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The diversity and complexity of the available approaches has identified the major difficulties associated with producing suitable guidelines that would allow comparisons between different laboratories and different countries in assessing metal mobility in the soil environment. This led to the development of single and sequential extraction procedures by the Standards, Materials and Testing (SM&T - formerly the (European) Community Bureau of Reference (BCR) Programme of the European Union (1987)). Single extractants evaluated included 0.05 moll-1 EDTA, 0.43 moll-1 acetic acid, and 1 moll-1 ammonium acetate at pH 7.
Take a look at the results of this study in Table 5.9 below. As a result of these findings, the use of ammonium acetate was excluded as a suitable single extraction method. Why do you think this was?
Ammonium acetate was excluded due to its poor reproducibility, probably as a result of the low concentrations of metals extracted.
5.7.3 Single Extraction Procedures
The procedures for the extraction of metals using the two single extraction methods (0.05 moll-1 EDTA and 0.43 moll-1 acetic acid) are shown in Figures 5.17 and 5.18, respectively.
Table 5.9 Metal contents of soil, determined by using single extractants (mg kg 1 dry soil): interlaboratory results (n = 35) 
Metal Cd Cr Cu Ni Pb Zn
EDTAa (0.05 moll-1) extracts
Mean 23.1 8.05 162.0 16.3 255.0 492.0
RSDb 9.2 25.9 8.5 13.0 11.8 5.8
Acetic acid (0.43 moll 1) extracts
Mean 19.3 26.1 29.2 15.7 3.36 522.0
RSDb 7.5 8.2 10.0 18.1 24.6 6.9
Ammonium acetate (1 moll-1) extracts
Mean 3.43 1.39 5.65 1.42 2.21 18.4
RSDb 10.9 40.6 23.4 22.5 26.8 22.5
a Ethylenediaminetetraacetic acid. bRSD, relative standard deviation (%).
Figure 5.17 Procedure adopted in the single extraction method for metals (employing EDTA), as applied to the analysis of soils and sediments.
For these procedures (Figures 5.17 and 5.18), it is important to note the following:
• All laboratory ware should be made of borosilicate glass, polypropylene, polyethylene or PTFE, except for the centrifuge tubes which should be made of either borosilicate glass or PTFE.
Methods for Environmental Trace Analysis
Figure 5.18 Procedure adopted in the single extraction method for metals (employing
acetic acid), as applied to the analysis of soils and sediments.
• Clean all vessels in contact with samples or reagents with HNO3 (4 moll-1) for at least 30 min, rinse with distilled water, clean with 0.05 moll-1 EDTA and then rinse again with distilled water. Alternatively, clean all vessels in contact with samples or reagents with HNO3 (4 moll-1) overnight and rinse with distilled water.
• Use a mechanical shaker, preferably of the ‘end-over-end’ type, at a speed of 30 rpm. Carry out the centrifugation at 3000 x G.
The reagents to be used are as follows:
• Water. Glass-distilled water is suitable. Analyse a sample of distilled water with each batch of extracts.
• EDTA (0.05 moll-1). Add in a fume cupboard, 146 ± 0.05 g of EDTA (free acid) to 800 ± 20 ml of distilled water and partially dissolve, by stirring, in 130 ± 5 ml of saturated ammonia solution (prepared by bubbling ammonia gas into distilled water). The addition of ammonia shall be continued until all of the EDTA has dissolved. The obtained solution should be filtered, if necessary, through a filter paper of porosity 1.4-2.0 ^m, into a 10 l polyethylene bottle and diluted to 9.0 ± 0.5 l with distilled water. The pH should be adjusted to 7.00 ± 0.05 by the addition of a few drops of either ammonia or hydrochloric acid (as appropriate). The solution should then be made up to 10 l with
distilled water to obtain an EDTA solution of 0.05 mol l 1. Analyse a sample of each batch.
• Acetic acid (0.43 moll-1). Add, in a fume cupboard, 250 ± 2 ml of glacial acetic acid (‘AnalaR’ grade or similar) to about 5 l of distilled water in a 10 l polyethylene bottle and make up to 10 l with distilled water.
The single extraction procedure can be carried out for the following trace metals: Cd, Cr, Cu, Ni, Pb and Zn. A separate sub-sample of the sediment or soil should be dried in a layer of approximately 1 mm in depth in an oven at 105 ± 2° C for 2-3 h and then weighed. From this, a correction ‘to dry mass’ can be achieved and applied to all of the analytical results obtained (quantity per g dry sediment/soil).
While the complete experimental details for the single extraction methods are shown in Figures 5.17 and 5.18, it is important to note the following:
• The calibrating solutions should be made up with the appropriate extracting solutions.
• For every batch of extractions, a blank sample, i.e. a container with no sediment/soil, should be carried through the entire procedure.
• The sediment/soil should be in complete suspension during the extraction. If this is not the case, adjust the shaking speed to ensure that the suspension is maintained.
The results obtained from the certification of two certified reference materials using this approach are shown in Table 5.10.