Why do you need to "blank" the spectrophotometer, and what does the blank consist of? You take a "blank" reading with no cuvet in the chamber, to subtract out the absorbance properties of the air The blank is a cuvet filled with water, to subtract out the absorbance properties of the solvent molecules. The blank is a cuvet filled with the stock solution, to serve as a basis for comparison for the more dilute solutions The blank is an empty cuvet to make sure the spectrophotometer is working properly You blank the spectrophotometer by pushing the "blank" button before you take a reading