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Old 11-17-2005, 02:33 PM   #1 (permalink)
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Default Help with calculating correct dosage of cacl2 and mgso4

Well, just when i thought everything was perfect in my tank (in terms of conditions) i test for gh and lo and behold the first drop turned green, indicating that the gh is either 0 or very close to it. i have the following chemicals: CaCl2 and MgSO4. interesting to note that the water out the faucet is also gh=0 so frequent water changes wont help

i would like some help in figuring out how much of each chemical (in grams) to add to my 55 gallon tank to raise the gh to by 1 degree of general hardness (~18ppm) in a 4:1 ratio between the calcium and the magnesium.

i would have done the calculations myself but i am finding this to be a very hard problem and i doubt my chemistry skills will be good enough to figure it out on my own, perhaps someone with lots of skill could help me out?
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Old 11-17-2005, 10:15 PM   #2 (permalink)
 
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Short answer: Niko's calc + Fertilator

Long answer:
Starting with Mg portion of target:
Ca ppm = 4 x Mg ppm
(2.497 x Ca ppm) + (4.119 x Mg ppm) = GH ppm
9.988 Mg ppm + 4.119 Mg ppm = 17.9 ppm
Target Mg ppm = 1.27
Target Ca ppm = 5.08

MgSO4+7H20 dose (note: using 55gal, but you should substract 15-20% for displacement if you are anal):
Target Mg ppm x 3.77L/gal x (mg MgSO4+7H2O4)/mg Mg x Tank gal = MgSO4 mg dose
1.27mg Mg/L x 3.77L/gal x (10.1mg MgSO4+7H2O4)/mg Mg x 55gal = 2660mg MgSO4 =~ 2.7g MgSO4

CaCl2 dose:
Target Ca ppm x 3.77L/gal x mg CaCl2/mg Ca x Tank gal = CaCl2 mg dose
5.08mg Ca/L x 3.77L/gal x 2.77mg CaCl2/mg Ca x 55gal = 2917mg CaCl2 = ~ 2.9g CaCl2
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Old 11-18-2005, 12:38 AM   #3 (permalink)
 
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Quote:
Originally Posted by Zapins
i would like some help in figuring out how much of each chemical (in grams) to add to my 55 gallon tank to raise the gh to by 1 degree of general hardness (~18ppm) in a 4:1 ratio between the calcium and the magnesium.
I haven't totally figured out czado's methods but he is incorrect. A degree of German Hardness is equivalent to 10 ppm of calcium oxide (CaO) or 17.86 ppm of calcium carbonate (CaCO3). CaO contains 7.1469 ppm of calcium and 2.853 ppm of oxygen. CaCO3 contains 7.1469 ppm of calcium and 10.7145 ppm of carbonate (CO3). See my work below.

To raise gH by 1 dgH in 1 gallon of water using calcium chloride dihydrate and magnesium sulfate heptahydrate while maintainig a 4:1 ratio of calcium to magnesium cations:

- Add 0.07939 grams of CaCl2*2H2O
- Add 0.03328 grams of MgSO4*7H2O

For 55 gallons, this would be 4.37 grams CaCl2*2H2O and 1.83 grams of MgSO4*7H2O.

Use this as a starting point. There are other factors that will affect the actual gH reading - calcium and magnesium cations may not register 100% equally on your test, the quality of the compounds used (agricultural grade from Greg Watson vs. reagent ACS grade from a lab supplier), compounds in your specific water chemistry - so simply adjust the measurement by using the same percent of each. (Example, if it raises it a bit more than you wanted, try multiplying each measurement by .75 to dose at 75% strength next time.)

DO NOT DISSOLVE CaCl2 and MgSO4 IN THE SAME CONTAINER! - The calcium and sulfate ions will bind and calcium sulfate will precipitate out. Calcium Sulfate is "insoluble" - it will dissolve but verrryyy sloowwllly.


