I use the NaNO2+AlCl3 method to quantify the TFC of cacao extracts. Sample+NaNO2+AlCl3 then NaOH. As sample blanks, I use the same reagents but replace AlCl3 with water. I notice that when I add AlCl3, the samples change color to yellow then when I add NaOH, the samples turn peach red. Some of my samples (those extracted at pH 3 to 5) however, exhibit a color change to peach red even when water is added instead of AlCl3. When I tried replacing both NaNO2 and AlCl3 with water however, no peach red formation occured. I was thinking which sample blank to use to subtract from the absorbance