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In my book it is written that in order to minimize systematic error in Mohr method for argentometry it is a common procedure to conduct a blank titration with $ \mathrm{Ag^+} $ against a $\mathrm{CrO_{4}^{2-}}$ solution (with the same concentration of the original solution).

The book then suggests to subtract the $ \mathrm{Ag^+} $ volume needed to observe precipitation (in the blank solution) from the volume obtained as a result of the 'standard' titration.

I don't understand why I have to subtract that number, specifically I do understand that there is a systematic error but I don't get why it is the volume of $ \mathrm{Ag^+} $ obtained by the blank titration.

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$\ce{AgCl}$ ppts before $\ce{Ag2CrO4}$. But in order to visually detect the $\ce{Ag2CrO4}$ you must add a slight excess of $\ce{Ag^+}$. Hence you use a "blank" with no $\ce{Cl^-}$ to account for the amount of $\ce{Ag^+}$ needed to get enough of the $\ce{Ag2CrO4}$ ppt to form so that it can be detected visually.

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