One of the oldest methods is using chromotropic acid. See this link

Clarke and Jennings. 1965. https://pubs.acs.org/doi/abs/10.1021/jf60138a023?journalCode=jafcau

You can also determine using a Nitrate ion electrode. Paper is attached.

The other one is using phenoldisulfonic acid method.

If you want to determine anion exchange capacity by using NO3 solution then you can use ion selective electrode, colorimetric methods given by the suggestions above, or steam distillation in the presence of devardo alloy. By the wayto determine NO3 content of biochar first of all you should extract NO3 on the biochar, if there is any. For this, treat the biochar with KCl solution. Then determine the nitrate by destillation procedure. I think there should not be any NO3 in the biochar it self because it is prepared under O2 deficit environment and high heat. Therefore any O2 source is consumed during the precedure. some ammonia can present in the biochar. You should take into account this ammonia if you use distillation method.

Dear,

First of all, biochar is prepared through pyrolysis i.e., burning residues under anoxic environment. During such process, most of the N contained in the residues would be lost. Moreover, plant residue contains N as NH4+ coming mainly through proteins. Thus, there will be little scope for you to obtain NO3- from the biochar.

Determination of NO3- will not substantiate AEC of biochar. Please, look for the methods of AEC determination in soil, which may be applicable for estimating AEC in biochar.

With regards,

SD