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Discussion Starter · #1 ·
Hi,
I recently bought a bag of magnesium sulfate (MgSO4) heptahydrate from AAAquatics and can't seem to work out how much I need to add to reach a desired PPM of Magnesium.
Considering GH test kits would only test for the magnesium in (MgSO4) heptahydrate, I must be making a mistake in my math... unless the GH kit tests for more than only magnesium... or something else is happening...

The MgSO4 heptahydrate (from AAAquatics) is composed of:
16% Magnesium
32% Sulfur

I am using RO water**

Let's say I start off with 1g of MgSO4·7H2O and 10L of water (10 000g) (2.64172 gal)
PPM = (Solute weight / Solution weight) * 1 000 000
PPM = (1g MgSO4·7H2O / 10 000g) * 1 000 000
PPM = 100
PPM of MgSO4·7H2O in 10L when using 1g of MgSO4·7H2O = 100


Out of the 100 ppm just found, 16% is Mg.
100 ppm * 0.16 = 16 ppm of Mg in 10L when using 1g MgSO4·7H2O


According to this result, if I want to reach 53.625 ppm of Mg in 10L :
53.625 ppm of Mg / 16 ppm of Mg = 3.352
3.352 * 1g of MgSO4·7H2O = 3.352g of MgSO4·7H2O to reach 53.625 ppm of Mg in 10L.


Verification:
16% of Mg in 3.352g of MgSO4·7H2O
3.352g * 0.16 = 0.53632g of Mg
PPM = (Solute weight / Solution weight) * 1 000 000
PPM = ( 0.53632g Mg / 10 000g) * 1 000 000
PPM = 53.632 ppm
PPM of Mg in 10L when using 0.53632g of Mg = 53.632 ppm


TESTING

3.352g of MgSO4·7H2O in 10L should = 53.625 ppm of Mg

Results
This is where I run into a problem... Since a GH kit tests mostly for Magnesium and Calcium, I should be able to add some MgSO4·7H2O to my RO water and have that GH kit tests only for the Mg, since it cannot test for anything else in MgSO4·7H2O.

That RO water was tested prior to adding the MgSO4·7H2O and showed:
0 TDS and 0 GH

When adding 3.352g of MgSO4·7H2O in 10L (after 12 hours) which should = 53.625 ppm of Mg:
My GH test kit (from API) takes 9 drops to turn green which is = to 9 degrees or 160 ppm of Mg...
TDS = 145 ppm...

The 145-160 ppm cannot possibly be only Mg... and so what could it be? Is it the Magnesium + Sulfur??
Since 16% Mg + 32% S = 48% of the composition of MgSO4·7H2O

PPM = (3.352g MgSO4·7H2O / 10 000g) * 1 000 000
PPM = 335.2 of MgSO4·7H2O in 10L of water

335.2 * 0.48 = 160ppm of Mg and S

So my questions are:
Wouldn't the GH Kit only test for Mg in MgSO4·7H2O?
How do I calculate the exact amount of Mg in PPM if my math is wrong? I need that to reach an appropriate GH and respect a 1:3 ratio of Mg to Ca.
Is something happening that I am completely looking over?

Thanks a lot, I tried to make it clear but I understand that anything is always clear to the author...
 

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Hi @Arsenios

Although I was a chemistry major it's been decades.

Yes, a GH test kit typically only checks for calcium and magnesium, for example API GH Test Kit.
Possibly one of my favorite websites for nutrient dosing calculations will help you determine the answer: https://rotalabutterfly.com/nutrient-calculator.php
Input the tank volume size, whether dry dosing or dosing a solution, and other information requested.

Based upon the information above of 3.325 g of MgSO4+7H2O added to 10 liters of water this is the results it calculated
Rectangle Font Parallel Software Screenshot


Now you tested 9.0 dGH, the calculation is there should have been a 7.57 dGH rise....I would guess the test kit may be a little off. We are not in a lab using calibrated testing / measuring equipment. Likely the test kit is slightly off. I have to remind myself regularly that growing plants in not a exacting science.....it's more like hand grenades......close is good enough to get the job done! Hope this helps! -Roy

Plant Plant community Terrestrial plant Aquatic plant Grass


Flower Plant Petal Terrestrial plant Vegetation
 

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The conductivity meter not misure on ppm but Siemens. Tds is calculated whit coefficient, every manufacturer use own coefficient.
Every ione have, with same concentration, different conductivity.
The test make right measure, the tds meter no.
Again the element on water can be consumed from plants or other
 

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Discussion Starter · #4 ·
Hi @Arsenios

Although I was a chemistry major it's been decades.

