Description of the voltammetric determination of different chromium species in solutions retrived from the vicinity of deep ocean hydrothermal chimneys
Hydrothermal chimneys are formed at tectonically active sites on the sea floor.
Cold sea water seeps into cracks and crevices of the basal-tic rocks and is heated to more than 300C in the sea bed.
During this process, trace elements are leached from the rocks and then transported into the surrounding sea water when the hot solution emerges from the sea floor.
The change in temperature causes the immediate precipitation of the less soluble compounds; these form chimneys that may be several meters high.
It is assumed that the hydrothermal solutions are one of the main sources of many heavy metals in sea water.
Little is known about the chemical valency and form of bonding of most of the metals in the hydrothermal solutions.
Due to the presence of hydrogen sulphide these solutions have reducing properties.
After their release they rapidly mix with the surrounding sea water, which contains oxygen.
As a result of this the valency and configuration of any metals present can alter very quickly. Oxidation by sea water components can occur.
In addition some of the metal ions form complexes very rapidly.
The presence of certain chromium species is used as an indicator for the redox conditions prevailing in the hydrothermal solutions and the surrounding sea water.
The various chromium species differ not only in their chemical behavior, but also in their effects on the biosphere.
Cr(III) is more strongly adsorbed on particles and less quickly taken up by sea organisms than Cr(VI), which is present in sea water mainly as dissolved chromate.
As it is a very strong oxidising agent, Cr(VI) also has a considerably more toxic effect on organisms.
There is thus a direct connection between the degree of oxidation of chromium and its biological availability.
Sea water sampling was carried out by means of a probe equipped with so-called Niskin bottles.
Sea water samples were taken at various depths down to approximately 2000m. Sampling directly at the hydrothermal vents was carried out with the aid of a special sampling system (Hydro Bottom Station, HBS). This allows water samples to be taken 15cm below and 100cm above the sea floor.
Thanks to the two sampling systems it was possible to obtain a depth profile of the chromium concentration in the water column above the tectonically active zones.
Instruments and accessories used included the 746 VA trace analyser with the 747 VA stand was used for the voltammetric analyses.
The chromium determination was carried out at the HMDE.
An Ag/AgCl system with c(KCl) = 3 mol/l was used as the reference electrode, a platinum electrode as the auxiliary electrode.
The 705 UV digester was employed for the UV digestions.
In the reducing environment at the hydrothermal vents chromium is mainly present as Cr(III).
The measured concentrations range up to 2.5ug/l.
In contrast, in normal sea water less than 0.5ug/l chromium is usually found, mainly as Cr(VI).
The reduced form could be detected up to 100m above the sea floor.
It could also be shown that Cr(III) is then mostly complexed by organic compounds or bound to particles.
In normal sea water a slow oxidation of Cr(III) to Cr(VI) occurs.
