Hi, everyone. Recently, I am trying to do some works with alkaline lignin which purchased from sigma-aldrich. However, when I use to distilled water to mix with alkaline lignin and adjust the pH of the suspension with 50% (w/w) sodium hydroxide to pH 10, it is so hard to adjust the pH from pH 5.8 to 10. It seems alkaline lignin is recalcitrant to the pH change. I wanna to know the mechanism of this phenomenon. Is alkaline lignin could react with sodium hydroxide? I am looking forward to receiving your reply. Sincerely！
Why you don't try to add the alkaline lignin to the basic liquid and see what will happen.
Generally, lignins are soluble in alkaline pHs, but this problem you having can be due to the fractionation of lignin in different pHs, I mean it is possible to fractionate the lignin varying the pH, obtaining fragments with different molecular weights in different pHs. I´ve seen some works that claim that high pHs, such as 12-13 favor the dissolution of lignin. But, I think that if the medium is too agressive something can happen to its structure, since a strong base will be present in the medium in a high concentration.
Therefore, this problem you having can be attributed to the fractionation of lignin in its different molecular weight fragments, specially if this lignin has a high polidispersity index value, which would corroborate with this hypothesis.
What is unusual in your observation? Lignin is a weak acid and contains typically ~1% of carboxylic groups (pK ~ 4.5) and several percent phenolic hydroxy groups with pK 7-8 (in structures with carbonyl conjugated with aromatic ring) and 10-11 (typical phenolic structures). The average pK of lignin in aqueous solution is about 10, so it easy to understand that you have to neutralize more than 50% of acidic groups to bring pH of solution to the value of 10. It demands a significant amount of alkali.
Dear Francisco
Thank you for your kindly reply. To my confusion, what kinds of fragment in alkaline lignin might be dissolved in distill water? Is it possible to form lignin-metal complex？Could Na+ have linkages with lignin which help lignin dissolving in NaOH solution just like humic acid and sodium humate.
Sincerely.
Xinxing Wu
Dear Dmitry
Yeah, your answer basically eliminated my doubts and broaden my horizon. Could you provide me some research papers which are related with this phenomenon, if it is convenient？ Thank you for your kindness.
Best regards
Xinxing Wu
Please, find attached paper on acid-base properties of lignin related phenols. Maybe it will be useful for your research.
Lignin also contain phanolic group. Lignin precipitated at pH ~ 2 and dissolve at higher pH ~ 13.
Rather than complete dissociation lignin can partially dissolve lower than pH 13 and oftentimes liberate H+. Therefor, it is difficult to fix up pH.
Dear Xinxing,
Since lignin is a big mixture of fragments of different molecular weights (MWs), we can fractionate it into its smaller or higher MWs using organic solvents or change the pH of the medium. It is not the distilled water that promotes the phenomena, but the change in the ionic strength of the medium. In your case, the difficult to dissolve it could be a range of factors, including the fractionation process, but also the other factors listed by the others colleagues, which make sense too since lignin in alkali medium would release H+ making the stabilization of pH very difficult. Perhaps, you can make this happen in a high pH (~12-13), where theorethically the lignin would be in the ionic form and the OH- would neutralize the H+ released from it.
In relation to the formation of lignin-metal complex, it is possible but I don´t think that it would help its dissolution, since it would be more easy to dissolve it through ionization of protic groups.
Hope this help you.
T.V. Lourençon et al. / Separation and Purification Technology 154 (2015) 82–88
This article represents an example of the influence of pH on the molecular weight of lignins.
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