I need to investigate the basic/acid character of my activated carbons. Which procedure/technique should I follow in order to do that? I have been told to apply Boehm's titrations. Is it the most reliable/accurate method or there are some alternative routes I could pursue?
If you just want to know about basicity, without any quantitative character, just introduce a given amount of AC in deionised water and measure the pH. Most of ACs are basic, except those prepared by phosphoric acid activation. If more accurate determinations are required, then I agree that Boehm titration or TPD may help. If you have a TGA coupled with MS, TPD is easier to handle than titration. The 2 methods show similar trends, but never identical results. Titration is more difficult but more suitable, as different groups can be separated. Whereas TPD only allows to see CO2 and CO desorption, which can originate from different moieties, and whose desorption peaks may overlap.
Alain
Thanks everyone...I'm specifically interested in total basic surface groups so I suppose Boehm's direct titration (using NaOH as titrant and HCl as analyte?) would be best...
By the way, I found from literature that for Boehm's titrations 1 or 1.5 g of carbon are generally used. Do you think that using a lower amount of material (e.g. 0.5 g) might significantly affect the results?
Titration is not a very sensitive method, so the higher the amount, the better it is.
0.5 g can be enough if your surface area is rather high. In all the cases, an automatic titrator is required. The errors can be huge if titration is performed by hand.
Alain
If you do not have much carbon and want to use less I think you could reduce accordingly the volume of HCl to be added to the carbon or use more dilute solutions of reactants (HCl and NaOH) so as to work with reasonable volumes of solution and minimise the errors in case your total basic surface groups are not many. In any case a first exploring experiments could be good to get a rough idea of how your carbon is (if you are lucky the first attempt could be good). You can then optimise your experimental conditions, if needed, based on your preliminary results. Repeating the experiment 3-4 times could help you make an idea on how good your data are.
You can calculate the PZC (point zero charge) of your carbons (the value of pH at which the sorbent surface charge takes a zero value). This value will give you the basicity, it shows the same trend than Boehm and TPD and you will need less amount of sample than for Boehm, but you will not have the contribution of the different groups as you will get with the other techniques. You can start with Babic, B.M., et al. Carbon 37, 477–481 (1999) or Carrott P.J.M., et al. Microporous and Mesoporous Materials 47, 243–252 (2001). I was using the inmersion technique.
Probably the most accurate method is immersion calorimetry with HCl and you only need 0.1 g of your sample
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