# My particles won’t separate without neutralizing the solution. I want to understand why?

I etched active carbon with a concentrated KOH solution and added HCl to neutralize the KOH. Separeted the particles with centrifuge at 4000 rpm. They should be micron sized.

My boss asked me to stop using HCl and just separated the KOH with centrifugation. But the separation didn’t work. What influenced this?

• Your active carbon particles may have been partially coated by carboxyl groups $\ce{-COOH}$, which have acidic properties. In contact with $\ce{KOH}$, these acidic groups are transformed into $\ce{-COO^-}$. So all particles are negatively charged and will repell one another. These repulsions are such that the particules will stay in emulsion and will not be separated by centrifugation. Mar 26 at 12:29
• @Maurice That makes sense Mar 26 at 12:41
• As a consequence, the only way of separating your charcoal particles by centrifugation is to first neutralize them by adding enough $\ce{HCl}$. Mar 26 at 12:50
• Also, what is the density of the KOH solution? If it approaches that of the activated carbon (which has air bubbles), centrifugation might be difficult. Mar 27 at 23:41
• I added diluted HCl until the pH of ~1 and that helped to separate the particles Mar 29 at 16:37