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FaNgS

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Anyone familiar with the Boehm titration procedure and calculations?

I've done the titrations based on what I was able to gather from literature and the calculations but the results don't make sense to me.

Even if you are unfamiliar with this method here's a quick idea about it: Boehm titration is an acid-base titration method which is used to determine the amount of surface oxygen groups (acidic or basic) present on carbon surfaces (activated carbon, carbon black, graphene, carbon nanotubes, etc). There are several authors that perform the titration in various techniques direct titration, back titration and potentiometric titration. But the most common is a back titration technique, which consists of mixing the initial base with an excess of acid and titrating with base, while in the case of an initial acid, it is mixed with an excess of base and titrated with an acid.

The method uses NaOH, Na2CO3, NaHCO3 and HCl and its main principle is that the number of acidic sites is determined under the assumptions that NaOH neutralizes carboxylic, lactonic and phenolic groups on the carbon surface; that Na2CO3 neutralizes carboxylic and lactonic groups; and that NaHCO3 neutralizes only carboxylic groups. The number of basic sites is calculated from the amount of HCl required in the titration.

I'll skip the procedure I followed, unless someone would like to know what I did, and I will just go straight to the results. I've calculated the amount of surface groups in mmol/g of carbon.

From the NaOH titration I found that there are 1.423 mmol/g of carboxylic, lactonic and phenolic groups.

From Na2CO3 titration there are 1.225 mmol/g of carboxylic and lactonic groups.

From NaHCO3 titration there are 1.575 mmol/g of carboxylic groups only.

So from the results there are 1.575mmol/g of Carboxylic groups, and by subtracting NaHCO3 titration and Na2CO3 titration this shows that there are 0.350 mmol/g of Lactonic groups. And finally the number of phenolic groups is calculated by subtracting the NaOH titration from the Na2CO3 titration to get 0.198mmol/g of phenolic groups.

Wouldn't it make sense that the NaOH titration would result in the highest amount in mmol/g since it neutralizes all 3 groups together? But from I've got, the highest amount was from the NaHCO3 titration 1.575mmol/g (only carboxylic groups neutralized).