# Use of pipette during experiments for determination of oxygen by Winkler's method

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?