Hi
I'm performing dissolved oxygen experiment for sea water sample. I check normality of sodium thiosulphate every time before titrating with sample. Normality is 0.0125 and titrated value for sea water is 0.7ml and blank value is 0.1ml. My results are 0.8ml/l and 1.2mg/l. But, sea waters average DO level is 5mg/l. I've attached the word file of the procedure that I follow. It will be help full if anyone can pin point my mistake.
Dear Umer,
This is enough distributed effect when using Winkler method! Can happen when your acidity is too high - started oxidation by iodine by oxygen from air! Should keep pH=2,6-2,7!
As well iodine as complex particle J3 minus when oversaturated sample can disappear.
As well manganese solution has oxygen!
As well exist some other.. How to avoid is written in the article:
Carpenter J.H. The Chesapeake Bay Institute technique for the Winkler dissolved oxygen method. // Limnol. Oceanogr. 1965, v. 10, _1, pp. 141-143. (Attached!)
Andrey
Dear umer,
Please flow the following SOP might be u can get correct result. instead of 125 ml sample container use 300 ml and prepare the normality of the STS 0.025 N and titrate with 200 ml volume of sample. The volume of sts consumed is equal to mg/l of DO present in your sample.
Links
https://www.google.co.ve/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=I'm+in+need+of+clarification+for+my+Dissolved+Oxygen+procedure%3F
http://nitttrc.ac.in/four%20quadrant/eel/quadrant%20-%201/exp10_pdf.pdf
http://serc.carleton.edu/microbelife/research_methods/environ_sampling/oxygen.html
ftp://data.nodc.noaa.gov/pub/data.nodc/...0/.../dickson.pdf
https://www.epa.ie/pubs/advice/water/wastewater/EPA_water_%20treatment_manual_primary_secondary_tertiary1.pdf
http://www.studyit.org.nz/Communicate/viewtopic.php?p=239296&sid=6dc026b55d6057d40313d5fcbc57d5df
Sometimes the problem is not the titration, but ti is the sample gathering. Do you fell sure of ir?
I suggest you look at your sample collection and also the time you take between collection and analysis ... once you have done that prepare fresh reagents and do the analysis ...
I think you took your sample from an area which has sewage discharge near shore or near algal bloom or may be you took it during windy weather.
if this not happened please do astndardization of all your reagent as mention in the book of strickland for seawater analysis and you will find it herre and you can download it for free
https://books.google.it/books/about/A_practical_handbook_of_seawater_analysi.html?id=sPNOAAAAMAAJ&redir_esc=y
kind regards,