Calculating Surface Oxygen Groups in Boehm Titration Procedure?

[FaNgS]

There is too much difference in literature regarding the procedure for Boehm Titration and my efforts to find the original article which describes the method developed by Boehm himself has not been fruitful.

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). This method is usually used complimentary with other methods such as Fourier Transform Infrared Spectroscopy (FTIR) and Temperature Programmed Desorption (TPD).

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; 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.

My main concerns are:

- Amount of carbon used
- Concentration of the bases and acid solutions
- Whether to titrate the filtrate directly or mix the filtrate with some base or acid followed by a back-titration
- Most importantly how is the amount of each surface oxygen group calculated after performing the titrations?

 

[DrDu]

...and my efforts to find the original article which describes the method developed by Boehm himself has not been fruitful.

I simply tried "author:Boehm titration" in Google Scholar.
The article from 1964 seems to be the first publication of the method:
http://onlinelibrary.wiley.com/doi/10.1002/anie.196406691/abstract
http://www.sciencedirect.com/science/article/pii/0008622394900310

 

[FaNgS]

DrDu thank you for your reply, however, during my search I also found the articles you mentioned but I could only get a copy of the second article from the Carbon journal which unfortunately doesn't provide details of the method. The first article, which I believe describes the titration method, I am not able to access even through my universities databases.

 

[FaNgS]

Maybe someone can help me workout the calculations and if the science checks out, this is what I've gotten so far:

After mixing separate batches of carbon with NaOH, NaHCO3 and Na2CO3 I will titrate directly the solution with HCl.

In the case of NaOH titrated with HCl, at equilibrium moles NaOH = moles HCl

I know the volume and concentration of HCl used from the titration, so I can calculate the number of moles. Is it necessary to subtract this from the initial number of moles of NaOH solution (before adding carbon)? Or is the value calculate the moles of the carboxylic, lactonic and phenolic groups present (this is based on the assumption that NaOH neutralizes those groups)?

Similarly for Na2CO3, I can calculate the moles of carboxylic and lactonic groups and for NaHCO3 I can calculate the moles of lactonic groups.

 

[chemisttree]

Something here.
...and here

 

[FaNgS]

Something here.
...and here

Thank you.

As you have noticed, even though Boehm titration is supposed to be a standard method, both papers have variations. Plus it seems they suggest a direct titration rather than a back-titration.

I've tried performing a direct titration but it was difficult to get the same or almost the same volume of titrant to achieve the end point between my titration runs. I'm considering re-doing the titrations.

 

[chemisttree]

Have you noticed the lengthy naoh soak for this procedure? How would you reconcile that with a titration procedure that occurs over a much shorter time frame?

 

[FaNgS]

Typically the time allowed for the carbon to remain in contact with the solutions is 24 hours and the maximum I've read is 72 hours. This is to allow enough time for the surface groups to get neutralized and from my experience to ensure that there is a measurable amount of titrant required to achieve the endpoint when doing the titrations.
I've tried doing a direct titration and even after 24 hours it was very difficult to achieve repeatable endpoint values compared to a back-titration.

If a shorter period of time is needed I would assume using a lower amount of carbon maybe 50 mg (I've used 1.50 g) with really good shaking.

 

[chemisttree]

What were you doing with the back titration? Did you add HCl and titrate remaining acid with NaOH? The hidden point of my question is this... if it takes time for NaOH or Na₂CO₃ or NaHCO₃ to diffuse to the various sites to neutralize their respective acidic groups, wouldn't take time for any excess base to diffuse out of those same spaces for titration? Remember that carbon black is the carbonized remains of once living tissues composed of tiny individual cells. These cells can remain largely intact after carbonizing so you have a group of somewhat porous individual cells into and out of which move the various base solutions. All this is controlled by diffusion so it is reasonable that it takes some time... 24 hours to 72 hours from what you have read. So you add the base and it moves quickly to the sites well exposed to the solution and somewhat slower into spaces that could be described as having a more tortuous path... perhaps areas where the cellular structure is still recognizable. The neutralization reaction occurs and you quickly filter and rinse the carbon, titrating the filtrate either directly or by a back titration. How much base remains caught up in those less accessable spaces? Did your rinse step allow for the base solution to diffuse back out of those 'sites'?

I haven't read the procedures you have access to so I may misunderstand what you refer to as back titration.

 

[FaNgS]

What were you doing with the back titration? Did you add HCl and titrate remaining acid with NaOH?
I haven't read the procedures you have access to so I may misunderstand what you refer to as back titration.

This is exactly what I meant by back-titration. I would add different amount of HCl depending on the initial solution. So for example I will add 20ml of HCl to NaOH while I would add 30ml of HCl to Na2CO3 to ensure complete reaction because 2 moles of acid required to react with 1 mole of sodium carbonate.

I would be very glad to send you some of the articles I have if you are interested.

The hidden point of my question is this... if it takes time for NaOH or Na₂CO₃ or NaHCO₃ to diffuse to the various sites to neutralize their respective acidic groups, wouldn't take time for any excess base to diffuse out of those same spaces for titration?
All this is controlled by diffusion so it is reasonable that it takes some time... 24 hours to 72 hours from what you have read.

Yes, you are correct absolutely. The activated carbons I'm using have both mesopores and micropores and diffusion is definitely a controlling factor and a sufficient amount of time would be required for firstly diffusion of the base into the pores, then reaction and neutralization to occur (mass transfer) and the diffusion back to bulk solution. There is a huge possibility that the groups in the smaller pores would not react with the base.

It could be possible to have shorter times but this has to be proven via several trials and controlling different parameters. But 24 hours was sufficient for my experiments.

Other techniques such as IR-Spectroscopy, Xray Photoelectron Spectroscopy and Temperature Programmed Desorption are used all together with Boehm titration, to get a good picture of the surface groups present. So Boehm titration alone is not sufficient.

The carbon precursor, production and activation method can greatly affect the properties of the carbon and different surface groups are produced. In some cases I've seen there were no carboxylic groups reported to be present at all after analysis by Boehm titration.

Did your rinse step allow for the base solution to diffuse back out of those 'sites'?

I did not have a rinse step. I would centrifuge my solution and decant out the solution. And then proceed on with the titrations.