Sulaiman
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Black and yellow Magnesium Sulphate impurities
I bought BP grade Epsom salts, some of it I re-crystalised to nice white/clear crystals
some of the re-crystalised magnesium sulphate I dehydrated in a pyrex bowl in a domestic oven
I got a slightly tan tone, very dry fine powder, which I stored as "Anhydrous MgSO4"
Today I re-hydrated some of my "Anhydrous MgSO4" and got a cloudy yellow liquid ... not good.
Vacuum filtering multiple times through a Buchner funnel with 2x slow papers with 1x medium paper on top,
I got a clear transparent yellow solution, and filtrate that is black when wet and changing to a dark grey as it slowly dries.
Shining a green laser through the yellow liquid shows that there is a lot of very fine suspended material.
Is the suspension just elemental sulphur and the dark grey stuff magnesium hydroxide ?
If so why?
Wikipedia informed me that by 200 C all hydrates should change to anhydrous MgSO4,
which does not decompose until 1,124 C, which could not be reached in a domestic gas oven.
How should I 'purify' my yellow MgSO4 solution ?
What simple test should I do on the dark grey filtrate ?
(only a VERY thin layer on a 90 mm diameter paper available)
EDIT: I have already ordered 99.5% MgSO4.7H2O to continue with battery experiments, it is cheap.
I want to continue purifying my c1.2 l of yellow solution for later use,
and as a matter of principle I want to find out what happened.
I should know what is going on by now 
[Edited on 10-4-2016 by Sulaiman]
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blogfast25
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Everything points to contamination incurred during drying in your domestic oven. That latter term translates to 'kitchen oven'. Have food odours or
carbonised remains transferred from the oven walls/floor/grill elements to your product?
I use garden grade Epsom (presumed less pure than BP) and never have that problem but I dehydrate simply on a hot plate in a silicone oven dish (for
baking).
Quote: Originally posted by Sulaiman  |
I got a clear transparent yellow solution, and filtrate that is black when wet and changing to a dark grey as it slowly dries.
Shining a green laser through the yellow liquid shows that there is a lot of very fine suspended material.
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By 'filtrate', did you mean filter cake?
Also, a 'clear transparent yellow solution' contradicts 'shining a green laser through the yellow liquid shows that there is a lot of very fine
suspended material': the latter points to an opaque liquid.
Try and add some peroxide to the yellow filtrate and warm gently. If the colour disappears that's an indication of organics being oxidised.
Certainly MgSO4 does not decompose in those mild drying conditions, so forget about sulphur causing the yellow hue.
[Edited on 10-4-2016 by blogfast25]
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Sulaiman
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Posts: 3625
Registered: 8-2-2015
Location: 3rd rock from the sun
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Update: slowly boiling down the yellow solution the yellow becomes more intense
and not just through concentration (I estimate 1.2 litre simmered to 1 litre now)
I realise it seems contradictory but a solution that appears 'crystal clear' in ambient light can reveal the path of a bright (laser) beam of visible
light. (Tyndall effect)
Even dH2O and tap water give a good indication of the technique that I learned playing with colloidal silver.
Inexcusable error, sorry.
The 'clear' yellow liquid was the filtrate, the grey stuff is a filter cake - but very thin
I'll try H2O2 in a few minutes, just tidying up ....
Update: c5ml 33% H2O2 had no observable effect on c50 ml of the now yellower solution within 1 minute .
. then I finished tidying up..
[Edited on 10-4-2016 by Sulaiman]
[Edited on 10-4-2016 by Sulaiman]
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