Organometallic compounds in soils 119
are extracted from 1g soil by shaking for 2 hours with 10 mL chloroform and in a
similar procedure [41] the sediment is wet oxidised with dilute sulphuric acid and
nitric acids in an apparatus in which the vapour from the digestion is condensed into a
reservoir from which it can be collected or returned to the digestion flask as required.
The combined oxidised residue and condensate are diluted until the acid concentration
is 1N and nitrate is removed by addition of hydroxylammonium chloride with boiling.
Fat is removed from the cooled solution with carbon dithizone in carbon tetrachloride.
The extract is shaken with 0.1M hydrochloric acid and sodium nitrite solution and,
after treatment of the separated aqueous layer with hydroxylammonium chloride a
solution of urea and then EDTA solution are added to prevent subsequent extraction
of copper. The liquid is then extracted with a 0.01% solution of dithizone in carbon
tetrachloride and mercury estimated in the extract spectrophotometrically at 485 nm.
A disadvantage of all the above procedures is that the lowest concentration of
mercury that can be determined in soil samples is of the order of 0.05–1 mg kg
−1 .
These high detection limits are in part due to high blanks caused by the multiplicity of
digestion reagents used in the procedures. Several investigators have liberated mercury
from soil and sediment samples by application of heat to the samples and collection of
the released mercury on gold surfaces. The mercury was then released from the gold
by application of heat or by absorption in a solution containing oxidising agents [41].
Bretthauer et al [42] and Anderson et al [43] described a method in which samples were ignited in a high-pressure oxygen-filled bomb. After ignition the mercury
was absorbed in a nitric acid solution. Pillay et al [44] used a wet-ashing procedure
with sulphuric acid and perchloric acid to digest samples. The released mercury was
precipitated as the sulphide. The precipitate was then redigested using aqua regia.
Gas chromatography
Longbottom et al [45] have described gas chromatography methods for the determination of alkylmercury in soils and sediments.
Atomic absorption spectrometry
Langmyhr et al [46] have applied cold vapour atomic absorption spectrometry to the
determination of organomercury compounds in soils and sediments.
Miscellaneous
Feldman et al [47] digested solid samples with potassium dichromate, nitric acid, perchloric sulphuric acid. Bishop et al [48] used aqua regia and potassium permanganate
for digestion.
The approved US Environmental Protection Agency [49] digestion procedure
requires aqua regia and potassium permanganate as oxidants. These digestion procedures are slow and often hazardous because of the combination of strong oxidising
agents and high temperatures. In some of the methods, mercuric sulphide is not adequately recovered. The oxidising reagents, especially the potassium permanganate,
are commonly contaminated with mercury, which prevents accurate results at low
concentrations.
As Umezaki and Iwamoto et al [61] have reported that organic mercury can be
reduced directly with stannous chloride in the presence of sodium hydroxide and
are extracted from 1g soil by shaking for 2 hours with 10 mL chloroform and in a
similar procedure [41] the sediment is wet oxidised with dilute sulphuric acid and
nitric acids in an apparatus in which the vapour from the digestion is condensed into a
reservoir from which it can be collected or returned to the digestion flask as required.
The combined oxidised residue and condensate are diluted until the acid concentration
is 1N and nitrate is removed by addition of hydroxylammonium chloride with boiling.
Fat is removed from the cooled solution with carbon dithizone in carbon tetrachloride.
The extract is shaken with 0.1M hydrochloric acid and sodium nitrite solution and,
after treatment of the separated aqueous layer with hydroxylammonium chloride a
solution of urea and then EDTA solution are added to prevent subsequent extraction
of copper. The liquid is then extracted with a 0.01% solution of dithizone in carbon
tetrachloride and mercury estimated in the extract spectrophotometrically at 485 nm.
A disadvantage of all the above procedures is that the lowest concentration of
mercury that can be determined in soil samples is of the order of 0.05–1 mg kg
−1 .
These high detection limits are in part due to high blanks caused by the multiplicity of
digestion reagents used in the procedures. Several investigators have liberated mercury
from soil and sediment samples by application of heat to the samples and collection of
the released mercury on gold surfaces. The mercury was then released from the gold
by application of heat or by absorption in a solution containing oxidising agents [41].
Bretthauer et al [42] and Anderson et al [43] described a method in which samples were ignited in a high-pressure oxygen-filled bomb. After ignition the mercury
was absorbed in a nitric acid solution. Pillay et al [44] used a wet-ashing procedure
with sulphuric acid and perchloric acid to digest samples. The released mercury was
precipitated as the sulphide. The precipitate was then redigested using aqua regia.
Gas chromatography
Longbottom et al [45] have described gas chromatography methods for the determination of alkylmercury in soils and sediments.
Atomic absorption spectrometry
Langmyhr et al [46] have applied cold vapour atomic absorption spectrometry to the
determination of organomercury compounds in soils and sediments.
Miscellaneous
Feldman et al [47] digested solid samples with potassium dichromate, nitric acid, perchloric sulphuric acid. Bishop et al [48] used aqua regia and potassium permanganate
for digestion.
The approved US Environmental Protection Agency [49] digestion procedure
requires aqua regia and potassium permanganate as oxidants. These digestion procedures are slow and often hazardous because of the combination of strong oxidising
agents and high temperatures. In some of the methods, mercuric sulphide is not adequately recovered. The oxidising reagents, especially the potassium permanganate,
are commonly contaminated with mercury, which prevents accurate results at low
concentrations.
As Umezaki and Iwamoto et al [61] have reported that organic mercury can be
reduced directly with stannous chloride in the presence of sodium hydroxide and