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Specialty Chemistry Forums => Citizen Chemist => Topic started by: wt2000 on May 19, 2010, 12:46:00 PM

Title: Re-Solidification
Post by: wt2000 on May 19, 2010, 12:46:00 PM

Hi,
I am having problem when doing my experiment with potash alum.
Alum such as the dodecahydrate form of potassium alum has a melting
point of 92.5° C. However, when melting the alum followed by re-solidification
transforms the alum into a weak and crumbly structure.
why is this happen? and how to solve it?

Thanks.

Title: Re: Re-Solidification
Post by: billnotgatez on May 21, 2010, 05:09:48 AM

I wonder if when you melt it the compound does not recrystallise the same way as it was created. Maybe it was in a solution which was dried to make a crystal, much like rock candy from sugar. I have not worked with potash alum so I am reading about it.

http://en.wikipedia.org/wiki/Potash_alum

Title: Re: Re-Solidification
Post by: billnotgatez on May 21, 2010, 05:22:27 AM

this is from WIKI

Quote

Potassium alum crystallizes in regular octahedra with flattened corners, and is very soluble in water. The solution reddens litmus and is an astringent. When heated to nearly a red heat it gives a porous, friable mass which is known as "burnt alum." It fuses at 92 °C in its own water of crystallization. "Neutral alum" is obtained by the addition of as much sodium carbonate to a solution of alum as will begin to cause the separation of alumina. Alum finds application as a mordant, in the preparation of lakes for sizing hand-made paper and in the clarifying of turbid liquids.

Title: Re: Re-Solidification
Post by: wt2000 on May 22, 2010, 12:19:40 PM

billnotgatez:
thank you for your info.

maybe the alum structure changed or loosing something during heat period.
so, i think the alum need to be added other substance which can be bond with alum.

as i know, alum which formulation in KAl(SO4)2·12(H2O), when heated, it is possible for water molecule to evaporates from alum.

is it because of the alum loosing the water, so the structure become weak?

Title: Re: Re-Solidification
Post by: skyjumper on May 22, 2010, 01:30:54 PM

To recrystallize the alum, dissolve in water and evaporate off the water?

Title: Re: Re-Solidification
Post by: wt2000 on May 23, 2010, 11:54:01 AM

skyjumper:
solution in water: same result, still weak n crumbly...
i wonder if i can add some substance that can really bond with the alum...
but i dont know what it is...

anyone know?

Title: Re: Re-Solidification
Post by: billnotgatez on May 26, 2010, 06:10:16 AM

When doing rock candy a super saturated solution of sugar is used.
I think that putting in 1 very small crystal starts the solidification.