Why is sulfuric acid used to seal flask with highly reactive chemicals? I the video https://youtu.be/saANxD0cqy0?t=200 (time 3:20) NileRed states that he uses sulfuric acid to seal off flask with a mixture of pottasium dichromate and table salt. He says that sulfuric acid is used to seal off flasks with highly reactive chemicals and I could not find any reasons why. 

I see sulfuric acid as a very reactive chemical, that could potentionally react with substrate causing some minor problems to purity of the final product. I also wonder why he does not use any other less-reactive seal that would not react with substarte what so ever.
Thank you for your contribution
Sulphuric acid reacts with dichromate and table salt, forming volatile and harmful chromyl chloride.
\begin{multline}
\ce{K2Cr2O7(s) + 4 NaCl(s) + 6 H2SO4 -&gt;}\\
\ce{2 KHSO4(s) + 4 NaHSO4(s) + 2 CrO2Cl2(l,g) + 3 H2O}
\end{multline}
Therefore it serves as a reagent and at the same time as protection against chromyl chloride escape.

Safety considerations
$\ce{CrO2Cl2}$ hydrolyzes to release $\ce{HCl}$ and hexavalent chromium $\ce{Cr^{\mathrm{VI}}}$.
Acute: Exposure to chromyl chloride vapour irritates the respiratory system and severely irritates the eyes, and the liquid burns the skin and eyes. Ingestion would cause severe internal damage.
Chronic: $\ce{CrO2Cl2}$ can produce chromosome abnormality/aberrations and is a human carcinogen via inhalation. Frequent exposure of the skin to chromyl chloride may result in skin ulcer/ulceration.