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RECEIVED for review July 2, 1987. Accepted April 18, 1988. We thank the Kawakami Memorial Foundation and the Ministry of Education for financial support.
Direct Determination of Iodide in Seawater by Cathodic Stripping Square Wave Voltammetry George W. Luther, III,* Christina Branson Swartz, and William J. Ullman
College of Marine Studies, University of Delaware, Lewes, Delaware 19958
A procedure for the dlrect determlnatlon of iodide in seawater is descrlbed. By use of cathodic strlpping square wave voltammetry, It Is posslble to determine low and subnanomoiar levels of iodide in seawater, freshwater, and bracklsh water. Precislon is typically f5 YO (la). The minimum detection limn Is 0.1-0.2 nM (12 parts per trllllon) at a 180-s deposition time. Data obtalned on Atlantlc Ocean samples show similar trends to previously reported iodine speclation data. This method is more sensltive than previous methods by 1-2 orders of magnltude. Triton X-100 added to the sample enhances the mercury electrode’s sensitlvlty to iodide.
Iodate and iodide are the two principal forms of iodine in seawater ( I ) . To date, the principal methods for determining iodine speciation in marine waters have centered on the analysis of the five-electron irreversible wave of iodate using differential pulse polarography ( 2 , 3 )or the colorimetric determination of IO3- after conversion to Is- in acidic solution ( 4 , 5 ) . In each method iodate is determined directly on a sample. Total iodine is determined by converting iodide and organic iodine compounds to iodate via chemical (3, 5 ) or photochemical means (2). Thus, iodide and organic iodine are determined by difference. The minimum detectable limit of iodate obtained by using the differential pulse polarography method is near 20 nM. For the colorimetric method, the minimum detectable limit (MDL) is 30-50 nM in seawater. However, in freshwater and estuarine water, the MDL is higher due to UV absorption by naturally occurring organic matter at the wavelength for Is- detection (350 nm). Total iodine concentration in seawater (35%0)is approximately 470 nM (2, 4-7). In surface ocean waters, iodate concentrations are near 300-400 nM and iodide concentrations can approach 100-200 nM ( 2 , 4 ) . At depth, iodate is the only
form that can be easily distinguished by these methods. Low levels of iodide (
