As a non chemist I tried to read to understand if active carbon acts in solutions as an acid or a base but I understood that it can act as both, depends on the material which used to make the solution.

For example, mixing a sodium bicarbonate with active carbon in a solution will make that solution with higher PH (hence a stronger base, per my understanding).

Is it correct to say that active carbon is an enhancer of either acid or base?

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    $\begingroup$ Could you please provide evidence, i.e., measured pH values, for the assertion in the second sentence? $\endgroup$ – Ed V Mar 25 '20 at 14:04
  • $\begingroup$ @EdV did you mean the second passage For example, mixing a sodium bicarbonate with active carbon in a solution will make that solution with higher PH (hence a stronger base, per my understanding).? $\endgroup$ – user61828 Mar 26 '20 at 0:58
  • $\begingroup$ Yes. I see no reason the pH would change unless adsorption was taking place. But then why? So if measured pH values, with and without the active carbon, displayed a significant difference, then an explanation would be warranted. But a small difference might just be experimental error. $\endgroup$ – Ed V Mar 26 '20 at 1:20
  • $\begingroup$ @EdV I don't know where to start - I don't have the equipment to measure the PH if I mix (the amount of) one capsule of Active carbon with say half a teaspoon of baking soda which I bought both in the supermarket; I can do that but don't have the equipment and will not purchase it for this question - if the question cannot be answered regarding this or a similar solution than I guess the question should be closed. $\endgroup$ – user61828 Mar 26 '20 at 1:24
  • $\begingroup$ Understood. Best to just wait and see how the answers shake down. $\endgroup$ – Ed V Mar 26 '20 at 1:29

Active carbon is neither an acid, nor a base. It cannot. It does not react directly in water and in acidic or basic solutions. But it may have a catalytic action on the solutes. It may accelerate the rate of chemical reactions. That is probably what you have observed when mixing sodium bicarbonate in water. Without knowing it, you must have increased the rate of the forward reaction : $$\ce{HCO_3^- <=> CO_2 + OH^{-}}$$ and this reaction is favored if the air in the room contains a low concentration of $\ce{CO_2}$, according to Le Chatelier's principle. As a consequence, the pH of the solution has slightly increased, due to this supplement of $\ce{OH^-}$ ions.


If one is already employing microwave (MW) assisted preparations, in this regard, per this 2007 source, 'Generation of hydroxyl radical in aqueous solution by microwave energy using activated carbon as catalyst and its potential in removal of persistent organic substances', by Xie Quan, et al, then one may, indeed, be able to increase the respective power of acidic reactions.

As to mechanics with bicarbonate at low pH in the presence of any created hydroxyl radicals (from MW irradiation of aqueous activated carbon):

$\ce{HCO3(-) + .OH = .HCO3 + H2O}$

the powerful hydrogen carbonate radical.

Similarly, working with a weak acid (for example, dilute acetic acid) and H2O2, accelerated activity via the formation of the more acidic hydroperoxyl radical from the action of the hydroxyl radical acting again on hydrogen peroxide:

$\ce{H2O2 + .OH -> .HO2 + H2O}$

Source: 'Radical-Enhanced Acidity: Why Bicarbonate, Carboxyl, Hydroperoxyl, and Related Radicals Are So Acidic’, referring in particular, to the hydroperoxyl radical, •HO2. To quote from the abstract:

Comparison of accepted pKa values of bicarbonate, carboxyl, and hydroperoxyl radicals, with those of models having the unpaired electron replaced by H atoms, implied the acidity of the radicals was greatly increased. A Density Functional Theory computational method of estimating pKas was developed and applied to a set of radicals designed to probe the phenomenon of radical-enhanced deprotonation (RED-shift) and its underlying causes. Comparison of the computed pKa values of 12 acid radicals to those of the corresponding model acids confirmed the intensified acidity of the title radicals..

More generally, in the realm of electrosynthesis (which relates to the chemistry of chemical synthesis of compounds in an electrochemical cell) and, in particular to the alkaline 'bleach battery', replacing the copper metal with more noble carbon, will also accelerate the reaction. Source: see comments and reference cited here.


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