The equilibrium concentration can not be greater than the initial concentration - if it is a substrate (adsorbate}. So what you are measuring is not an equilibrium concentration!!! Regards,

Dear M.Grzesik,

You are right, the equilibrium concentration can not be greater than the initial concentration. And I am sure that I measured the equilibrium concentration. I am also using Nesslers' reagent method using the UV/VIS spectrophotometer at 420 nm.

But the problem might be from the following:

1. My adsorbent is char and it has a color, and it gave a high absorbance

2. My adsorbent might have NH4+-N

Professor, can you suggest me what can I consider when I am preparing and measuring?

I thank you very much.

Dear Sebastian Hippmann,

Yes, I have used the dry char.

Do you mean that the moistened char adsorb more NH4+-N than the dry one?

I do not know what error you are making and so you get such a result. But as I wrote, what you measure is not an equilibrium concentration, because it would be against the law of mass conservation. Discussions, whether something is dry, or something wet or, more wet, do not make sense here, because the problem is that there is "an additional mass", which is of course impossible. And one more thing, how can anyone know that your adsorbent may contain an adsorbate. It would have to start with this.

Regards,

So probably the experiment was performed badly. And as a consequence, the concentrations were measured/calculated incorrectly (with respect to changing volume of solvent). - And also blocking the char surface with a solvent molecules. This is the case if incomplete information is given. Regards,

I think you need to perform blank experiment (without adding the analyte), there is possibility that the charcoal containing ammonium ion. if that is the case you have to find a method to eliminate the ammonium from the adsorbent prior it uses.