[stdacet]

Blue silica gel that is used as desiccant is impregnated with cobalt chloride which change color when combine with water molecules. I want to confirm that which one of these two has higher affinity for water.
Is the mechanism is  that when water absorbing power of the silica gel is fully utilized then cobalt chloride combine with water and change color?
thanks

[Hunter2]

Dry Cobaltchloride CoCl₂ is blue. If it add water then a Complex will beformed. Co(H₂O)₆Cl₂ this is pink red.

[stdacet]

The actual absorbent is silica gel. Cobalt chloride will absorb water when silica gel is saturated, is it right?

[Hunter2]

No it willl absorb already, the colour changes slowly.

[stdacet]

Perhaps my question is not clear. Does silica gel and cobalt chloride have equal affinities for water molecules?

[Hunter2]

They are similar otherwise an indication is difficult.

[Intanjir]

Affinity for water isn't a single number. It is a continuous function depending on how much water has already been sorbed.
This is often examined via the moisture sorption isotherm.
It is easy enough to find a graph of this for silica gel:
http://www.multisorb.com/products-and-systems/natrasorb-bulk-silica-gel/

I couldn't immediately find the same graph for cobalt chloride, but this is ok since what you want to know isn't the weight of water absorbed but the color.
The affinity that cobalt chloride has for water, that is the ability of cobalt chloride to bind the water and prevent it from becoming vapor, is not the immediately practically relevant feature.
The color change is.
So you normally just want to cross reference a CoCl2 humidity indicator color key:
https://www.microessentiallab.com/ProductInfo/F30-SPLTY-HUMIDI-SRD.aspx

You can see that the linked silica gel has a smooth knee around 50% humidity for adsorption where additional water becomes a lot more effective at increasing humidity.
The cobalt chloride color strips I linked also distinctly change color from blue to purple at 50% RH, making this a good choice of indicator.
For desorption, the gel's knee is sharper and at 40%. A fair amount of hysteresis is evident.
 
Of course perhaps you really are instead interested in the affinity of cobalt chloride.
It would be an interesting graph to see sorption isotherms overlaid with the color...
We should certainly expect that rapid color change corresponds to structural changes which we would then also be expected to correspond to a change in affinity.
So I would think that both substances have knees in their sorption isotherms at around 50%, and that this results in a good practical match, as Hunter2 suggested.

[stdacet]

Ok, got it, thanks a lot Intanjir and Hunter2.