I need to wash some KOH-activated carbons with HCl. From literature different procedures are reported. Usually the concentration is between 1 and 5 M but just a few studies (almost none of them really) mention about the amount of acid to be used. Is this parameter not so significant? How could I work out the optimal concentration/volume of acid I should wash my samples with?
Should I refer to the KOH/char ratio I used (for instance 4:1) to work out the amount of moles of KOH present onto the final carbon (though I suppose after the heat-treatment this figure is not correct anymore?), thus calculating the moles of HCl needed to neutralise it?
Dear Antonio, you can calculate the moles of KOH and use the moles of HCL need it to neutralize them. Although after the heat treatment the moles of KOH will not be exactly the same it can be taken as a good approximation. After neutralization with the acid solution , wash with aboundant warm/hot DW (80-85 degreeC). You can roughtly check the pH of your filtrate with some pH paper indicator or use a pHmeter for better acuracy.
When you say huge change of pH you mean from highly basic (say 12) to highly acid (say 2? right? If you mean that, then I am supposed to keep washing with HCl until I get a strongly acid pH?
However, I tried to wash my samples through the filter first with acid and then with DI water. I stopped rinsing with HCl when I first got a pH strongly acid (ca. 2). Anyway, after adding some DI water the pH came back to be basic (say 10-12). Then, I presume that the preliminary rinsing with acid was not efficient?
To summarise:
1) initial pH = 12
2) pH after rinsing with HCl= 2
3) pH after rinsing DI water (at the beginning) = 12
4) pH after rinsing DI water (at the end) = 7
I suppose that 3) occurred because the HCl rinsing was not sufficient? Should I soak/stir my sample in acid rather than washing through the filter?
For alkali activation, 4M KOH is added to the carbon in the solid-tio-liquid ratio of 10:1 and soaked for overnight. Then, excess alkali can be removed by washing with either distilled water or dilute HCl (0.1 M). The washing continues to constant pH.
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