I am currently trying to determine labile carbón with permanganate, and I am using the following process. Firstly, I am taking 5 gr of soil sample, adding 20 ml of permanganate, shaking for 2 minutes, and after waiting 10 minutes to leave particles precipitate, I am taking 0.5 ml and filling with distilled water until 50 ml, and then I am measuring the absorbance in a spectrophotometer at 550 nm. The problem is that in some samples, there are so much turbidity in water, and I am sure that it has an influence in my measurement of absorbance. I do not wanna centrifugation because I think it might be so strong, and some other fractions of carbon besides labile carbon might react, giving me a wrong measure. To solve the problem, I was thinking to prepare a blank test with the soil, following the same procedure mentioned, but this time without permanganate, only to measure the absorbance of the particles. In your opinion, what should I do?. I will really appreciate your help.
Consider filtering your samples through a 0.45µm membrane filter.
Because some of the colloids that are suspended in your solution after the shaking are very small (nm-µm range), it will take a very long time for them to settle out of solution. I think you either have to centrifuge or filter (or both). I hope this is useful.
Dear Asmeret, thank you for your answer. Filtering was something that we were thinking to do at the lab, however, our boss has an ecological point of view, and he does not want to use filters, so therefore, I was suggesting to do that blank test, and my main issue is about it, if it sounds reasonable or not
Luciana, I'm currently working on POXC determination... it seems that you are using the Weil method. Are you preparing the 0.2M KMnO4 solution with 1M CaCl2?! Try a higher molarity. Although I run hundreds of samples and I never had a turbidity problem. Filtering the samples won't help since the nature of POXC doesn't necessarily represent the dissolved organic carbon (those particles that pass 0.45 um). Plus, the method was proposed as a cheap and easy alternative to measure labile C (those filters are expensive). If you centrifuge the samples, you will probably release recalcitrant compounds. So, try a higher molarity of CaCl2. Also, after those 10 min, you should pipette from the top!
Hope it helps.
Carlos
Dear Carlos, thank you so much for your answer. The information was indeed useful for us. I just want to ask you about your opinion about the blank test we want to do, do you think that it makes any sense?
Luciana, the blank test could be useful to see how your CaCl2 solution works flocculating your soil. I am not sure how measuring the absorbance of your particles will help...
I got it Carlos, and I think i will follow your former suggestion. Thank you so much for your help :)
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