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GSC cold-extractable “heavy metals”
On-site semi-quantitative measurement of “heavy metals”
This method of geochemical prospecting is based on the premise that some of the metal in a sample of soil or stream sediment is loosely attached to the surfaces of the mineral grains or organic materials. This adsorbed or 'loosely bonded' metal, which may amount to as much as 20 per cent of the total metal in the sample, may be removed by leaching the sample with a dilute solution of ammonium citrate, (NH4)2HC6H5O7, or even water. The heavy metal thus removed is then determined by reacting it with dithizone, C13H12N4S, to form a coloured product. The colour produced is a measure of the metal content of the sample.
Because the method is designed for rapidity and ease of use in the field, samples are not weighed but are measured with a small scoop. A scoopful of the sample is placed in the analysis tube with five millilitres (ml) of the ammonium citrate buffer solution. The function of the buffer solution is to dissolve the 'loosely-bonded' metal referred to above, and to maintain the acidity of the sample solution at a pH of 8.5, the point at which the dithizone reacts most rapidly and completely with the greatest number of heavy metals. The buffer solution contains hydroxylamine hydrochloride to prevent iron from interfering with the test. Five millilitres of a dithizone-benzene solution is then added to the tube. The tube is corked and shaken vigorously for exactly 30 seconds. The benzene layer is allowed to separate and is observed for a colour change which may range from th original bright green to red, through the sequence, blue-green, blue, blue-purple, and purple. Any colour other than green is an indication that the sample contains an appreciable quantity of metal. Additional arnounts of the dithizone-benzene solution are added with shaking until a standard colour, or end point, usually blue, is reached. The amount of dithizone required to reach this point is then a measure of the cold extractable heavy-metal content of the sample.
[Smith, A.Y. (1964). Field and laboratory methods used by the Geological Survey of Canada in geochemical surveys, No. 5 Cold extractable “heavy metal” in soil and alluvium. Geological Survey of Canada, Paper 63-49.]
Sample material: solid
Sample weight: unstated
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