calibration blank
A calibration blank is the "zero" point of your calibration set — a sample made just like your standards but containing none of the analyte. It tells the instrument what "nothing" looks like, so the curve has an honest starting point. Picture setting a kitchen scale back to zero with the empty bowl on it before you begin to pour.
Concretely, it is the solvent or matrix used to dissolve your standards, carried through the same measurement but with the analyte concentration set to zero. Its signal becomes the bottom anchor of the calibration curve, and the instrument is often "zeroed" or "blanked" against it before standards and samples are read.
It matters because even pure solvent and clean glassware give a small background signal, and the calibration line must account for it. The calibration blank fixes where the line begins; without it, every reading would inherit that hidden offset. A note of care: a calibration blank carries only the matrix used for standards, which may not capture contamination introduced by the full sample-preparation procedure.
Before running iron standards, the analyst measures pure dilute acid (the standards' solvent) with no iron added. The spectrophotometer is zeroed on it, fixing the bottom of the calibration curve.
The zero-analyte point sets where the calibration curve begins.
Distinguish the calibration blank from the reagent blank. The calibration blank is essentially the standards' solvent at zero analyte; the reagent blank goes through the full procedure with all reagents. The reagent blank catches contamination the calibration blank can miss.