# Quick question about MgSO4



## dmachado (26 Jan 2021)

I have been using heptahydrated magnesium sulphate MgSO4.7H2O, which states MgO 16% and SO3 32%, giving us the approximate 10% mg (I think).
Now I got my hands on some magnesium sulphate which states MgO 23% and SO3 45% - I guess it is the "dry" salt.
Simple math tells me to use from this "dry salt" two thirds of the quantity I use from the heptahidrate version.
Is this correct?
Thank you for any validation or correction.


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## dw1305 (26 Jan 2021)

Hi all,


dmachado said:


> heptahydrated magnesium sulphate MgSO4.7H2O, ..... giving us the approximate 10% mg (I think)


It does.  You can only keep the salt dry (anhydrous) by heating it (to about 300 oC) and then storing it in water free conditions (in a plastic bag in, the freezer?) before it cools. If it has access to the atmosphere <"it will revert to the heptahydrate form"> or "Epsom Salts".

I'd just use it as 10% magnesium, it is a lot easier.


dmachado said:


> Simple math tells me to use from this "dry salt" two thirds of the quantity I use from the heptahidrate version.


Yes, you have to subtract the weight of the "7H2O" from the RMM of MgSO4.7H2O.  The RMM of MgSO4 is 120.4 (24.3 (RAM Mg) + 32.1 (S) + (4*16) (O)) and of the heptahydrate 120.4 + ((14*1) + (7*16)) = 246.5

So it isn't quite 2/3 it is ~ 1/2 (120.4/246.5).

cheers Darrel


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## dmachado (26 Jan 2021)

Thanks for the explanation, Darrel.

I found this page in the meanwhile, and it seems to work very well, just adding the chemical formula and it shows the results:





						Molecular Weight Calculator (Molar Mass)
					

Online Molecular Weight Calculator that computes the molecular mass of any molecule or element.




					www.lenntech.com
				




A few questions I have:
After opening the bag and starting to use it, it will eventually turn to MgSO4.7H2O - will I need to get back to the previous quantities I was using?
Is there an intermediate moisture content, or does it get back to heptahydrate very fast?
Maybe even gaining weight from absorbing water, if this makes any sense...?

Best regards,
Daniel


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## dw1305 (26 Jan 2021)

Hi all,


dmachado said:


> I found this page in the meanwhile, and it seems to work very well, just adding the chemical formula and it shows the results:
> Molecular Weight Calculator (Molar Mass)


Yes, there is a lot of <"good stuff"> on the <"Lenntech website">.  I've used it a lot for for pretty much anything <"water treatment related">.

I do a bit of <"bucket chemistry"> so I have a spreadsheet with a periodic table embedded etc, but I'll probably make more use of the <"IFC spreadsheet in future"> (it would seem rude not to), and I often check things via <"Rotala Butterfly">.


dw1305 said:


> ........ @Zeus. and @Hanuman have taken all the fun out of this, by providing a spreadsheet that provides all the answers at the click of the button. They have removed all the mystery and totally deskilled potion making, it is an absolute disgrace and they should be very pleased with themselves.





dmachado said:


> After opening the bag and starting to use it, it will eventually turn to MgSO4.7H2O - will I need to get back to the previous quantities I was using?
> Is there an intermediate moisture content, or does it get back to heptahydrate very fast?
> Maybe even gaining weight from absorbing water, if this makes any sense...?


So it will take up water pretty quickly until it reaches the stable heptahydrate state, "Epsom Salts" MgSO4.7H20.

Anhydrous magnesium sulphate will pretty much double in weight (246.5/120.4).

So 10g of MgSO4 will become 20.5g of MgSO4.7H2O, the extra 10.5g is just the "7H2O", the <"water of crystallization">.

Hopefully that makes sense.

cheers Darrel


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## Andy Pierce (26 Jan 2021)

It's not really worth messing around with the anhydrous form if you want to get all precise about amounts of things (not saying that degree of precision is actually useful either ).  You can easily pick up the hydrated form from e.g. Magnesium Sulphate MgSO4 - Dry Salts - Dry Chemicals - Fertilisers then you know what you have.


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## dmachado (26 Jan 2021)

I know that, I bought I convinced it was the hydrated form... and I bought a LOT...

And EI is not about precision, agreed, but one needs to know where one's aiming at... two fold dosage is not within my tolerance range.

I'll leave 100g in open air and check for crystal growth and weight gain vs time... it will be a fun experiment. 

I'll probably double my stock and keep the same dosage in the end...


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## Andy Pierce (26 Jan 2021)

dmachado said:


> I have been using heptahydrated magnesium sulphate MgSO4.7H2O, which states MgO 16% and SO3 32%, giving us the approximate 10% mg (I think).
> Now I got my hands on some magnesium sulphate which states MgO 23% and SO3 45% - I guess it is the "dry" salt.
> Simple math tells me to use from this "dry salt" two thirds of the quantity I use from the heptahidrate version.
> Is this correct?
> Thank you for any validation or correction.


Just checking on what you've got, does it actually state 'MgO' and 'SO3' as components on the label?  There is such a thing as these molecules, but not in the context of magnesium sulphate, i.e. MgSO4 does not have MgO or SO3 as components.  If you bought a big amount it should have a CAS number (CAS Registry Number - Wikipedia) on it somewhere that will identify it unambiguously.


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## dw1305 (26 Jan 2021)

Hi all, 


dmachado said:


> know that, I bought I convinced it was the hydrated form...


Yes it will be.  You can actually <"use it as a desiccant">.

It is quite difficult to stop it picking up atmospheric moisture and requires you to put it in a desiccator (while it is still very hot) to keep it in anhydrous.


dmachado said:


> I'll leave 100g in open air and check for crystal growth and weight gain vs time... it will be a fun experiment.


You will have to do it the other way around, weigh out 100g and see how much weight it has lost after 30 minutes in a very hot oven. You will find the weight will rise pretty quickly as as it cools down.

cheers Darrel


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