# Reconstituting RO Water



## cfreeman (Mar 19, 2008)

Hello,

Would somebody tell me the specific relation of the ppm of Ca and Mg with the GH in water, please?

I am interested in increasing the GH of RO water. If I want to maintain a Ca:Mg ratio of 4:1, how much Calcium Sulfate and Magnesium Sulfate do I add per gallon to raise the GH by 1 degree?

Also, is Sulfur a component of GH? I seem to recollect reading here that it is, but I would like to confirm this.

Thank you very much for your time and consideration.


----------



## cfreeman (Mar 19, 2008)

Anybody?


----------



## ed seeley (Dec 1, 2006)

The amount of Calcium and Magnesium in water will vary to make up the GH. Some water out of the tap over here has very high GH but almost no Magnesium which can lead to stunting issues so some people, even with sky high GHs have to dose Mg.

You don't need to worry about the sulphur as this is not part of the GH measurement.

To work out what dosing levels you need for the desired ratio simply use the atomic weights of the elements in each chemical and work out the weights needed to get that ratio.

e.g.
CaSO4 = Ca (40) + S (32) + 4 x O (4 x 16) = 136
MgSO4 = Mg (24) + S (32) + 4 x O (4 x 16) = 120

So for a mixture of 4 parts Calcium to 1 part Magnesium you will need a weight ratio of 4.53 units of CaSO4 : 1 unit MgSO4. How much you add will depend on how much GH you need to add. The simplest way to do this, IMHO, is to add a teaspoon of your mix then test the GH.

To be honest, unless you have the Mg and Ca already I'd buy some Kent RO Right as it contains these and other required minerals, dissolves quickly and easily and even gives directions on how much to add to get the desired GH with their mixture. When I costed out making my own it wasn't much cheaper either.


----------



## JamesC (Jul 26, 2005)

Ed, you forgot that both calcium sulphate and magnesium sulphate that we buy are hydrated. this means the molecular weights are as follows:

CaSO4.1/2H20 = 145
MgSO4.7H20 = 246

So for your 4:1 Ca to Mg ratio you'd need 2.4 parts calcium sulphate for every 1 part magnesium sulphate.

Thanks
James


----------



## cfreeman (Mar 19, 2008)

James and Ed,

Thank you very much for your responses. I appreciate this very much. 

I think I understand fertilizer calculations ok. What I am really looking for is the relationship between ppm Mg and Ca with the resulting GH measurement in water. More specifically, I am wondering if there is a calculation for how much of these 2 elements to add to deionized water to raise the GH by 1 degree (or x # of ppm).

If anybody can shed some light here I would be grateful.

Thanks!


----------



## JamesC (Jul 26, 2005)

1 GH is defined as 10mg/litre of CaO which equals 7.143 mg/litre of Ca

For calcium sulphate hemihydrate (CaSO4.1/2H2O) this equals 7.143/40 x 145 = 25.9mg

So adding 25.9mg of calcium sulphate to 1 litre of water will raise the GH by 1


Magnesium is more complicated as GH is defined using calcium so a conversion will need to be done. For your purposes though adding 26mg calcium sulphate and 11mg magnesium sulphate per litre of water will raise the GH by a little over 1 and give you a Ca:Mg ratio of about 4 to 1

James


----------



## ed seeley (Dec 1, 2006)

JamesC said:


> Ed, you forgot that both calcium sulphate and magnesium sulphate that we buy are hydrated. this means the molecular weights are as follows:
> 
> CaSO4.1/2H20 = 145
> MgSO4.7H20 = 246
> ...


I did indeed completely forget that. Good catch James!


----------



## cfreeman (Mar 19, 2008)

Thank you both for all of your insights. This is indeed very helpful.

Cheers!
Chris


----------



## ray-the-pilot (May 14, 2008)

*This may be more chemistry than you need*

but I'll give you the simple answer at the end.
First here are some values you will need:

Atomic Weight of Ca: 40.078
Atomic Weight of Mg: 24.305
Molecular Weight of CaCO3: 100.086
Molecular Weight of CaSO4(H2O)2: 172.17
Molecular Weight of MgSO4(H2O)7: 246.472

General hardness is caused by the presence of Ca(+2) and Mg(+2) ions. 1 ion of Mg(+2) is equal to 1 of Ca(+2) in terms of general hardness.

There are a number of different ways to define deg. of hardness but one way is:

1 deg. = 17.9 ppm CaCO3 or 17.9 mg. CaCO3/Liter
First convert this to ppm of Ca(+2)
17.9 *40.078/100.086 = 7.17 ppm Ca(+2)
Now you need a little algebra:
Let x be the number of millimoles of Ca needed:
Then
40.078 x + 24.305 * (1/4)x = 7.17
x = .1553 millimoles of Ca
(1/4)x = .0388 millimoles of Mg.

