Desorption instead of adsorption from solution using activated charcoal I was just trying to see the effect of concentration on adsorption in solutions, i.e. I was trying to verify the Freundlich isotherm. I used oxalic acid and a batch of homemade activated charcoal (using this site).

First I prepared solutions of different concentrations. Then I added the activated charcoal, stirred it for 5 minutes and filtered the solution. When I titrated the filtrate using KMnO4, after calculating the concentration, I found that the concentration of the solution had been increased!

I tried the same process with different concentrations of oxalic acid solution and the result was the same. Can anyone tell me where did I go wrong?

Note: I used the same batch of activated charcoal in all the experiments. (Even then, how did the concentration of the solution increase? Shouldn't it be the same or lower than the initial concentration?)

For titrating oxalic acid, I heated the solution to about 60 degrees C.( After filtering the solution)
In such experiments you typically run a blank. So add the homemade activated charcoal to just some water, filter, then titrate that filtrate as a blank. 

There are two possibilities for the increase. 


  
    (1) Your batch of homemade activated charcoal was contaminated with who knows what soluble compound. (This seems the most likely...)
    
    (2) Quite possible that some of the charcoal particulates are small enough to not be trapped by the filter. 
  


Another way to run the experiment would be to add the homemade activated charcoal to just some water, filter, then wash the wet charcoal into the oxalic acid solutions (this would remove soluble compounds from the charcoal). Then dilute all the solutions to some co0nstant volume, filter, and titrate the filtrates. (You should still run a blank.)