Could someone check my chemistry? - The Planted Tank Forum
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post #1 of 18 (permalink) Old 06-26-2014, 08:59 AM Thread Starter
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Could someone check my chemistry?

I'm trying to make a solution of KNO3 from Ca(NO3)2 and K2SO4. I've only ever done high school chemistry, and that was five years ago so I really need someone to check this in case I screwed up horribly

K2SO4 + Ca(NO3)2 --> CaSO4 + 2KNO3

Room temperature here is always around 28-30*C so solubility for each is:

K2SO4: 0.13 g/mL
Ca(NO3)2: 1.52 g/mL
CaSO4: 0.00264 g/mL
KNO3: 0.48 g/mL

I used this site to figure out how many grams of the reagents I need to get 50 g of KNO3 in solution.

K2SO4: 43.0894 g; needs at least 331.4572 mL to dissolve
Ca(NO3)2: 40.5743 g; needs at least 26.6936 mL to dissolve

So I'm thinking I'll just dissolve them in 400 mL of distilled water.

What I should get, theoretically, is 50 g of KNO3 in 400 mL (after I filter out the CaSO4). Which is 125000 mg/L, or 0.125 g/mL. Since 50 g of KNO3 only needs around 104 mL of water to dissolve, I guess I could evaporate it off to around 110 mL? (I wonder, would evaporation to dryness work with KNO3?) If I can evaporate it off it'd be 454545.45 mg/L or 0.454545 g/mL. But since I don't have any proper equipment to make sure I'm evaporating the right amt of water I'll just assume I won't evaporate it off for the rest of this:

My tank will have around 34-36 gallons of water in it. According to this calculator , for EI and with 36 gal I should add 2.1 g of KNO3 per dose. Which should be 16.8 mL of the solution.

Is this right? Idk if dosing in solution works this way
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post #2 of 18 (permalink) Old 06-27-2014, 11:00 AM
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Theoretically, the math is correct, but in reality, you will end up having some calcium sulfate dissolved in solution, so you will not end up getting exactly 50 grams of potassium nitrate.

Furthermore, calcium nitrate is probably not sold in the non-hydrated form; most likely, you have a tetrahydrate (which would need to be accounted for when doing your stoichiometric calculations).

Yes, you can evaporate to dryness if you want, but you will probably need the help of a desiccant, since KNO3 is slightly hygroscopic. In addition, during evaporation, you can expect some calcium sulfate to precipitate out as well, slightly contaminating your potassium nitrate.

However, evaporating to dryness is moot, especially if you are trying to dry dose.

If you evaporate to dryness, you will not have 0.454545 g/mL; by definition, evaporating to dryness means you cannot measure the concentration (of a solution).

Aside from that, the math looks correct.

You could probably just get away with dosing calcium nitrate, if your water is not too hard. This would keep things much easier.

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post #3 of 18 (permalink) Old 06-28-2014, 08:50 AM Thread Starter
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Yeah, I'd forgotten to count the fact that it's a tetrahydrate. I don't think I'd evaporate to dryness or even evaporate off most of the water since I don't have the equipment. Thanks for the comments!

If I made a solution with these amounts I'd run out after around 24 dosings so I wouldn't want to do this in the long term, but I figured it might be worth a one-off shot. Otherwise I'll just dose Ca(NO3)2 for the nitrate. My main issue is dosing potassium without having a bunch of insoluble CaSO4 in the tank....

Would it work to use an EI dosing regime with calcium nitrate, monopotassium phosphate and magnesium sulphate, and dosing Seachem Flourish Potassium for the K? Along with Flourish Comprehensive for traces.
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post #4 of 18 (permalink) Old 06-28-2014, 12:59 PM
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Quote:
Originally Posted by poissonej View Post
My main issue is dosing potassium without having a bunch of insoluble CaSO4 in the tank....
It is unlikely that you will observe precipitation of CaSO4 in your aquarium when you dose your fertilizers. The amount we dose is fairly low, and unless you are dosing them immediately one after another (i.e. calcium nitrate followed immediately by potassium sulfate), you should not encounter any problems.

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Originally Posted by poissonej View Post
Would it work to use an EI dosing regime with calcium nitrate, monopotassium phosphate and magnesium sulphate, and dosing Seachem Flourish Potassium for the K? Along with Flourish Comprehensive for traces.
Yes, this would work fine.

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post #5 of 18 (permalink) Old 06-30-2014, 06:03 AM Thread Starter
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Okay, thanks.

