Answer:Because When you're cleaning your glassware, you use water to rinse it off. If the burette is not completely dry by the time you use it, the remaining traces of water on the inside will make your titrant more dilute and thereby change its concentration.Before a burette is used, it needs to be cleaned. During the cleaning process, usually, water is introduced into the burette. Be the water clean or not, if the burette is then used without rinsing it with the solution that it is going to be filled with, the result of that analytical exercise will not be precise and accurate. The reason is that water residue in the burette would dillute the solution when it's filled in the burette which would make it impossible to determine the exact concentration of the solution moved by the burette. By rinsing and re-rinsing the burette several times with the solution it is going to be filled with, residue water from the cleaning process would be successfully removed from the burette as are other leftover substances from the cleaning process.
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