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I found online several sources that state 100 °C for a rapid dehydration of cobalt chloride in its various forms. I'm wondering if that's a minimum, or would it be possible to dehydrate it at a much lower temperature, provided ambient humidity is controlled.

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The minimum I could find is 110 °C (Chemical forums) although not verified. The same is written in this link although there are two temperature, I am assuming the hexahydrate start to lose water above 110 °C.

mp 86 °C; loses $\ce{6H2O}$ at 110 °C

[...]

Anhydrous cobalt chloride is blue in color. It can be prepared by dehydration of the hexahydrate either at 150–160 °C in vacuo.

Also, you can find this paper useful. The temperature mentioned here is 200 °C.

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One of the "various forms" of cobalt chloride is Drierite, a mixture of cobalt chloride and calcium sulfate which is a common dessicant. It is blue when active and pink when hydrated (Ref 1). "Drierite can be regenerated by spreading the granules in a tray and heating them in an oven at about 425°F (220°C) for 1 to 1.5 hours."

A more informative website (Ref 2) explains; "The temperature at which DRIERITE desiccants are regenerated is crucial in restoring DRIERITE to its original condition. Absorbed moisture is water of hydration and is chemically bound to the calcium sulfate of DRIERITE. Temperatures in the range of 400° - 450° F are required to break these bonds and release absorbed moisture. Lower temperatures, regardless of heating time, will not regenerate DRIERITE unless applied under vacuum (26" Hg, 325° F or 28" Hg, 275° F). Care should be taken not to overheat DRIERITE Desiccants. High temperatures can alter the crystal structure and render the desiccants permanently inactive."

There are three well-defined hydrates of CaSO$_4$: the dihydrate, the hemihydrate (Plaster of Paris) and the anhydrous material, which can be especially useful (i.e., reactive) when not quite totally anhydrous. Wikipedia explains (Ref 3): "On heating to 180 °C (356 °F), the nearly water-free form, called γ-anhydrite (CaSO4·nH2O where n = 0 to 0.05) is produced. γ-Anhydrite reacts slowly with water to return to the dihydrate state, a property exploited in some commercial desiccants. On heating above 250 °C, the completely anhydrous form called β-anhydrite or "natural" anhydrite is formed. Natural anhydrite does not react with water, even over geological timescales, unless very finely ground." Apparently, some porosity remains in the anhydrite until all the water is gone, and then the lattice is closed up tight.

Although the times and temperatures mentioned above apply to CaSO$_4$ rather than to CoCl$_2$, and stray from the actual question, they indicate that a mixture including CoCl$_2$ performs its function (color change vs dessicant capability) usefully when the properties of the actual dessicant are taken into account.

Ref 1. https://secure.drierite.com/catalog3/faq.cfm

Ref 2. https://www.foxscientific.com/PDF/Drierite.pdf

Ref 3. https://en.wikipedia.org/wiki/Calcium_sulfate

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