This International Standard specifies two methods for the determination of mercury, one using tin(II) chloride and the other sodium tetrahydroborate as reducing agent. The methods are suitable for the determination of mercury in water, for example in ground, surface or waste water, in the concentration range 0,01 μg/l to 1 μg/l. Higher concentrations may be determined if the water sample is diluted.

The total mercury content of the water is determined after digestion of the sample. If only soluble mercury compounds are to be determined, the sample is filtered through a 0,45 μm membrane filter prior to digestion.

Mono- or divalent mercury is reduced to the elemental form by a reducing agent such as tin(II) chloride, SnCl₂, or sodium tetrahydroborate, NaBH₄, in an acid medium. Elemental mercury is then stripped from the solution with theaid of a stream of inert gas with negligible mercury content and transported over a noble-metal surface with a large area, such as gold/platinum gauze, on which the mercury is adsorbed. The mercury is released by rapid heating of the adsorbent and further transported in a stream of carrier gas to a suitable cuvette. Absorbances are measured at a wavelength of 253,7 nm in the radiation beam of an atomic absorption spectrometer. Concentrations are calculated using a calibration curve.

In order to fully decompose all of the mercury compounds, a digestion procedure is required. Digestion can be omitted only if it is certain that the mercury concentration may be measured without this pretreatment.

Careful consideration should be given to whether, and to what extent, particular problems will require the specification of additional conditions.

It is absolutely essential that analyses conducted in accordance with this International Standard are carried out by suitably qualified staff.

In natural water sources, mercury compounds generally occur only in very small concentrations of less than 0,1 μg/l. Higher concentrations may be found, for example, in waste water. Both inorganic and organic compounds of mercury can be present. Mercury may accumulate in sediment and sludge.