Determination of the reagent blank

The reagents added to the seawater sample contain oxygen. This amount of oxygen is minimized by using reagents that are almost saturated with respect to their salt content. According to Murray el al. (1968) 1 mL of the reagents contains approximately 0.0017 mL 

As the ratio of reagent addition and sample volume (Vr / Vs) is constant (2 100 in nearly all modifications of the Winkler method), DOr is 0.034 mL/L or 1.52/anol/L. 

About 15 mL of pure water are placed in a sample bottle. A 1 mL volume of the 50 % sulphuric acid solution and the amounts of alkaline iodide solution and manganese(/7) chloride used for fixation are added separately. After each reagent addition the solution is thoroughly mixed to avoid any precipitation of manganese hydroxides. 

Hien add l.OOmL of the iodate standard solution, fill up to just below the bottle neck (about 5 mL less) and titrate with the thiosulphate solution until the exact equivalence point. 

Add another 1.00 ml of the iodate standard and titrate again until the equivalence point. The reagent blank is the difference between the first and the second thiosulphate titration volume, i.e., 

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