I need to measure the chemical oxygen demand for cooking oil that I am using for Anaerobic digestion. I tried high range, high range plus, and low ranges COD kits, and still no luck. After digestion, the sample transforms into pitch black kind of color. Also, I have doubt with the dilution as oil doesn't mix with the water. So, I need to know the best way to measure COD of cooking oil. I'll appreciate your kind suggestions.