I m trying to estimate the concentration of lignin in rice husk. I m using TAPPI T222 method for this purpose. On treating with 72% sulphuric acid, the husk powder is still visible. Is it correct or I m missing somewhere.
Since after this water will be added and 4 hrs boiling will be done and then protocol says to filter and weigh the filtrate. Under such condition the filtrate will have husk powder as well.
The sugars in wood and pulp are hydrolyzed by sulfuric acid and made soluble; the acid-insoluble lignin is filtered off, dried, and weighed. This type of analysis indicates that lignin is a part of wood or pulp that is insoluble in 72% sulfuric acid. I believe you skipped a few stages along the way. So be sure to read the entire guideline and adhere to it strictly.
You should not have any "husk" powder remaining, just lignin and ash. Your filtrate will contain acid-soluble lignin which also must be determined to obtain your total lignin content. I have attached a couple of NREL methods you may find helpful alternatives to the TAPPI method. If the husks are very recalcitrant - you could macerate the fibers a bit more using a Wiley mill. I always treat my biomass this way to ensure that the acid fully penetrates my sample.
It is not normal. Please grind their samples and extract them with methanol-water (2:1), and adjust the sulphuric acid concentration to 24 Normal.
Rice husk silica content is very high, so the lignin content must need ash correction.
Best regards
Some fractions of non-lignin components may remain insoluble in sulfuric acid, depending on your feedstock. How about trying the same procedure (reagents + apparatus) for a standard sample of wood powder (if you can get it somewhere) to check if the problem is indeed with the feedstock?
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