Mgso4 Vs Mgso4*7H20

[andeekaii]

After researching James' great information about home made TPN+, I have a few questions (http://www.theplante...uk/allinone.htm)

He says to add 17g Magnesium Sulphate Heptahydrate (aka MgSO4*7H20). This is supposed to add 0.39ppm of Magnesium as per the dosing he specifies.
I have worked this out and it only actually supplies 0.335274ppm. With a percentage ratio of 0.08%, compared to the 0.39% it is supposed to be.

Whereas if you take MgSO4 (Magnesium Sulphate - the non Heptahydrate version) it makes 1.58ppm, but it makes the 0.39% ratio that it is supposed to.

It doesn't appear to be possible to make 0.39ppm and 0.39%

It is DIY TPN+ (3) that I am specifically talking about, but I think it applies to all of them.
The only way I can get the ppm and % to be correct is to use MgSO4 and combine the molecular weights of the Magnesium and Sulphur, which gives the exact results that James specifies.

Have I missed a trick here? It is pretty late...so it's probably likely, but all my other calculations match with James information perfectly

 

[andeekaii]

After re-examining my calculations this morning I have found my problem. I wasn't according for the atomic weight of the sulphur and oxygen when calculating the Mg PPM.

Is it correct that
17g of MgSO4*7H20 in a 500ml bottle
1ml dose contains 0.31302644ppm of Mg

Even though the atomic mass for Mg is only 9.2952%
And atomic mass for the MgSO4 is 40.0333%

Why is it that the ppm of Mg is calculated including the entire atomic mass (MgSO4), rather than just the mass for the actual Mg?

Is it the same reason the ppm of Nitrogen is calculated from NO3? Rather than just the 'N'?

 

[dw1305]

Hi All,
Andy you are making this more difficult than it needs to be. All you need to do is add up all the RAM's for the elements to give you the RMM of the compound, the % of any element is just the RAM for that element divided by the RMM.

So in this case MgSO4.7H2O = 24.3 + 32.1 + (4*16) = 120.4 for the MgSO4. and 7H2O = 7*((1+1)*16)) = 7 * 18 = 126 so the RMM for MgSO4.7H2O is (120.4 +1216) = 246.4. The percentage magnesium is 24.5/246.4 = 9.86% Mg.

So "Epsom Salts" are 10% Mg. If that doesn't help PM with your email address and I'll send an Excel spreadsheet that allows you to convert wts, and dilutions into ppm and vice versa.

Why is it that the ppm of Mg is calculated including the entire atomic mass (MgSO4), rather than just the mass for the actual Mg? Is it the same reason the ppm of Nitrogen is calculated from NO3? Rather than just the 'N'?

The reason that some of the elements are quoted as compounds goes back to the standards for fertilisers, you only need to use the elemental ppm N, Mg etc.

From Wikipedia.

The number for "P" is actually the weight of an equivalent quantity of P2O5 and not elemental phosphorus. In order to calculate the weight of P in the formulation, the weight of P2O5 can be multiplied by 0.44 to compensate for the weight of the oxygen in the molecule. For example, a bag of 10-10-10 has 10 pounds of nitrogen, 10 pounds of P2O5, but only 4.4 pounds of P. Likewise, the number for "K" is actually the weight of an equivalent quantity of K2O, and not elemental potassium. In order to calculate the weight of K in the formulation, the weight of K2O can be multiplied by 0.83 to compensate for the weight of the oxygen in the molecule. For example, a bag of 10-10-10 has 10 pounds of K2O, but only 8.3 pounds of K.

cheers Darrel