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When we are titrating acid/base using ph meter we add distilled water to immerse the ph electrode. Won't this affect the concentration of the acid/base: I mean isn't this dillution. Won't this affect our calcultion of the concentration at the end.

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What's important to note in this situation is that the number of moles of the titrand isn't changing. This matters because all acid/base reactions go to completion, so every drop of titrant that falls into the solution will react with the titrand until there are no moles of titrand left regardless of concentration, at which point you've reached the equivalence point.

Adding a little water here or there doesn't really change the pH at the equivalence point, because ideally there are no moles of acid or base present in solution.

The reason this doesn't mess up your measurement for the concentration is because you measured the volume of titrand before the titration, and then recorded the volume of the known titrant needed to reach the equivalence point. From this information, you can calculate the moles of titrant needed to reach equivalence, which in most cases equals the number of moles of titrand present, and with the original volume of titrand noted previously, you can calculate the original concentration of the solution the sample was taken from. voila!

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    $\begingroup$ I should note: titrant--dropping solution of known concentration; titrand--solution of (originally) known volume, whose concentration you are seeking to find out. $\endgroup$ – Gavin Kramar Jun 18 '13 at 15:11

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