[baboom]

The titrant is silver nitrate, and the analyte is the unknown chloride solution. The indicator used is pottasium chromate:
Determine an indicator blank by suspending a small amount of chloride-free
CaCO3 in 100 mL of distilled water containing 2 mL of K2CrO4.

Why do I need to add CaCO3 to the indicator blank? Isn't the blank for purposes of determining the amount of chlorine in the distilled water which is used to dissolve the unknown chloride sample? Can't I just perform a simple blank on distilled water without using CaCO3?

[baboom]

Never mind guys, I found the answer after an extensive search:

In principle, an amount of chromate to give this concentration should be added, in order
to develop the red precipitate after the equivalence point. However, a chromate ion
concentration of 6.6 x 10-3 M imparts such an intense yellow color to the solution that
formation of the red silver chromate is not readily detected. Thus, lower concentrations of
chromate ion are generally used. An excess of silver nitrate is therefore required before
precipitation begins. An additional excess of the reagent must also be added to produce
enough silver chromate to be seen over the heavy white precipitate of silver chloride.
These two factors create a positive systematic error in the Mohr method that becomes
significant in magnitude at reagent concentrations lower than about 0.1 M. A correction
for this error was made by a blank determination. In a blank determination, all steps of
the analysis are performed in the absence of the analyte. In this experiment, blank was
determined by titrating a solution of a small amount of chloride free calcium carbonate
and indicator potassium chromate with standard silver nitrate solution. Calcium
carbonate was used to imitate the white silver chloride precipitate. Reagent volumes were
corrected for blank in the calculations.