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We were performing estimation of $\ce{O2}$ by Winkler's method and we were told by the instructor that while adding $\ce{MnSO4}$ and $\ce{NaOH}$ / $\ce{KI}$ the pipette tip should be well below the surface of the solution.
When asked as to why this should be done, he said that if the reaction took place at the surface then there's a possibilty that the atmospheric $\ce{O2}$ may get dissolved.
Is it true?
[...] the pipette tip should be well below the surface of the solution [...]
Usually, one would indeed keep the pipette above the solution to avoid that a (minor) part of the solution forms a drop (or film) on the tip of the pipette and is carried away.
Here, when the amount of oxygen dissilved in the solution is to be determined, another effect prevails.
If you add your reagents from greater height, you might create a splash, just like throwing a rock into a lake. As a result, you will mix some oxygen from the gas phase into the solution, which might taint the measurement.
I was always taught that, no matter what the solutions are, never immerse the pipette. Always touch it to the inside wall of the target container as close as possible to the receiving solution's surface.
