Dear Jo-Anne van der Knaap, 

I suggest you using Glass Microfiber filter instead of normal filter paper. Normal Whatman filter papers are made of cellulose which will be affected by the acid and will be losing weight when drying for your ADF determination.

Regard,

Yes I've suggested using acid-proof filters instead, which is what we'll do now

However, I still am curious what causes these phenomenoms

But is it really the cellulose? Since I'm measuring cellulose and lignin with the ADF determination, the sulfuric acid (0.5M) shouldn't affect the cellulose. If it does, then my measurements won't be reliable, would it?

Thank you for your answer

I actually think it does affect your cellulose because of the elevated temperature of drying (100 degree right?)

1-"At room or lower temperatures, the sulfuric acid with concentrations greater than 62-63 wt.% caused swelling and dissolving of cellulose samples that was accompanied by hydrolysis process. Increased temperatures of the acidic treatment promoted both to hydrolysis and dissolving of cellulose [6, 7]. Cellulose samples having the distorted structure dissolved in 63–65 wt.% SA better than the structural-ordered celluloses [8]. After dilution of the solution, the cellulose precipitated in a modified swollen structural form called “hydrocellulose” [9]"

If concentration of SA was high enough, usually 72 wt.%, then after acidic treatment at room temperature the cellulose hydrolyzed with forming of water-soluble oligomers [7, 10]. The following boiling of the diluted acidic solutions permitted to convert the oligomers mainly into glucose. (Check https://www.hindawi.com/journals/isrn/2012/428974/)

2-Sulfuric acid is an extremely strong oxidizing agent, and dehydrates many organic compounds to produce carbon in the form of graphite, and water. For example, the reaction of sulfuric acid with paper (cellulose) is:

(C6H10O5)n(s)−→−−−H2SO46n C(s)+5n H2O(l)

Try it yourself and add one drop of SA on a paper, it will turn black.

Hope that explains the confusion.

Best of Luck

Thank you so much!

I'll try drying at a lower temperature and see what that does to the filters (out of curiosity). I'll also work with acid-proof filters for the ADF determination of the samples.

You are most welcome, Jo.

This is really a very good discussion. But my doubt here is unsolved, can anyone put the light on "If sulfuric acid turned cellulose black, is it really a reliable technique to determine ADF, since it should not affect cellulose in biomass." I have an observation, "I did ADF of alkali treated biomass, and when I put it in oven to dry it turned black even at 70 degree Celsius." I am new in exploring this technique, can please explain me in detail what I should do for accurate measurements?