Question for chemists: calculating K - The Planted Tank Forum
 
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post #1 of 8 (permalink) Old 10-07-2011, 06:27 PM Thread Starter
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Question for chemists: calculating K

Im doing calculator for ferts and ive done NO3 and PO4 but with K im missing the mark, comparing results with , lets say http://www.theaquatools.com/fertilization-calculator

this is my calculation:

((weight of fert in grams/weight of water)*10^6)*percetage of K in kno3 molecule= x ppm

i do that for k2so4 too , and combine results.

This are percentages of K atoms in molecules:

K% in kno3= 38,672%
K2% in k2so4=44,874%


Is that calculation and data ok?
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post #2 of 8 (permalink) Old 10-07-2011, 08:27 PM
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that appears to be accurate

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post #3 of 8 (permalink) Old 10-08-2011, 03:39 AM
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Hey majstor76.

Your calculations figure pure/distilled/RO water at perfect density? (this is 1g/mL H2O.). If so, this totally works and is accurate enough for our purposes, but it is important to understand that really you are making an approximation that you have pure water, when in reality you probably don't (we have fish and substrate and plants (organics and inorganics), we add fertilizers, and we may/may not have leeching ornaments (driftwood/rocks) and/or tap water). So, another way to do your calculation is to keep it simple:

ppm K is really mg K/L tank
We know:
Tank volume
How much K is on KNO3
How much K is in K2SO4

[(mg KNO3 * mg K / mg KNO3) + (mg K2SO4 * mg K/mg K2SO4)] / tank volume in L = K mg/L in your tank = ppm K in your tank

But again, ballpark is totally good enough. You just want an idea if what you're adding to the tank; your plants just care they get all the nutrients they need to grow. Nobody (plants, people, or fish) should care about exact values! How far off has your calculation been?

Cool site btw! I had not seen it before.
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post #4 of 8 (permalink) Old 10-08-2011, 01:04 PM Thread Starter
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Quote:
Originally Posted by wet View Post
Hey majstor76.

Your calculations figure pure/distilled/RO water at perfect density? (this is 1g/mL H2O.). If so, this totally works and is accurate enough for our purposes, but it is important to understand that really you are making an approximation that you have pure water, when in reality you probably don't (we have fish and substrate and plants (organics and inorganics), we add fertilizers, and we may/may not have leeching ornaments (driftwood/rocks) and/or tap water). So, another way to do your calculation is to keep it simple:

ppm K is really mg K/L tank
We know:
Tank volume
How much K is on KNO3
How much K is in K2SO4

[(mg KNO3 * mg K / mg KNO3) + (mg K2SO4 * mg K/mg K2SO4)] / tank volume in L = K mg/L in your tank = ppm K in your tank

But again, ballpark is totally good enough. You just want an idea if what you're adding to the tank; your plants just care they get all the nutrients they need to grow. Nobody (plants, people, or fish) should care about exact values! How far off has your calculation been?

Cool site btw! I had not seen it before.
tnx,

my results vs aquatools vs fertilator

no3 18.4 18.4 18.4

po4 2.79 2.8 2.79

k 25.06 27.4 26.21

So, no3 and po4 are spot on but K is drifting a bit so i was thinking that my math is off.
For weight of water im using 1000g per litre, i think that should be weight of distilled water we us for mixing ferts.

Ill try to use your formula to see what ill get

EDIT ok, i used your formula and got 25,06. Then i checked Chucks calc and got 25,08. So , fertilator and aquatools got it wrong Thanks for help!
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post #5 of 8 (permalink) Old 10-08-2011, 09:42 PM
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Quote:
Originally Posted by majstor76 View Post
tnx,

my results vs aquatools vs fertilator

no3 18.4 18.4 18.4

po4 2.79 2.8 2.79

k 25.06 27.4 26.21

So, no3 and po4 are spot on but K is drifting a bit so i was thinking that my math is off.
For weight of water im using 1000g per litre, i think that should be weight of distilled water we us for mixing ferts.

Ill try to use your formula to see what ill get

EDIT ok, i used your formula and got 25,06. Then i checked Chucks calc and got 25,08. So , fertilator and aquatools got it wrong Thanks for help!
My guess is you are not putting in the correct information. I get the same answer all three ways and you can check this by putting in a 1000 L tank and 1000 g KNO3. The result is the same 386.7 which is 10 x the percent of K in KNO3.

If you play with chemicals you should know some chemisty.
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post #6 of 8 (permalink) Old 10-09-2011, 05:04 AM Thread Starter
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Quote:
Originally Posted by ray-the-pilot View Post
My guess is you are not putting in the correct information. I get the same answer all three ways and you can check this by putting in a 1000 L tank and 1000 g KNO3. The result is the same 386.7 which is 10 x the percent of K in KNO3.
Im getting 386,72 too. How much do you get for 1000g of k2so4 and 1000L?
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post #7 of 8 (permalink) Old 10-09-2011, 08:35 PM
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If the other calculators are using a more accurate density for water and converting liters to grams, they would be dividing by a smaller number, giving a slightly higher result than yours. The density of water is slightly less than 1. http://en.wikipedia.org/wiki/Propert..._water_and_ice

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post #8 of 8 (permalink) Old 10-10-2011, 09:18 AM Thread Starter
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Quote:
Originally Posted by KevinC View Post
If the other calculators are using a more accurate density for water and converting liters to grams, they would be dividing by a smaller number, giving a slightly higher result than yours. The density of water is slightly less than 1. http://en.wikipedia.org/wiki/Propert..._water_and_ice

Kevin
In formula I entered weight of water of 997,0479 g/L (thats water on +25C) and got 25,14 ppm which is 0.08 ppm more than with 1000 g/L , not enough to explain the difference.
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