.After thoroughly mixing the contents of tubes2,3,5 and 6, place a portion of each of the six solutions in six correspondingly numbered tubes that fit the spectrophotometer . (Fill the spectrophotometer tubes about 80-90% full.) Measure the absorbance of each of these solution at 645 nm as demonstrated by your instructor and as outlined in the " Spectrophotometry" material in Section G of this manual. The infinite absorbance setting ( left end of the scale ) is made with nothing in the sample chamber of the spectronic 20, and the 0 absorbance setting (right end of the scale) is made using the solvent in which you dissolved your compound, that is pure 1 M HNO3 (sample tube no.1).