in an acid-base titration?
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Because strong alkalis (Such as NaOH) react with glass - acids (apart from HF) do not.
If you leave NaOH(aq) in a burette with a glass tap for any length of time without thorough rinsing then it is more than likely that the tap will become irreversibly stuck. Small amounts of sodium silicate go into solution and then, as the water evaporates, are redeposited between the two pieces of glass in very close proximity effectively sticking them together. Not the problem it used to be, as many/most burettes have PTFE taps nowadays.
If you leave NaOH(aq) in a burette with a glass tap for any length of time without thorough rinsing then it is more than likely that the tap will become irreversibly stuck. Small amounts of sodium silicate go into solution and then, as the water evaporates, are redeposited between the two pieces of glass in very close proximity effectively sticking them together. Not the problem it used to be, as many/most burettes have PTFE taps nowadays.