I want to determine the protein content in my sample of Azolla Pinnata that was oven-dried for 12 and 24 hours and 2 days sun-dried. I found that my sample turned black when I do the Kjedahl method during digestion. I added 3 Cu tablets and 15 mL of H2so4 together with the sample in the digestion tube but it still turning black. What should I do?