What causes oxygen readings to vary with Winkler titration in an aquarium?

[Ryan4]

I’ve been trying to measure dissolved oxygen in my aquarium water using a homemade Winkler titration setup, but my readings are consistently lower than my test kit’s results. I’m careful with my sodium thiosulfate solution and the titration itself, so I’m wondering if my issue might be with the initial sample collection or the manganous sulfate addition introducing oxygen from the air.

[Luna69]

I had the same issue. The bottle would look perfectly full but tiny air gaps in the cap let oxygen exchange happen after collection. I started filling the bottle underwater and capping it under water so there was no air and the readings lined up more with the kit.

[NicholasT]

Another time I waited too long between collecting and fixing the sample. The ambient oxygen kept diffusing away and the titration reading drifted low. Fixing immediately helped.

[ZoeyC]

I’m careful with the mixing step too. If you twist the cap or shake a lot you pull air into the sample. I found gentle inversion to mix reagents makes the cap stay sealed and the result stayed steadier.

[JeffreyUG]

Temperature and acid normality bite too. If your room temp vs the sample temp isn’t accounted for, the endpoint can shift and you’ll end up undervaluing the DO compared to the kit that does some compensation.

[Joseph92]

I’ve also wondered whether the reagents themselves degrade. MnSO4 or the iodide solution that you add later can pick up contaminants or moisture and then the chemistry doesn’t go as planned. Fresh bottles helped a bit but it’s a pain.

[Matthew_G]

Could the real issue be that the kit is overestimating rather than underestimating, and my Winkler setup is actually fine?