Determination of labile iron at low nmol L-1 levels in estuarine and coastal waters by anodic stripping voltammetry

A new method is presented for the determination of electrochemically labile iron in estuarine and coastal seawater. The method is based on differential pulse anodic stripping voltammetry (DPASV) at a rotating silver-alloy disk electrode. The voltammetric parameters include a plating potential of -1.5 V and an activation potential of -5 V for 10s; the seawater is at the original sample pH. The main finding is the presence of a peak for low nmol L-1 levels of iron at -0.55 V ascribed to elemental iron deposited on the bare silver alloy electrode. The peak increased linearly with the iron concentration between <1 and 14 nmol L-1 using a 900 s plating time. At higher concentrations an additional iron peak appeared at -0.7 V which was also found to increase linearly with the iron concentration but at a higher concentration ran-e from ca. 15 to 90 nmol L-1 using a 300 s plating time. The second peak was ascribed to iron deposited on iron. Additions of chelating agents (EDTA and a siderophore) to seawater caused the iron peak to be masked indicating that this method is suitable for iron speciation as only the electrochemically labile fraction is determined. The detection limit was 0.3 nmol L-1 using a 900 s plating time. The method was used to determine iron in the range of 5 to 50 nmol L-1 in samples from the Mersey estuary near Liverpool and its potential use for in situ monitoring was demonstrated by using it to monitor labile iron (at 2-3 nmol L-1) over a period of 4 days at I It intervals in coastal waters in the Trondheim fjord, Norway.