it has been many years since i have done a Kjeldahl analysis, but have you neutralised enough after the original hot acid digestion (with alkali), ie is the product still acid-. if so back titrating with an acid solution will never give a green endpoint..

James Leigh Thanks sir for your valuable discussion

The receiver solution must be green before distillation. It turns red when distilling. To know how much titrant is due to the adjusted boric acid, you do a couple of blank determinations first.

The blank should be approx. 0,02-0,1 ml titrant.

Do a blank and repeat the process. What do you use as digesting agent?

Cyril Ogbuanu Ok sir, digesting agent as sodium sulfate & copper sulfate 10:1 sir.

Bala, did you add selenium dioxide

No sir , I didn't because some literature they used selenium in optional.

use 1:10 cuso4:k2so4 and 10 ml of h2so4 for a 0.5 gram sample digestion. digest for 45 min in heating mantle to get a green color solution. after digestion add 100 ml distill water after cooling. now add 40ml 40% naoh. then collect distillate in 30 ml boric acid 4%. let me know still it is green or red

The use of selenium dioxide is not optional. Try it and report back.

Cu and Se are catalysts. Some users prefer Cu and some prefer Se. The important thing is that you have one of them, otherwise the digestion time will be much longer than 1 h.

K2SO4 is to raise temperature of the H2SO4.

You need to make some blank determinations (Add 100 ml distilled water and 40 ml NaOH) to a digestion tube. Add 30 ml receiver solution to an E-flask. Distill 100 ml and note the colour. Receiver solution should be redish from the beginning and green after distillation). Titrate with 0,2 N HCl or H2SO4 and note volume.

Digest and distill your samples. Note titrate volume. Withdraw blank volume from sample volume and then calculate protein/nitrogen content.

Afsaor Uddin : Thanks sir and sure let you know.

Helene Sjöström : Thanks Sir

Cyril Ogbuanu : Thank sir