Blanks for colorimetric determination The blank may be used repeatedly for measurements using the same lot of vials. Store the blank in the dark. 1. Monitor decomposition by measuring the absorbance at the appropriate wavelength. Refer to the Range-specific test wavelengths table. 2. Zero the instrument in the absorbance mode. Use a vial containing 5 mL of deionized water and measure the absorbance of the blank. Record the value. 3. Prepare a new blank when the absorbance has changed by about 0.01 absorbance units.