I am studying about digestibility of animals by using chromic oxide (Cr2O3) as an indicator in their diet and feces. To find the amount chromium in the diet and feces samples by FAAS, The amount of Cr2O3 was approximately 2-5 mg/g. I prepared the samples using acid digestion as described by EPA 3050B (https://www.epa.gov/sites/production/files/2015-06/.../epa-3050b.pdf) in order to make Cr in the samples to be soluble form. After the samples were digested with conc.HNO3, 30%H2O2, and conc.HCl, respectively, I got pale green suspended particles in some digested sample solutions after they were cooled to room temperature. Is it usual for this method? What is the green particle? Did the Cr turn to be solid form?