Some blank titration values are 0, so you can't make one for it. It doesn't matter whether it's good or not.

There is a problem: if we don't use indicators here, the error will definitely be greater than that without deducting blanks! How can this method be the same as that used by middle school students? ! We use potassium dichromate method to determine COD, and the indicator is phenyl o-aminobenzoic acid or sodium diphenylamine sulfonate.