[AnnaQ]

Hi,
I'm using the Winkler method to test for dissolved oxygen in water as the distance from a waterfall increases. I know it is usually necessary to titrate the water within 6 to 8 hours of collection, but for me, this is not possible. I have done the titrations over 2 weeks. I had fixed the water samples.
I had added 9mol L-1 sulphuric acid at the beginning of the 2 week period to all bottles, but I noticed that the titre values decreased after a few days - e.g. for point A, I did the first lot of titrations immediately after adding sulphuric acid and the titre was around 29 mls. However, after 5 days, to check for reliability, I returned to point A, but the titre became lower at an average of 25ml. All my bottles were sealed.
Would anyone know why this happens?
Thanks.

[Nobby]

Are the bootles filled to the top? If not oxygen and also other gases will released in the volume above water level.

I guess after opening first time the process started if the solution wouldn't stored in another smaller container.

[enahs]

Also, the solubility of oxygen in water varies greatly with the temperature of the water. For a true to life measurement, you need to either do it right away, or maintain them at the same temperature as the water in the waterfall.


But you might have microorganisms in the water causing the oxygen volume to change over time, as well. Protecting them from light might help limit that effect, but will not completely stop it. 


In short, do it ASAP.