Yes, a GH test kit typically only checks for calcium and magnesium, for example API GH Test Kit.
Possibly one of my favorite websites for nutrient dosing calculations will help you determine the answer: https://rotalabutterfly.com/nutrient-calculator.php
Input the tank volume size, whether dry dosing or dosing a solution, and other information requested.

Based upon the information above of 3.325 g of MgSO4+7H2O added to 10 liters of water this is the results it calculated
View attachment 1043781

Now you tested 9.0 dGH, the calculation is there should have been a 7.57 dGH rise....I would guess the test kit may be a little off. We are not in a lab using calibrated testing / measuring equipment. Likely the test kit is slightly off. I have to remind myself regularly that growing plants in not a exacting science.....it's more like hand grenades......close is good enough to get the job done! Hope this helps! -Roy

View attachment 1043782

View attachment 1043783
Hi @Seattle_Aquarist
Thanks! That calculator seems to be on part with the subsequent GH tests I've done. Looks like my math approach is completely flawed then... even in relation to the concentration of Mg... and I have no idea why...

Before I make some costly mistake, if the calculator measures 32.79 ppm of Mg for a dGH of 7.57 and I am being recommended to follow a ration of 1:3 Mg to Ca, I would have to add 98.37 ppm of Ca regardless of Sulfur if I was to use CaSO4? I'm asking because I did not think the Sulfur would play a role in GH like the calculator seems to be implying and now perhaps I have to consider it in the Mg:Ca ratio.

Note: I won't be using that much Mg, Ca and S, it's simply as an example.

Thanks again!
 

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Hi @Seattle_Aquarist
Thanks! That calculator seems to be on part with the subsequent GH tests I've done. Looks like my math approach is completely flawed then... even in relation to the concentration of Mg... and I have no idea why...

Before I make some costly mistake, if the calculator measures 32.79 ppm of Mg for a dGH of 7.57 and I am being recommended to follow a ration of 1:3 Mg to Ca, I would have to add 98.37 ppm of Ca regardless of Sulfur if I was to use CaSO4? I'm asking because I did not think the Sulfur would play a role in GH like the calculator seems to be implying and now perhaps I have to consider it in the Mg:Ca ratio.

Note: I won't be using that much Mg, Ca and S, it's simply as an example.

Thanks again!

Hi @Arsenios

Yes, obviously you would not use that much. I try to target somewhere in the range of 3:1 - 5:1 calcium (Ca):magnesium (Mg) ratio for my tanks. Usually my tanks are about 5 ppm Mg and 20 ppm Ca and most species seem to thrive well. -Roy
 

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3.352g MgSO4 in 10 liters of water will be 33.05 ppm of Mg or 7.63 dGH (previous poster did a slight mistake - calculated for 3.325 g).
Rectangle Font Parallel Number Screenshot


Now, if you want 33.05 x 3 = 99.15 ppm of Ca than you'll need to add 4.25 gm CaSO4.

Results:

Ca 99.15
Mg 33.05
dGH 21.498552
S 122.932328

MgSO4.7H2O 3.351505 gm (approximately 3/4 teaspoons)
CaSO4.2H2O 4.259358 gm (approximately 1 1/4 teaspoons)

The levels above are for a 10 liter tank using a dry dosing method.


Calculated using Zorfox's Planted Tank Calculator (Zorfox's Planted Tank Calculator).
 

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3.352g MgSO4 in 10 liters of water will be 33.05 ppm of Mg or 7.63 dGH (previous poster did a slight mistake - calculated for 3.325 g).
View attachment 1043819

Now, if you want 33.05 x 3 = 99.15 ppm of Ca than you'll need to add 4.25 gm CaSO4.

Results:

Ca 99.15
Mg 33.05
dGH 21.498552
S 122.932328

MgSO4.7H2O 3.351505 gm (approximately 3/4 teaspoons)
CaSO4.2H2O 4.259358 gm (approximately 1 1/4 teaspoons)

The levels above are for a 10 liter tank using a dry dosing method.


