French phrase
À quel pH doit-on ~ ?
Meaning
At what pH should one ___?
When to use it
Asks which pH is suitable for a given procedure. It is polite and somewhat technical, common in laboratory work and solution adjustment.
Example dialogue
À quel pH doit-on régler la solution ?
At what pH should one adjust the solution?
Il faut l'ajuster à pH 7.
It needs to be adjusted to pH 7.