Iridium (Ir)

Atomic number:	77
Atomic weight:	192.22
Average concentration in ocean:	680 amol/kg
Residence time:	2000 to 20,000 years

Distribution in ocean
The vertical profile of Ir is not well resolved due to the very small number of samples (5) that have been analyzed (data). Ir has the distinction of having the lowest mean concentration of any stable element in the ocean (680 amol/L = 680x10^-18 mol/L = 6.023x10²³ atom/mol x 680x10^-18 mol/kg = 409x10⁶ atoms/L, which is a very small number of atoms).

Speciation
Iridium probably exists in the +III oxidation state as chloro- or hydroxy- complexes.

Residence time
The upper limit on the iridium residence time was determined from the global average river input and the lower limit from the rate of removal to sediments (Anbar et al., 1996).

Analysis
Iridium is determined by isotope dilution using thermal ionization, mass spectrometry after concentration with anion exchange resins.

References