Why do I need to perform a ‘blank’ before measuring my samples in my photometer

The photometer will measure the combined absorbance of the vial (or cuvette) and the test sample. As we are only interested in the absorbance of the test sample, the absorbance of the cell needs to be subtracted from the total absorbance. This is done with a ‘blank’ vial – usually a cell filled with purified water. Most photometers will automatically subtract the blank’s absorbance from the test sample readings. For improved measurement accuracy, matched cells that are certified as having identical optical properties should be used for photometric testing.

For some test methods, e.g. COD, the unreacted test reagents have a significant absorbance at the measurement wavelength. In these instances, the blank is prepared by treating purified water with the reagents and the blank can be used to set the zero point.