonium chloride plus 25 % acetic acid mixture is not a strong enough
complexing agent to compete in complex equilibria, nor acidic enough to cause
dissociation of the natural complexes. The result of this effect is seen with copper
(which correlates highly with organic matter), where ethylenediaminetetraacetic acid,
which is the weakest extracting agent, shows higher values for copper than the