Colorless silica gel heating in 120-140 oC for resuscitation. After it was observed that some of the granules change the color and turned to brown. Does it change color on absorption is effective or not?
It sounds like either an absorbed organic material such an alcohol or VOC material or that some of the granules had some sort of organic binder in them. Pure silica gel should either be translucent or white regardless of heating.
If you have large quantities of this silica gel, then you can dry at lower temperatures under vacuum to avoid the brown color most likely caused by decomposing organic material of some type.
Was the initial intention for adsorption of a gas on the silica gel surface? Most likely some form of carbon compound on the surface. Usually you cannot tell by eye that adsorption has occurred unless there is a redox process happening. For instance, CuCl is a white solid, but when left in air, oxidizes to CuCl2 and becomes greenish-blue (Cu+ to Cu2+ transition).
Since silica is relatively inert, you will not see color change for the adsorption, but will see it go brown at higher temperature because of the adsorbent being (lightly) combusted.
Normally to clean/activate the surface, you would heat up slowly in either an inert atmosphere (N2, Ar or He) or in a vacuum. This will help drive off the adsorbent if that is the objective.
If you can perform a TGA-MS, you should be able to see how much was adsorbed, and detect the outgassing product which will be water and your contaminant (likely to be in the form of carbon).
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