-------------------------


Calculations - Assumes an EDTA chelation gH test in which calcium and magnesium cations register equally


- Calcium = 40.078 g/Mole

- CaCl2*2H2O Dihydrous Calcium Chloride = 147.01396 g/Mole

- Magnesium = 24.305 g/Mole

- MgSO4*7H2O Heptahydrate Magnesium Sulfate = 246.47556 g/Mole


1 dGH (Degrees of German Hardness) for calcium = 7.14691 ppm or mg/Liter / 1000
= .00714691 g/Liter / 40.078 g/Mole = .00017832 Moles

1 dGH (Degrees of German Hardness) for magnesium:
.00017832 * 24.305 = .0043342 g/Liter * 1000
= 4.33419 mg/Liter or ppm


Increasing 1 dGH of Calcium (7.14691 ppm) in 1 liter of water using Dihydrous Calcium Chloride:
7.14691 / (40.078 / 147.01396) = 26.21626 mg/Liter

Increasing 1 dGH of Magnesium (4.33419 ppm) in 1 liter of water using Heptahydrate Magnesium Sulfate (Epsom Salt):
4.33419 / (24.305 / 246.47556) = 43.95274 mg/Liter


Converted to Gallons:

Increasing 1 dGH of Calcium in 1 gallon of water using Dihydrous Calcium Chloride:
26.21626 * 3.785412 = 99.23934 mg/Gallon

Increasing 1 dGH of Magnesium in 1 gallon of water using Heptahydrate Magnesium Sulfate (Epsom Salt):
43.95274 * 3.785412 = 166.37924 mg/Gallon


Converted to a 4 : 1 ratio of calcium : magnesium

99.23934 mg/Gallon CaCl2 = 1 dGH
166.37924 mg/Gallon = 1 dGH

79.39147 mg/Gallon = .80 dGH
33.27585 mg/Gallon = .20 dGH

Last edited by Salt; 11-18-2005 at 02:09 AM..
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Old 11-18-2005, 02:04 AM   #4 (permalink)
 
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Salt,
If you are actually trying to figure out my mess, (2.497 x Ca ppm) + (4.119 x Mg ppm) = GH ppm comes from a bookmark and I checked the numbers against linked calcs before posting. I am beginning to understand why you are correct from your post and the krib archive but need to try again later. Appreciate explanation.
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Old 11-18-2005, 01:01 PM   #5 (permalink)
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hey thank you both for helping with the calculations! it obviously took you a long time to complete them, thanks for taking the time to help me! just one thing i need to clarify on though.
Salt: when you said "For 55 gallons, this would be 4.37 grams CaCl2*2H2O and 1.83 grams of MgSO4*7H2O."

i assume you ment add 4.37 grams CaCl2*2H2O and 1.83 grams of MgSO4*7H2O to raise 55gal by 1 Gh?

Also, thanks for the warning about not mixing these chemicals together in the same container. i didnt know they reacted. though if they react in a container wouldnt they just react in my fishtank anyway? should i replace the Gh every week or just wait until water change day?
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Old 11-18-2005, 01:10 PM   #6 (permalink)
 
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Quote:
Originally Posted by Zapins
i assume you ment add 4.37 grams CaCl2*2H2O and 1.83 grams of MgSO4*7H2O to raise 55gal by 1 Gh?
Correct



Quote:
Originally Posted by Zapins
though if they react in a container wouldnt they just react in my fishtank anyway?
I am not a chemist, this is only my understanding... they don't really "react." The ions just sort of "bump up" against each other in such a small space and make the dissolve time take longer. In a tank, it's much more dilute to cause precipitation.



Quote:
Originally Posted by Zapins
should i replace the Gh every week or just wait until water change day?
Totally up to you. Adding the compounds raises GH. If you do a water change and there is little to no GH in your water, then a water change will lower GH.

Be aware that it is possible that your test kit may be bad, especially if it's very old. Always shake the testing reagents before using them. You may want to buy a new GH test kit just to be sure.
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