I am doing oxygen measurements using the Winkler method in marine waters under the influence of riverine inputs. Salinity never goes down 30. Recently, I am experiencing some difficulties to dissolve the precipitate. I read that "In running determinations of dissolved oxygen on fresh or slightly brackish waters, considerable difficulty may be experienced in obtaining complete solution of precipitate" (Codispoti 1988). Why it that so? Organic matter in suspension (I prefiltered under 200 um)? Salinity?

I tried adding some more drops of sulfuric acid (R3), but it doesn't change. I tried to change R3 with HCl 37 %, but it doesn't work either. I found some ref about using AIK(SO4)2.12H2O and NH3) to separate material in suspension, is this working?

Thanks in advance for your help!