Cruise Report, RV ATAIR, Cruise 0671021-1, Bremerhaven - Thyboron - Aberdeen, 18 Jul —- 10 Aug 2023 | 
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settle, the sample is acidified (pH -1-2.5). The precipitate dissolves and the iodine jons added 
during fixation are oxidised to an equivalent amount of iodine. The concentration of the 
released iodine is determined by titration with a standardised sodium thiosulphate solution. 
The consumption of thiosulphate is proportional to the original amount of dissolved oxygen. 
immediately after sampling, 1 ml of manganese(ll) chloride solution and 1 ml of sodium 
jodide/sodium hydroxide solution were added to the sample. The sample bottle was then 
sealed with the appropriate ground glass stopper without trapping any air bubbles. It was 
subsequently shaken for approx. 2 minutes, during which time a white to brown precipitate 
formed (depending on the oxygen concentration). The samples were darkened, and after 
approx. 20 to 30 minutes the precipitate had settled. Before titration, a titre determination was 
sarried out every working day, followed by a blank value determination once a week or when 
changing reagents. Calibration/standardisation for oxygen determination is carried out by 
titrating the titrant solution (sodium thiosulphate solution). Potassium iodate (KlOs) is used as 
the standard titrant. The software used to operate the DOA performs the calculations to obtain 
values for dissolved oxygen internally. The calculations are based on internationally accepted 
recommendations. In order to assess the resulting data quality, double samples have been 
analysed. Unfortunately, any certified reference material (CRM) or interlaboratory performance 
studies (like QUASIMEME) do not exist for the parameter ‘dissolved oxygen in seawater’. The 
mean range for 20 samples taken as duplicates on this cruise was less than 0.03 mI/l DOXY. 
KH(IOs)2-solution was measured from 33 samples as an IRM (irnternal reference material) and 
resulted in a deviation of less than 0.01 mI/I for the titrant. 
Oxygen data obtained from water samples were subsequently used for comparison with and 
verification of the CTD-based oxygen sensor SBE43 
5.5. 
Water Sampling with respect to Densıty 
Water sampling with respect to density analysis was carried out on selected stations. Similarly 
to salinity, water samples were taken from the Niksin bottles that were closed near the surface 
at 5 dbar and close to the bottom. Numbered glass bottles were used and rinsed three times 
with seawater before finally filling the bottle. The sample was made air-tight by closing the 
bottle with a rubber plug and attaching a metal seal on top. All samples were given a BSH 
sample Identification number similar to Bedford numbers. Back at the home lab, density 
samples were analysed with a densitymeter of type Mett/er Toledo Excellence D6. In total, 36 
samples taken from station locations distributed all across the North Sea were analysed for 
density. Figure 5.2 presents the station locations with density sampling carried out during 
zruise 067/021-1. Sampling and analysis is done with respect to building up a density database 
for the North Sea for the sake of obtaining absolute salinity data, Sa. These should be based 
on density measurements that can be traced back to SI units, a prerequisite when considering 
the Thermodynamic Equation of Seawater 2010 (TEOS-10).