What is the difference between a reagent blank and a sample blank?

Document ID

Document ID TE770

Published Date

Published Date 11/11/2019
Question
What is the difference between a reagent blank and a sample blank?
Answer
A reagent blank refers to a small positive error in test results that comes from the reagents themselves. Hach Company makes every effort to manufacture reagents that have negligible blank values, and typically the values are so small that they do not affect test accuracy.

 The reagent blank value is most important to measure and subtract from test results when measuring low concentrations. For example, subtracting a reagent blank value of 0.02 mg/L from a test result of 0.06 mg/L changes the result by more than 30 percent. On the other hand, subtracting a reagent blank value of 0.02 mg/L from a result of 1.23 mg/L changes the results by less than 2 percent.
 To measure the reagent blank, use good quality deionized water in place of your sample and run the test as usual, adding reagents and waiting any timed steps. Then subtract this value from your sample results. The reagent blank value can change from one reagent lot to the next. Therefore, measure the reagent blank each time you use a different lot of reagent.
 A sample blank refers to using the sample for zeroing an instrument during a test procedure. A sample blank can correct for potential error from existing color or turbidity in the sample before reagents are added.
 When zeroing the instrument on a sample blank, only the color that develops from reaction with the reagents is measured. Because background color and turbidity can vary from sample to sample, a sample blank is most commonly used to zero the instrument.