Question

Why is blank titre value higher than back titre value in chemical oxygen demand (COD) experiment?

Answer 1

To review for likely sources of error in the blank solution, a blank titration is performed without the presence of the analyte present.
Blank titration provides the estimation of the measure of reactive substance within the solvent which is to be determined and consequently provides a measurement of the error in later titration experiments using the same solvent.

When the endpoint of the normal titration is difficult to find, then a back titration is practiced. The analyte can be reacted with an excess of titrant followed by titration of the excess titrant.
The number moles of titrant responded with the analyte can be calculated by subtracting the moles of excess titrant from the original amount used gives and the moles of the analyte can be figured out.

Now as, blank titration uses a free run to get the end point its value is higher than back titration.