# Epsom Salts



## Aqua sobriquet (28 Jun 2012)

I've just got some dry ferts from one of our sponsors but I didn't order any Epsom Salts (Magnesium Sulphate - MgSO4) as I already had some. Now I've looked at the packet it says: 49% MgSO4 = 16% MgO. Is this the right stuff?!


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## ceg4048 (28 Jun 2012)

Yes, MgSO4 = Magnesium Sulphate = Epsom Salts.

Cheers,


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## Aqua sobriquet (28 Jun 2012)

But why does it say 49% Clive?


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## ceg4048 (29 Jun 2012)

Hi mate,
            It is highly probable that the powder being supplied is actually what's called "Epsomite" which is the much more common hydrated form. 

Bonded to the MgSO4 molecule are seven water (H2O) molecules, so that it is really "Magnesium Sulphate Heptahydrate".  The formula looks like: MgSO4·7H2O

The molecular weight of the anhydrous MgSO4 is about 120 grams per mole.

One "mole" is 0.602 trillion trillion of something, kind of like one dozen is 12 of something. So if I gather 0.602 trillion trillion MgSO4 molecules they will collectively weigh 120 grams. The Mole is a standard unit of measure for chemists in exactly the same way as the dozen is a standard unit of measure for chefs.

The molecular weight of MgSO4·7H2O is about 246 grams per mole. That's because the 7 bonded water molecules weigh about as much as the MgSO4 alone. 

So the percentage of the mass of MgSO4 within the hydrated MgSO4·7H2O is about 49%.

Hope this makes sense...  

Cheers,


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## ian_m (29 Jun 2012)

Excellent reply..

Next question...why is the atomic weight of magnesium a non integer (24.31) ?


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## ceg4048 (29 Jun 2012)

Hi Ian,
          Well there's no reason for the weight or mass of atoms to be integers because the weight/mass numbers are describing the inertial properties associated with the combines masses of the protons, neutrons and electrons of that atom. The weights are average weights because individual atoms of the same element have slightly different weights due to the fact that they have varying amounts of neutrons in the nucleus. These are called isotopes. The published atomic weights are calculated by averaging the individual weights of all isotopes and then that weight is reference to the 1/12 the weight of a single carbon atom (a carbon 12 atom). The arithmetic seldom, if ever works out to be an integer. It would be a miracle if it did. Anyway, that's why the atomic weight is a dimensionless unit and is not expressed in terms of grams.

The integer value associated with atoms is the atomic NUMBER. The atomic number is simply a count of the number of protons bound together in the nucleus. That HAS to be an integer. Magnesium has 12 protons, so that the atomic number is the integer 12. Ordinarily it would also have 12 neutrons, but as mentioned above, different isotopes will have either less than 12 or more than 12 neutrons.





Cheers,


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## Aqua sobriquet (29 Jun 2012)

Very comprehensive answer that went way over my head but thanks for taking the time.    I suppose I should have asked, can I use this stuff in the fert recipe in the same way or do I need to adjust the amount?


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## dw1305 (29 Jun 2012)

Hi all,
You always end up with MgSO4·7H2O what-ever it was sold as. You can only get the anhydrous form by heating MgSO4·7H2O to drive the water of crystallization off, and then cooling and storing the crystals of MgSO4 in a desiccator, as soon as the salt is exposed to any moisture it will absorb H2O until it becomes the stable heptahydrate form again. 

The magnesium oxide content (MgO) is quoted because fertiliser values were traditionally quoted as the oxides, you can ignore it. 


> I suppose I should have asked, can I use this stuff in the fert recipe in the same way or do I need to adjust the amount?


It doesn't matter where you start from, as long as you know the chemical formula you can work out percentages of each element in a compound easily. In this case (from the periodic table at <http://www.webelements.com/>)
Element & RAM ("Relative Atomic Mass")
Mg = 24
S = 32
O = 16
H = 1

24+32+(4*16) + (14*1)+(7*16) ~ 24+32+64 = 120 + 14+112= 126 ~ 120 + 126 = 246  (this is the RMM "Relative Molecular Mass" of MgSO4.7H2O from Clive's answer). 

So "Epsom Salts" have 24/246 ~ 10% Mg. 
10g of MgSO4·7H2O, supplies 1g of Mg. 
1g of Mg (from 10g MgSO4·7H2O) in 1litre of H2O = 1000mg in 1,000,000mg = 1000ppm Mg sol.
1g in 10litres = 100ppm Mg 
1g in 100litres = 10ppm Mg

cheers Darrel


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## ceg4048 (29 Jun 2012)

Yeah, to make life simple just dose per whatever recipe you have. Even though it's a crtically important element, you really don't need a whole lot of Mg. It's a micronutrient, and so even though you only get a small amount of Mg from the hydrated Epsom salt this is not really a big deal. In fact, Mg does appear in some tap water and you may not need to add more at all, but it's better to dose and to find out. 

Try dosing it for about 3 weeks then withdrawing for 3 weeks and see if there is a difference. If no difference is noted then you can just stop adding it, so there's one less thing to buy. People using RO have to be more attentive of course, if they are not using any other re-mineralizer. Mg can be dosed just after the water change and it will hang around quite happily in the water. It doesn't need to be chelated, it's not affected by UV, has no precipitation issues like Iron and has no exotic idiosyncrasies to worry about. Just dump a few teaspoons in the tank after a refill of your water and get on with it. Mg is a low maintenance, low worry trace. Very easy. Only anoraks care about all that other stuff. Background info is offered so that you realize that we're not just coming up with some arbitrary dogma imported from The Matrix. 8) 

Cheers,


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## Ady34 (29 Jun 2012)

I presume adding mineral salts with mg in is the same as dosing mgs04? The plants will use it the same?


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## ceg4048 (29 Jun 2012)

Sure mate, MgSO4 is MgSO4. If it's an ingredient of some remineraizing powder mixture such as Seachem's Equilibrium it is exactly the same, but that stuff costs 100X more than Epsom Salt. Quantities are different, that's all.

Please, will someone go to Epsom Surrey and confirm that they were in fact the first people to market this salt from their saline springs? I need to know these things (I don't know why, but I just do).

Cheers,


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## Ady34 (29 Jun 2012)

dont know whether you were joking Clive but: http://en.wikipedia.org/wiki/Epsom_salts
Wiki matrix never lies  
a few more:
http://www.surreyforum.co.uk/threads/ep ... salts.892/
http://www.epsomandewellhistoryexplorer ... omSpa.html


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## ceg4048 (29 Jun 2012)

Well, I'd rather hear it from someone in Epsom, preferably from a high ranking official in the Chamber of Commerce. The Matrix isn't real...  

Cheers,


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