I was performing the COD of paracetamol sample prepared in acetone. As soon as I added the digestion solution to my sample, the color turned black with yellowish hint. I have performed COD test several times for different samples but the color always remains yellow before digestion.
I would much appreciate if anyone can help me understand the reason behind the black color and/or my possible mistake.
Thanks in advance.
Many COD reagents contain Mercuric Sulfate to eliminate the interference in the COD test due to Chlorides during digestion. It appears that you are producing Mercuric Sulfide which is precipitating in the Acetone solvent. Try using Methanol instead.
Bruce made everything clear.
Why you use aceton - it will participate in the COD value. What is the concentration level of paracetamol? I assume that you use COD to measure its mineralistion in some degradation experiment?
Regards
GB
Shakir, how does your comment help to the cause? Be wise at using RG please.
Thank you so much Bruce Neagle for your response.This clarified a lot.Could there be any other possibility besides formation of mercuric sulfide?
Yes, Grzegorz Boczkaj we are studying the mineralisation of paracetamol for which we have taken 1mM concentration of paracetamol in acetone.
Nishat,
Yes! Iron sulfide is also a black precipitate. In both cases you are reducing them from their sulfates during the digestion. You can try acidifying the acetone before COD analysis with dilute sulfuric acid (but I don't know if this will help).
You can also try acidifying with dilute 0.05 N Nitric Acid. If it is reduced it goes to Nitrogen gas.
Nishat make an analysis of "blank" sample i.e. your model effluent without paracetamol but containing acetone. Compare this result with sample with paracetamol. You need to include how the aceton affect the COD measurement.
Regards
GB