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I had the (perhaps wrong) notion that silica gel wasn't that strong of a dessicant, especially compared to CaO, MgO, CaSO4, H2SO4, KOH, Mg(ClO4)2), but I've been exposed to information* that might give me wrong.

Can silica gel beat most of those dessicants in terms of residual water in a dried solid? Can silica gel really be used to dry salts down to only twice the residual water level that can be achieved with Mg(ClO4)2 and half the residual water level than can be achieved with KOH as dessicant? Or does that only apply to air/gases?

Will anhydrous silica gel allow to dessicate compounds that have high affinity to water down to very low residual water levels? (when used at say 5-20C at atmospheric pressure on salts pre-dried with MgSO4)

What are the conditions that give silica gel its greatest capacity to dessicate salts to ultra low residual water levels?


*Merck Memento, p14, says:

Residual humidity in mg H2O /L of air after dehydration at 25°C:

  • White CuSO4 1,4
  • Molten ZnCl2 0.8
  • CaCl2 0.14-0.25
  • CaO 0.2
  • Molten NaOH 0.16
  • MgO 0.008
  • Anhydrous CaSO4 (plaster of Paris) 0.005
  • Concentrated H2SO4 (95-100%) 0.003-0.3
  • Dry Al2O3 0.003
  • Molten KOH 0.002
  • Silica gel (SiO2) 0.001
  • Anhydrous Mg(ClO4)2 0.0005
  • P2O5 below 0.000025
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    $\begingroup$ I've always thought that silica gel was one of the best dessicants in terms of its capacity, but not necessarily it's affinity, for water absorption. In other words, I thought it was preferred in many applications because it can suck up a lot of water from fairly wet materials before becoming saturated, not because the residual water it leaves behind under optimal conditions was so low. But maybe I've always been wrong! $\endgroup$
    – Curt F.
    Sep 4, 2022 at 17:07
  • $\begingroup$ @CurtF. I was under a similar impression (i.e. I thought it absorbed more water per weight than the likes of KOH but with much higher residual water levels) but these numbers seem to contradict that. I don't know if the numbers are correct and if they are, if this order holds true for residual water in dehydrating solids / salts as well. $\endgroup$
    – Hans
    Sep 4, 2022 at 17:37
  • $\begingroup$ Without you telling what you want to dry, there is no good answer. $\endgroup$
    – Karl
    Sep 4, 2022 at 21:27
  • $\begingroup$ @Karl: chemistry.stackexchange.com/questions/167576/… $\endgroup$
    – Hans
    Sep 4, 2022 at 21:32

1 Answer 1

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Silica gel is really one of the best desiccants known. But it has to be thoroughly dehydrated at high temperature. Usually silica gel contains a small amount of cobalt chloride $\ce{CoCl2}$ : blue when anhydrous, and red when hydrated. Silica gel should not be used if reddish. It has to be blue to act as desiccant.

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  • $\begingroup$ Thank you for your answer Maurice! Will thoroughly dehydrated silica gel dessicate salts to half the residual water level as Mg(ClO)4 would? $\endgroup$
    – Hans
    Sep 4, 2022 at 17:32
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    $\begingroup$ The residual water amount in air from my tables (from Germany) are not the same as yours. The maximum amount of water in air is $0.006$ mg/L for silica gel, and $0.0005$ mg/L for magnesium perchlorate. So Magnesium perchlorate is more efficient, in my tables. $\endgroup$
    – Maurice
    Sep 4, 2022 at 19:05
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    $\begingroup$ Magnesium perchlorate is not $\ce{Mg(ClO)4}$, but it is $\ce{Mg(ClO4)2}$ $\endgroup$
    – Maurice
    Sep 4, 2022 at 19:07
  • $\begingroup$ Thank you! Sorry for the mistake (I can't correct the comment anymore though). Your numbers are more in line with what I would expect. I wonder how Merck came up with a 6x (!) smaller figure. It can hardly be a mistake because it's not just the figure that's very different, but with it the ranking of silica gel in that list which becomes completely different from what seems to be common expectations amongst chemists. $\endgroup$
    – Hans
    Sep 4, 2022 at 20:36

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