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Iodine speciation measured from samples taken from CTD niskin bottles on SSD-024 (BoBBLE)

Originator's Protocol for Data Acquisition and Analysis:

Sampling methodology

Samples of 50-200 mL were taken of a standard CTD using Niskin bottles or from the underways seawater supply in acid washed HDPE bottles. Samples were filtered on GF/F filters and frozen at -20°C in acid washed HDPE bottles and kept frozen during storage and transport until analysis in the lab in 2017 (as suggested in Campos et al., Marine Chemistry, 1997). Part of the samples was used for iodide analysis, the remaining volume was used to determine iodate.

Analytical methodology

Iodide was analysed by cathodic stripping square wave voltammetry using a µAutolab III potentiostat connected to a 665VA stand (Metrohm) with a hanging mercury drop electrode, using a Ag/AgCl reference electrode and a carbon or platinum auxiliary electrode. 12 mL of the sample was introduced in a glass cell and 90µL of Triton X-100 (0.2%) was added. The sample was purged with oxygen free grade N ₂ for 5 minutes before each measurement. The deposition potential was set at 0 V and deposition times were typically 60 seconds, scan step was 2 mV with a 75 Hz frequency and 0.02 V wave amplitude and ranged from 0 to -0.7 V. Each scan was repeated 6 times. Repeatability was equal to or better than 5%. Iodide concentrations were determined using 2 or 3 standard additions of a KI solution, aiming to cover the end of the linearity domain with the last addition. Confirmation of the high concentration samples (station AR_10& AR_25 and UWIBS_3 & UWIBS_8) was performed by ion chromatography on a Dionex ICS-2000 equipped with EGC III KOH elugen cartridge, CR-ATC continuously regenerated trap column, ASRS 300 2-mm suppressor, DS6 heated conductivity detector and coupled to a Dionex AS40 autosampler. A IonPac AG18 guard column (2 mm id x 50 mm length) and IonPac 18AS anion column (2 mm id x 250 mm length) were used and the oven temperature was kept at 30°C. The eluent was a KOH solution and two different gradients were used: 2-20 mM (0-18 min) and then then 20-41 mM (18-19 min, hold 2 min). The sample volume injected was 5 µL with a flow rate of 0.25 mL/min. Values reported for these samples are averages of both methods. Iodate was analysed in triplicate with a 4 decimal places spectrophotometer after reduction to iodonium (UV-1800, Shimadzu). 2.3 mL of the sample was introduced in the 1 cm UV quartz cell, 50µL of sulfamic acid (1.5M) was added to lower nitrite interferences and the first absorbance value was obtained after 1 minute. Then 150µL of KI (0.6M) was added and the second absorbance read after 2.5 minutes. The absorbance value used is the difference between both absorbances. Samples were measured at least in triplicate; reported values are means. Repeatability was equal to or better than 5%. Calibration was performed daily and correction for instrument drift was accounted for by measuring a standard every 5 samples. This method measures all inorganic iodine in oxidation states from 0 to +5 but this fraction is dominated by iodate and is taken as a measure of iodate (Chance et al., 2007). Reported errors are calculated using the standard deviation on the repeated measurements, the errors on the fit of the calibration and error on the volumes pipetted.

References Cited

Campos, M., 1997. New approach to evaluating dissolved iodine speciation in natural waters using cathodic stripping voltammetry and a storage study for preserving iodine species. Marine Chemistry 57 (1-2), 107-117

Chance, R., Malin, G., Jickells, T., Baker, A.R., 2007. Reduction of iodate to iodide by cold water diatom cultures. Marine Chemistry 105 (1-2), 169-180.

BODC Data Processing Procedures

Data received were loaded into the BODC database using established BODC data banking procedures. A parameter mapping table is provided below: