# MgSO4.7H20???



## Alastair (2 Aug 2011)

I'm just wondering if it's ok to use the above instead of mgs04? I've managed to find it cheaper than buying mgso4 and adding postage onto that too. If it is ok, would I have to use more or less of this stuff compared to what I'd normally use? 
My tanks 120 gallon so I'd normally add 36 tsp of mgso4, but if I buy this stuff, I don't want to end up not using enough. I'm useless when it comes to this  

According to to the last post in this thread viewtopic.php?f=11&t=16874 it states that mgso4.7h20 is 9.8% magnesium and on our sponsors sights their mgs04 is the same so I presume I'd use the same amount??


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## Bobtastic (2 Aug 2011)

I believe that you are able to use it instead, but you need to work out the exact molecular weight so you know how much more/less you need to add. I think there is a different amount of the required for MgSO4.7H20. Clive will be able to explain it all better.


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## dw1305 (2 Aug 2011)

Hi all,
Magnesium sulphate is hygroscopic, and you can only keep MgSO4 anhydrous, by heating MgSO4.7H2O, and then cooling it, and storing it, in a desiccator. Any "MgSO4" you buy will be "Epsom Salts", the heptahydrate form.

Like it says in the other post you can work out the % of any element in a compound from its RMM,to calculate the RMM you add all the RAMs together (1 Mg, 1 S, (4 x O). (14 x H), 7 x O) and divide the RAM of the element you are interested in by the RMM of the compound.

RAM Mg = 24.3, RMM of MgSO4.7H2O = 246.5  24.3/246.5 = 0.0986 = 9.86% Mg.

cheers Darrel


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## Alastair (2 Aug 2011)

Errr darrel does that mean I have to heat it before I use it then?  
The stuff I've bought says it's hydrated.


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## dw1305 (2 Aug 2011)

Hi all,
PM replied to. All the "MgSO4" you buy will actually be MgSO4.7H2O. This is "hydrated" because of the .7H2O, this H2O known by a lovely term "_the water of crystallisation_". 

MgSO4 would be "anhydrous", but it is highly hygroscopic and will suck up any water from the atmosphere (like salt or sugar, but more so) until it is the stable .7H2O hydrated "Epsom Salts". 

You can only get MGSO4 by heating "Epsom Salts" to drive the water off, and then storing the anhydrous compound in entirely dry air (which usually means storing in a desiccator).  Realistically it is almost impossible to stop hygroscopic compounds picking up moisture, so usually if you need the anhydrous version, you weigh out the weight you would need of the hydrated compound to give you the anhydrous mass you need, heat it up and re-weigh until it reaches constant weight (hopefully the weight you've worked out) and then add it to your reaction before it cools. If you are making solutions this is obviously nonsensical.

cheers Darrel


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## ceg4048 (2 Aug 2011)

Bobtastic said:
			
		

> I believe that you are able to use it instead, but you need to work out the exact molecular weight so you know how much more/less you need to add. I think there is a different amount of the required for MgSO4.7H20.


No you don't need to do any of that. As Darrel mentions, every Epsom Salt you buy is hydrated. That's just how life is and you don't need to make any corrections or to make your life more complicated than it is already. The Mg content percentage differences are not enough to worry about for this micronutrient. If we were talking about NPK that would be a different story, so just get on with the dosing and stop worrying.

Magnesium is ultra important because it is the "queen bee" inside a hive that is the Chlorophyll molecule. If you look at the chemical formula for  a typical Chlorophyll  molecule you'll see it's something like C55H72O5N4Mg. That means it's constructed of 55 Carbon atoms, 72 Hydrogen atoms 5 Oxygen atoms, 4 Nitrogen atoms and 1 Magnesium atom. Here's a schematic of a Chlorophyll molecule. Can you see that orange ball in the middle? That's the one and only Mg atom in the entire structure. The molecule weighs about 894 grams per mole. Magnesium alone weighs only about 24 grams per mole. So, less than 3% of the weight of the Chlorophyll is Mg. That means you don't need a lot of Mg. So, whether you add 10% or 20% Mg by weight of powder does not make much of a difference.

There are certain conditions under which it makes a critical difference whether a compound is hydrated or not. Epsom Salt dosing is not one of those conditions.

Here's a quiz: How much Carbon does a plant need in comparison to the amount of Mg it needs?






Cheers,


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