Now multiply your millimoles by the molecular weights to get the mg.

.1553 millimoles of Ca * 172.17(CaSO4(H2O)2) = 26.7mg. (CaSO4(H2O)2)
.0388 millimoles of Mg. * 246.472(MgSO4(H2O)7) = 8.35mg. (MgSO4(H2O)7)

This is the quantity per liter so multiply by 3.78:

26.7mg. * 3.78 = 100.9 mg. CaSO4(H2O)2 per gal
8.35mg * 3.78 = 31.6 mg. MgSO4(H2O)7 per gal.

OK Simple answer:

Adding 
31.6 mg MgSO4(H2O)7
And 
100.9 mg. CaSO4(H2O)2
per gallon of RO water
will increase total hardness by 1 deg.

Sulfates have no affect on Hardness.

BTW These chemicals will NOT increase carbonate hardness. This means that your water will have no buffering properties and will rapidly change pH. You will need to add NaHCO3 to increase you buffer capacity. I'd try about 275 mg. of NaHCO3 per gallon of water.

BTW I'd check my water for hardness, pH and KH before adding any fish even if I was using tap water.


----------



## JamesC (Jul 26, 2005)

Hi Ray, hate to say but somewhere along the line I think your maths might have an error as your results don't seem quite right.

With a bit of head scratching this is how I remembered to calculate it.
One decimal place is more than accurate enough for our calculations so these are the amounts I worked with:

Ca: 40.1
Mg: 24.3
CaCO3: 100.1
MgCO3: 84.3
CaSO4.2H2O: 172.2
MgSO4.7H2O: 246.4

*For Calcium*

1 GH = 17.9 mg/l CaCO3
= (40.1 / 100.1) x 17.9
= 7.17 mg/l Ca

So if using CaSO4.2H2O

1 GH = (7.17 / 40.1) x 172.2
= 30.8 mg/l CaSO4.2H20

*For Magnesium*

As GH is measured in equivalents of calcium carbonate we first need to convert to equivalents of magnesium carbonate.

1 GH = (84.3 / 100.1) x 17.9
= 15.1 mg/l MgCO3
= (24.3 / 84.3) x 15.1
= 4.4 mg/l Mg

So if using MgSO4.7H2O

1GH = (4.4 / 24.3) x 246.4
= 44.6 mg/l MgSO4.7H2O

So from this:

*30.8mg of Calcium Sulphate will raise 1 litre of water by 1 GH
44.6mg of Magnesium Sulphate will raise 1 litre of water by 1 GH*

Or if you want per US Gallon then just multiply the amounts by 3.8

nb. Noticed first time I did the calcs for Calcium Sulphate I used the hemihydrate rather than the dihydrate. Hopefully I've got all the calcs right now, if not please let me know.

James


----------



## ray-the-pilot (May 14, 2008)

*You're right but*

I think the question is how much CaSO4.2H2O and MgSO4.7H2O you need to add to increase GH 1 deg. and Ca/Mg are in the ratio of 4/1.

Also (as far as I know) GH is always expressed as mg./l (ppm) of CaCO3 not mg./l (ppm) of MgCO3. I think if you make those two changes you will get the same answer as I did.

Then again I could be wrong about the question and the definition of GH in which case, I'd get the same answer as you.


----------



## JamesC (Jul 26, 2005)

Ah, I'm with you now. Didn't realise you had already done the 4:1 ratio. If I do the 4:1 ratio on my results then I get the same answers as you. I should have finished off my calcs.

Sorry for any confusion
James


----------



## Edward (May 25, 2004)

Hi cfreeman
Plants don't care about ratios, so why bother. For Ca use CaSO4, harmless, cheap and efficient. Not sure what your aquarium size is so let's say 4 tsp into 50 gallon aquarium will increase Ca to ~20 ppm. The next day test for Ca and if you need more add more. The recommended level is 20 - 30 ppm. This is easy to maintain, plants don't uptake much. As for the GH degrees, 7.1 ppm Ca = 1 dGH.

Now about Mg. Unlike Ca, Mg is a mobile element, in a sense plants uptake much more then they actually need, so daily dose at small accurate amount works the best. Why we don't want to add too much of it is because Mg raises dGH faster then Ca and also some plants don't do well in higher Mg levels. We have resolved this by not adding any extra at water change except what comes with the daily PPS-Pro dose. This amount is consumed in a day not causing GH fluctuations. For information, 1 tsp MgSO4 in 50 gallon = 2.5 ppm Mg, 4.4 ppm = 1 dGH.

Edward


----------