I'm wondering about dosing with Flourish Potassium, because the instructions on the back recommend dosing 5 mL for every 125 litres/30 gallons, which raises the K by 2 mg/L. For a 34 gallon tank, the EI dose is 2 g of potassium nitrate, which I think is just 0.77344 g of potassium.

I'm not sure if I'm calculating this right but this is what I figure:

5 mL of Flourish Potassium adds 2*125 = 250 mg (0.25 g) of potassium.

For an EI dose I'd need to add 0.77344/0.25 = approx 3 times the recommended dose = 15 mL.

Is that right?
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post #6 of 18 (permalink) Old 06-30-2014, 12:31 PM
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That looks correct, from a brief look.

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post #7 of 18 (permalink) Old 06-30-2014, 01:58 PM
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I suppose your method would be close. However, when doing calculations for solutions there is a formula you can follow.

Desired ppm * Volume (liters)
___________________________

% of element in fertilizer x 10 (10 is the constant for grams per liter or PPM)

Since law requires manufacturers to report potassium as K2O equivalent we need to convert K2O to K.

To convert K2O to K multiply by 0.8301

Flourish Potassium is 5.8% K2O so,

5.8 * 0.8301 = 4.81458% K

36 gallons * 3.78541 (liters per gallon) = 136.27476 liters

Now plug in the numbers...

7.5ppm * 136.27476 liters
_____________________

4.81458% * 10

1022.0607 / 48.1458 = 21.2

So to raise potassium to 7.5ppm in 36 gallons we add 21.2ml of Flourish Potassium.

All that said there is a MUCH easier way to do the math...Yet Another Nutrient Calculator.
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post #8 of 18 (permalink) Old 07-02-2014, 09:05 AM Thread Starter
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Quote:
Originally Posted by Zorfox View Post
All that said there is a MUCH easier way to do the math...Yet Another Nutrient Calculator.
Ah thanks! I'd forgotten this calculator did premixes too lol. Thanks for the help!

I used the calculator to see how much of Flourish Comprehensive I should add, and it gave me this:

"To reach your target of 0.5 ppm Fe you will need to add 20 mL Flourish Comprehensive to your 34.0 US gal aquarium, 2-4 times a week"

It seems to be basing the dose on Fe, but Comprehensive has a whole bunch of other elements. I have no clue, should I be adding so much just based on Fe? I also have a separate fertilizer for Fe which I could use.
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post #9 of 18 (permalink) Old 07-02-2014, 01:00 PM
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Originally Posted by poissonej View Post
A
It seems to be basing the dose on Fe, but Comprehensive has a whole bunch of other elements. I have no clue, should I be adding so much just based on Fe? I also have a separate fertilizer for Fe which I could use.
There's no harm in dosing those levels based on iron. How much other trace elements do we need? Honestly, I have no idea. I've never see a definitive range for each trace element in the planted tank.

You could always try dosing the Flourish using the manufacturers suggestion and then add enough iron to raise levels to the EI recommended levels 0.2-0.5 ppm.

Most of us use Plantex for traces and dose based off of the iron content. It seems to work quite well. Of course this method is far cheaper and cost is not an issue. I personally mix 3 parts Plantex and 1 part DTPA iron. I've never had a trace deficiency using that mix which theoretically is 25% less traces than usual. How low can I go? Never bothered. I'm not aware of anyone who has.
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post #10 of 18 (permalink) Old 07-02-2014, 04:55 PM
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Everything looks fine, theoretically, you'll not even have a gram of CaSO4 left over at room temp, so it's near negligible. Pop the solution in the fridge and you might precipitate a little more, but it's probably not worth it. I would also think you could dose Ca(NO3)2 on its own and not go through all of this trouble. Though I can never argue against a little chemistry...

Glad to see an opinion on the plantex dosing based on iron, I've actually wondered the same thing.

My real question is how do you have Ca(NO3)2 and not KNO3 on hand? Ca(NO3)2 is much more obscure...and probably more expensive.

Just had to comment on a chemistry related post.
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post #11 of 18 (permalink) Old 07-02-2014, 06:15 PM Thread Starter
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Originally Posted by Zorfox View Post
There's no harm in dosing those levels based on iron. How much other trace elements do we need? Honestly, I have no idea. I've never see a definitive range for each trace element in the planted tank.

You could always try dosing the Flourish using the manufacturers suggestion and then add enough iron to raise levels to the EI recommended levels 0.2-0.5 ppm.

