These results are not really in contradiction. By Boehm's titration, you have the number of acidic and basic surface fonctionnalities but you don't know the strength of each group (although you can have this information by boehm's titration using acids or bases of different strength for the "reaction" between activated carbon and the acid or base). The PZC gives you the pH at which the number of negative and positive charge is equal. For example, if you have a basic group which is three times stronger than a acidic group, the PZC will be approximatively 7 if the number of basic groups (by boehm's titration) is three times smaller than the number of acidic groups.

So, if you want to have a more precise result, you can do Boehm's titration by using différent kind of acid or base for the reaction between the activated carbon and the acid or base. For the acidic groups, you can use NaHCO3, Na2CO3 and NaOH to have more details on the strength of the different acidic groups.

These results are not really in contradiction. By Boehm's titration, you have the number of acidic and basic surface fonctionnalities but you don't know the strength of each group (although you can have this information by boehm's titration using acids or bases of different strength for the "reaction" between activated carbon and the acid or base). The PZC gives you the pH at which the number of negative and positive charge is equal. For example, if you have a basic group which is three times stronger than a acidic group, the PZC will be approximatively 7 if the number of basic groups (by boehm's titration) is three times smaller than the number of acidic groups.

So, if you want to have a more precise result, you can do Boehm's titration by using différent kind of acid or base for the reaction between the activated carbon and the acid or base. For the acidic groups, you can use NaHCO3, Na2CO3 and NaOH to have more details on the strength of the different acidic groups.

Thank you for your answer, Nicholas. You confirm my assumption . In fact, I determined the nature of the acidic groups and I found 1.27 of carboxylic groups, 0.055 of lactonic groups and 0.22 phenolic groups (from a total of 1.545 acidic groups). As basic groups I found only amines (by IR spectroscopy). 

These results show that the surface having weakly acidic groups.

I have a question pertaining to the back titration itself. When it is performed, with the solutions containing the bases to determine the acidic groups against NaOh, to what pH is it run down?

The pH is acidic. For more information about the boehm's titration and optimization of different parameters (back or direct titration, CO2 expulsion, ...), you can read those papers:

- Standardization of the Boehm titration. Part I. CO2 expulsion and endpoint determination by S.L. Goertzen et al.

http://www.sciencedirect.com/science/article/pii/S0008622309007854

- Standardization of the Boehm titration: Part II. Method of agitation, effect of filtering and dilute titrant by A. M. Oickle et al.

http://www.sciencedirect.com/science/article/pii/S0008622310003271

I hope it will help you!

Nicolas

Hey Nicolas,

Thank you so much for your response. I have read those 2 papers and from that they mention making use of indicators. I have used a pH meter instead.

So what I did for the basic solutions, I first titrated with HCl to a pH of 2 and then back titrated with NaOH to a pH of 7. Whereas if I am to use the same procedure for the acidic (HCl) solution must I titrate it with NaOH to a pH of 12 and then back titrate with HCl to a pH of 7?

Hi,

One of the advantage of using back-titration in this case is that in acidic pH, H2CO3 is easy to remove via degazing with nitrogen (H2CO3=>H2O+CO2). As you can see in the papers, H2CO3 makes a lot of interferences with the method so it is really important to remove it. That's why you add HCl.

For example: if at the beginning, you put 50 mL NaOH 0.05N; at the end, you put 50 mL HCl 0.05N; then you degas and finally you back-titrate HCl with NaOH. Using an indicator or a pH-meter doesn't change the result. In all cases, it is important to know the exact quantity of HCl you add for your calculations, either if you add 50mL or if you go at pH 2.

For the acidic solution, you don't need to add NaOH because you are already at acidic pH and H2CO3, if present, is easy to remove. So in this case, it is a direct titration.

Oh wow! This saves me a great deal of re experimentation! This is awesome! Interestingly I didnt know that the HCl was used for that. I included N2 purging as well for CO2 removal!

Amyway, now in case of direct titration then, the mathematical expression I use would differ right?

For back titration, I am using this formula:

Fx= ((Vex-Vbx )* Mt*DF*1000)/m  μmol/g

Where

Fx = concentration of Functional Groups donating protons to Boehm Reactant with a pKa of x

Vex = volume of titrant (ml)

Vbx = volume of titrant used to titrate the blank (ml)

m = mass of sample (g)

Mt = Molarity of Titrant

DF = Dilution Factor

What must the expression be for direct titration?

I join you the formula that I use :)

I have a question about the calculations. In the original version of the equation posted by Upasna Ray the following units are used: ml - for the volume and mol/l for the molar concentration. It doesn't make sense -we can't mix the units such as ml and l.  And another thing: what does the dilution factor means in this case - is it the ratio of the final volume (aliquot+HCl) to aliquot volume? Could someone explain it to me, please?

O have also a question to Nicolas. Don't you think that in your considerations the mol ratio of HCl to bases should be taken into account? NaOH and NAHCO3 are monoprotic while Na2CO3 is diprotic. Such modification has been done by Goerzen in the part II of the paper about Boehm standarization procedures.