Dataset: iodine
Deployment: TT045

Iodide methodology, Luther, Lewis

Iodide, total iodine

ANALYSES
Iodide and iodate concentrations were determined using polarographic
and voltammetric methods.  
Iodide (I-) was measured using cathodic stripping square wave
voltammetry (CSSWV) [Luther et. al., 1988].

Iodate (IO3-) was measured using differential pulse polarography (DPP)
[Herring and Liss, 1974].

Total iodine (...Ired) was measured using CSSWV [Campos (in press)]. 

Total iodine (...Iox) was measured using method of Takayanagi and Wong,
1986 following sample oxidation with 0.2% NaOCl.

The instrument minimum detection limits in seawater for I-, IO3-, ...Iox
and ...Ired using polarography are 0.2, 20, 20, and 5 nM respectively.
Total iodine methodologies gave statistically equivalent values and are
therefore simply reported as Tot_I.

For detailed comparisons please consult (Farrenkopf,1997 --
Dissertation University of Delaware).

Precision for iodide based upon triplicate measurements of individual
samples is within 5-10% in samples greater than 200 nM and within 1-2%
for iodide concentrations less than 200 nM.  Method precisions in 3.5%
NaCl were +/- 1%.  Precisions for the total methods tend to vary
significantly from sample to sample and so reported errors "stdev
Tot_I" reflect the standard deviation of at least three replicates with
three distinct standard addition curves (n>3).

EQUIPMENT 
Electrochemical measurements were made in 10 mL glass polarographic
cells.  EG & G Princeton Applied Research model 384 B polarographic
analyzers equipped with 303A hanging mercury drop working electrode
(HDME) stands were used throughout.  Potentials were measured vs. a
saturated calomel reference electrode (SCE). A platinum counter
electrode was used for current measurements in a standard three
electrode voltammetric arrangement.  Iodide gives rise to a peak at a
potential of -0.306 V, and iodate has a peak potential of -1.08 V.
Aboard ship 10.0 mL aliquots of sample were dispensed into glass
voltammetric cells and purged of dissolved oxygen with ultra pure
nitrogen gas.  The concentrations of iodine species were determined by
the method of standard addition.  A minimum of three standard additions
were made for each determination. Tot_I measurements were also made
with an Analytical Instrument Systems (AIS) DLK-100 with version 3.4
software equipped with a 303A hanging mercury drop working electrode
(HDME) stand.  The analyses on the DLK-100 were the same as with the
384B (Luther et al., 1988) with the exception that the frequency was
200 Hz (as compared to 100 Hz on the 384B).

References:

Campos, M.L.A. (in press) New approach to evaluating dissolved iodine
speciation in natural waters using cathodic stripping voltammetry.
Marine Chemistry

Luther, G. W., III, C. Branson Swartz and W.J. Ullman (1988) Direct
determination of iodide in seawater by cathodic stripping square wave
voltammetry. Analytical Chemistry.  60: 1721-1724.

Luther, G.W., III (1991) Sulfur and iodine speciation in the water
column of the Black Sea, in Black Sea Oceanography, E. Izdar and J. W.
Murray, Editors.  Kluwer Publishers:  Netherlands. p. 187-204.

Herring, J.R. and P.S. Liss (1974) A new method for the determination
of iodine species in seawater. Deep-Sea Research I. 21: 777-783.

Farrenkopf, A.M., G.W. Luther, III, V.W. Truesdale and C.H. van der
Weijden (in press) Sub-surface iodide maxima: Evidence for biologically
catalyzed redox cycling in Arabian Sea OMZ during the SW intermonsoon.
Deep-Sea Research.

Takayanagi, K. and G.T.F. Wong (1986) The oxidation of iodide to iodate
for the polarographic determination of total iodine in natural waters.
Talanta. 33(5): 451-454.

Theberge, S.M., III G.W. Luther and A.M. Farrenkopf (in press)  On the
existence of free and metal complexed sulfide in the Arabian Sea and
it's Oxygen Minimum Zone.  Deep-Sea Research.