Determination Of Metallic Elements

A. Determination by Ashing

When a metallic element is a constituent of an organic compound, it will leave a solid residue on complete decomposition of the sample by strong heating. This provides a simple technique, known as ashing, for determining metallic elements, except for mercury and osmium, which form volatile products. The organic substance can be heated in a crucible or microboat: decomposition by heat is aided by moistening the sample with nitric or sul-furic acid. After heating, noble metals such as platinum leave the pure metal:

Organic platinum compound

HNO3

Other metallic elements are converted to oxides, for example,

Organic copper compound hno3

CuO,

or are recovered in the form of sulfates, for instance, h2so4

Organic sodium compound-> Na2SO4. (32)

If the organic sample contains a single metallic element, the residue is weighed and its metal content can be calculated. If two or more metallic elements are present in the original substance, the residue is brought into solution and subjected to inorganic analysis.

B. Determination of Mercury

Because of its volatility, mercury in organic compounds cannot be determined by the ashing technique described above. Instead, the organic sample is placed in a porcelain microboat and heated in a combustion tube packed with granulated calcium oxide. The mercury vapor that forms is conducted to a tube containing gold foil, which traps the mercury to produce amalgam. The increase in weight of the gold tube gives the content of mercury in the sample.

Amalgamation in a gold trap is also employed in environmental analysis of organic mercury compounds such as methyl mercury. The mercury is commonly determined by cold vapor atomic absorption spectroscopy.

When an organic mercury compound is decomposed by digestion in a mixture containing nitric acid, sulfuric acid, and potassium permanganate, the resultant mercuric ions are preferably determined by titration with standardized potassium thiocyanate solution:

Organic mercury compound and

H2SO4,HNO3,KMnO4

Ferric ammonium sulfate is used as the indicator, and the end point is located spectrophotometrically.

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