Most of us use Plantex for traces and dose based off of the iron content. It seems to work quite well. Of course this method is far cheaper and cost is not an issue. I personally mix 3 parts Plantex and 1 part DTPA iron. I've never had a trace deficiency using that mix which theoretically is 25% less traces than usual. How low can I go? Never bothered. I'm not aware of anyone who has.
Ah okay. Yes the cost is one of the main things for me with dosing so much of Comprehensive. I'd get Plantex but greenleafaquariums only ships it to US destinations and from other providers the shipping cost is way too much. I think I'll try dosing a bit more than the recommended dose of Comprehensive and adding more iron separately. Thanks!

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Originally Posted by ChemGuyEthan View Post

My real question is how do you have Ca(NO3)2 and not KNO3 on hand? Ca(NO3)2 is much more obscure...and probably more expensive.
It's illegal to buy/sell it in Singapore There is one legal provider who sells 1000 mL bottles with 5% KNO3, which would last for quite a while, but his price is almost the same as an amount of Ca(NO3)2 which I think would last even longer if I dosed Ca(NO3)2. I might give the KNO3 provider a shot anyway, though I figure it's useful to know what to do otherwise just in case he's not allowed to sell it anymore some day.
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post #12 of 18 (permalink) Old 07-02-2014, 06:45 PM
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Quote:
Originally Posted by poissonej View Post
It's illegal to buy/sell it in Singapore There is one legal provider who sells 1000 mL bottles with 5% KNO3, which would last for quite a while, but his price is almost the same as an amount of Ca(NO3)2 which I think would last even longer if I dosed Ca(NO3)2. I might give the KNO3 provider a shot anyway, though I figure it's useful to know what to do otherwise just in case he's not allowed to sell it anymore some day.
Ah, I understand. That's unfortunate you have to go through this round-about method. Best of luck with your tank(s)!
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post #13 of 18 (permalink) Old 07-06-2014, 12:29 PM Thread Starter
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Originally Posted by ChemGuyEthan View Post
Ah, I understand. That's unfortunate you have to go through this round-about method. Best of luck with your tank(s)!
Thank you I decided to get KNO3 from the provider anyway just to keep things easy while KNO3 is available. But now I'm trying to figure out dosing for EI. Here's what it says on the bottle:

"1000 mL, 5% KNO3 = 50,000 ppm. (contains 19,000 ppm K & 31,000 ppm NO3 nitrate nitrogen or 7,000 ppm pure N)

Dose: Adding 10 mL of this solution to 100 L of water will give 19 ppm K and 31 ppm NO3 nitrogen"

According to the calculator Zorfox linked earlier, the aim for EI is to have 7.5 ppm of NO3 for one dose. If I play with the ratios for my tank which is around 129 litres, I get that I should dose 3 mL of the solution.

But I'm also confused because I can't figure out how that works out.

If there's 50,000 ppm of KNO3, converting to grams/L that's around 50 g/L, which is 0.05 g/mL

So if I wanted to add 1.574 g of KNO3 as suggested by the calculator above to get 7.5 ppm, I'd have to add 1.574/0.05 =~ 31 mL? Not sure if I worked something out wrong
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post #14 of 18 (permalink) Old 07-06-2014, 01:06 PM
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You're partially correct. The solution is 50g/L. However, you're calculating for NO3 which is not 50g/L. There is only 31,000 ppm of NO3 NOT 50,000ppm.

So for 129 liters of water you need to add 6ml to raise NO3 7.5 (roughly, it's actually 7.417).

oops, I selected teaspoons rather than grams lol. You are in fact correct.


Last edited by Zorfox; 07-06-2014 at 01:20 PM. Reason: Corrected error
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post #15 of 18 (permalink) Old 07-06-2014, 03:35 PM Thread Starter
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Quote:
Originally Posted by Zorfox View Post
You're partially correct. The solution is 50g/L. However, you're calculating for NO3 which is not 50g/L. There is only 31,000 ppm of NO3 NOT 50,000ppm.

So for 129 liters of water you need to add 6ml to raise NO3 7.5 (roughly, it's actually 7.417).

oops, I selected teaspoons rather than grams lol. You are in fact correct.

Ah thank you for checking it out for me! But I'm still confused about the labelling of the bottle--even if I calculate for KNO3, using the instructions on the label, I still end up with the dose being 3 mL? I worked it out like this:

1) 10 mL -- 100 L -- 50 ppm KNO3

2) 12.9 mL -- 129 L -- 50 ppm KNO3

3) 3.2 mL -- 129 L -- 12.23 ppm KNO3 (7.5 ppm of NO3 + 4.73 ppm of K)
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