Why does a blank, unreacted test strip not match the blank color on the bottle label?

Document ID

Document ID TE6780

Published Date

Published Date 27/05/2017
Question
Why does a blank, unreacted test strip not match the blank color on the bottle label?
Summary
Unreacted test strip paper color
Answer
Sometimes the dry, unreacted paper on the test strips is darker than the blank block on the label.
This is not a reason for concern. Test strips should only be compared to the label after they have been reacted. 

The Hach Quality Control team has checked the blanks of a number of test strips. The quality control specification for the strips is that they must read about ± 1/2 a color block from the nominal level. This means that the blank can be a little darker than the zero block on the label and still pass specifications.