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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: curiouscat on July 22, 2016, 10:36:08 PM

Title: CaSO4 as drying agent; which form? anhydrous or hemihydrate
Post by: curiouscat on July 22, 2016, 10:36:08 PM
When CaSO4 is used as a drying agent for solvents (MeOH in particular), it is the anhydrous form, correct? i.e. CaSO4.0 H2O?

It's what makes intuitive sense to me, but since there's also a hemihydrate CaSO4. 0.5 H2O I wanted to be sure.


Title: Re: CaSO4 as drying agent; which form? anhydrous or hemihydrate
Post by: AWK on July 23, 2016, 01:24:06 AM
As desiccant - the γ-anhydrite (drierite) is used. It is almost anhydrous CaSO4 (CaSO4·0.05H2O - dried at ~ 170 C). Hemihydrate shows lower water capacity. Completely anhydrous CaSO4 (dried at  ~300 C) does not show drying properties.
Title: Re: CaSO4 as drying agent; which form? anhydrous or hemihydrate
Post by: curiouscat on July 24, 2016, 01:12:58 AM
Thanks @AWK! That explains why my attempts are failing.

A followup question: The table I found (https://www.erowid.org/archive/rhodium/chemistry/equipment/dryingagent.html) reports the drying capacity of CaSO4 as 0.066 gm H2O/gm dessicant.

If I use the drying transition as

CaSO4.0.05 H2O  :rarrow: CaSO4.2H2O

then shouldn't I get a drying capacity of 0.25 gm H2O / gm dessicant?

What gives?

Have I gotten the final state wrong? Is it only hydrated to a hemihydrate, i.e. CaSO4·0.5 H2O? That would explain the lower drying capacity reported.
Title: Re: CaSO4 as drying agent; which form? anhydrous or hemihydrate
Post by: NewmanProj on August 19, 2016, 12:22:04 AM
Curious, do you have any magnesium? Does a wonderful job drying methanol.
Title: Re: CaSO4 as drying agent; which form? anhydrous or hemihydrate
Post by: curiouscat on August 20, 2016, 10:48:51 PM
Quote from: NewmanProj on August 19, 2016, 12:22:04 AM
Curious, do you have any magnesium? Does a wonderful job drying methanol.

Thanks. I can get some & try.
Title: Re: CaSO4 as drying agent; which form? anhydrous or hemihydrate
Post by: phth on August 23, 2016, 03:16:02 AM
Quote from: curiouscat on August 20, 2016, 10:48:51 PM
Quote from: NewmanProj on August 19, 2016, 12:22:04 AM
Curious, do you have any magnesium? Does a wonderful job drying methanol.

Thanks. I can get some & try.

Try finding a book on the subject of purifying/drying chemicals: e.g. https://www.google.com/#q=purification+of+laboaraotry+chemicals is a good book.  All common ones are in it with literature references, discussion of dessicants you can use, etc.  I would guess someone may have a copy in your department/workplace.
Title: Re: CaSO4 as drying agent; which form? anhydrous or hemihydrate
Post by: Babcock_Hall on August 24, 2016, 04:01:27 PM
Going on memory here, but I seem to remember series of papers by Burfield and Smithers in the 1970s-1980s comparing various desiccants and various solvents.  One of the papers was a reconsideration of Dri-Rite after someone from the company objected to their heating of commercial Dri-Rite.  Maybe the authors were over-drying it.  If anyone is interested, I'll go through what I have.
EDT
"Desiccant Efficiency in Solvent and Reagent Drying  9. A Reassessment of Calcium Sulfate as a Drying Agent:" David R. Burfield. JOC !984 49, 3852.