Calculated using Zorfox's Planted Tank Calculator (Zorfox's Planted Tank Calculator).
but surely you don't want to raise hardness to 21???? Wouldn't you want to do at most around half a teaspoon/ 3/4 teaspoon of CaSO4 and then 3-4 times less Magnesium? So the total dGH is like 6-7?
 

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I see two problems:

First, I have Mg as 9.8% of MgSO4*7H2O: 100* (24.31g (Mg) / 246.48g (MgSO4*7H2O) = 9.8%.
Also this:
"When adding 3.352g of MgSO4·7H2O in 10L (after 12 hours) which should = 53.625 ppm of Mg:
My GH test kit (from API) takes 9 drops to turn green which is = to 9 degrees or 160 ppm of Mg...
TDS = 145 ppm..."

The test kit uses units of CaCO3 equivalent. So it is telling you there is 160ppm CaCO3 equivalent. Converting that back to ppm Mg goes like this:

160mg CaCO3/L * (1 mole CaCO3/100.09g CaCO3) * (1mole Mg / 1 mole CaCO3) * (24.31g Mg / 1 mole Mg) = 38.9ppm Mg.

So the test kit closely agrees with the corrected calculation (using 9.8% Mg): 3.352g MgSO4*7H2O * 0.0986 (fraction Mg) * 1000mg/g * 1/10L = 33.0ppm Mg expected.

Kevin
 
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Discussion Starter · #10 ·
I see two problems:

First, I have Mg as 9.8% of MgSO4*7H2O: 100* (24.31g (Mg) / 246.48g (MgSO4*7H2O) = 9.8%.
Also this:

The 16% Magnesium above in my post comes directly from the website and package. - Arsenios

The test kit uses units of CaCO3 equivalent. So it is telling you there is 160ppm CaCO3 equivalent. Converting that back to ppm Mg goes like this:

160mg CaCO3/L * (1 mole CaCO3/100.09g CaCO3) * (1mole Mg / 1 mole CaCO3) * (24.31g Mg / 1 mole Mg) = 38.9ppm Mg.

So the test kit closely agrees with the corrected calculation (using 9.8% Mg): 3.352g MgSO4*7H2O * 0.0986 (fraction Mg) * 1000mg/g * 1/10L = 33.0ppm Mg expected.

This I completely ignored! Perhaps it's supposed to be obvious but still I went over that. Thanks. - Arsenios

Kevin
 

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The calculations of KevinC are correct.
The instructions foil is wrong, because sulfate of magnesium Eptahydrate have 16% of magnesium oxide not magnesium.
The hardness test is sensitive to all metals dissolved, not only calcium and magnesium, but results is calculated in calcium carbonate.
If you use a stock solution follow the instructions you not have a problem.
 

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Hi,
I recently bought a bag of magnesium sulfate (MgSO4) heptahydrate from AAAquatics and can't seem to work out how much I need to add to reach a desired PPM of Magnesium.
Considering GH test kits would only test for the magnesium in (MgSO4) heptahydrate, I must be making a mistake in my math... unless the GH kit tests for more than only magnesium... or something else is happening...

The MgSO4 heptahydrate (from AAAquatics) is composed of:
16% Magnesium
32% Sulfur

I am using RO water**

Let's say I start off with 1g of MgSO4·7H2O and 10L of water (10 000g) (2.64172 gal)
PPM = (Solute weight / Solution weight) * 1 000 000
PPM = (1g MgSO4·7H2O / 10 000g) * 1 000 000
PPM = 100
PPM of MgSO4·7H2O in 10L when using 1g of MgSO4·7H2O = 100


Out of the 100 ppm just found, 16% is Mg.
100 ppm * 0.16 = 16 ppm of Mg in 10L when using 1g MgSO4·7H2O
Magnesium is only 9.86% of the mass of the MgSO4.7H2O molecule, not 16%.

Total mass of MgSO4.7H2O is 14*H + 11*O + 1*Mg + 1*S where H=1.008, O=16, Mg=24.305, and S=32.065.
If you add all that up you get 246.4746 grams/mol
The one magnesium is 24.305/246.4746 of that, or 9.86%

Start from there and see where you get. I didn't read beyond that point.
 
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