Calculating PPM

[Munchy2007]

I've been trying to work out how much of a solution I need to add to my (roughly 200 litre) aquarium to raise the Nitrate concentration by 2 ppm. I tried to work it out manually, but it's been a long time since my last chemistry lesson and I wasn't confident of my methodology, so I asked ChatGPT to give me an answer.

I tried multiple times and got stupidly varied answers of having to dose from as much as 1616 Litres to as little as 16 microlitres of the ferts solution!

From my own workings out, I felt that the following exchange resulted in the most accurate answer.
Me

If I add 50 grams of Potassium Nitrate to 2000ml of distilled water what will be the concentration of nitrate in the solution expressed as parts per million, using the molar mass of Potassium Nitrate to calculate this.

ChatGPT

To calculate the concentration of nitrate in the solution in parts per million (ppm), we need to first calculate the number of moles of potassium nitrate in the solution, and then use this to determine the concentration of nitrate ions.

The molar mass of potassium nitrate is 101.1032 g/mol (39.0983 g/mol for potassium and 14.0067 g/mol for nitrogen, plus 3 x 16.00 g/mol for oxygen).

To calculate the number of moles of potassium nitrate in 50 grams, we divide the mass by the molar mass:

moles of KNO3 = 50 g / 101.1032 g/mol = 0.494 moles

Next, we need to calculate the concentration of nitrate ions. Since each molecule of potassium nitrate contains one nitrate ion, the concentration of nitrate ions is equal to the concentration of potassium nitrate.

The volume of the solution is 2000 mL, or 2 L.

Concentration of KNO3 = moles of KNO3 / volume of solution = 0.494 moles / 2 L = 0.247 M

To convert the concentration to ppm, we need to multiply by the conversion factor 10^6 ppm / 1 M:

Concentration of KNO3 in ppm = 0.247 M x 10^6 ppm / 1 M = 247000 ppm

Therefore, the concentration of nitrate in the solution expressed as parts per million (ppm) is 247000 ppm.

Me

How much of the above solution would I need to add to 200 litres of distilled water to give it a nitrate concentration of 2ppm?

ChatGPT

To calculate the amount of the potassium nitrate solution needed to add to 200 liters of distilled water to achieve a nitrate concentration of 2 ppm, we can use the following formula:

Volume of KNO3 solution needed = (target concentration * volume of water) / concentration of KNO3 solution

Plugging in the values:

Volume of KNO3 solution needed = (2 ppm * 200 L) / 247000 ppm = 0.00162 L or 1.62 mL

Therefore, you would need to add 1.62 mL of the potassium nitrate solution to 200 liters of distilled water to achieve a nitrate concentration of 2 ppm.

Would anyone with a a better understanding of this care to weigh in and confirm or correct this for me?

Thanks

[SPACKlick]

I don't think you need to use mols as ppm usually just refers to mg/L And I disagree by a factor of 10.

50g / 2L = 25,000mg/L So the high strength solution = 25,000ppm. To add 2ppm to 200L you need to add 2*200 = 400mg. 400mg /25,000mg/L = 0.016L (which is 16ml)

[Munchy2007]

Thanks SPACKlick, I was beginning to think 1.6ml was a bit low comparing it to the standard EI dosing instructions, which aims to add around 20 ppm per week as I recall, so 16ml makes much more sense.

[Munchy2007]

I don't think you need to use mols as ppm usually just refers to mg/L And I disagree by a factor of 10.

50g / 2L = 25,000mg/L So the high strength solution = 25,000ppm. To add 2ppm to 200L you need to add 2*200 = 400mg. 400mg /25,000mg/L = 0.016L (which is 16ml)

My limited understanding of this leads me to believe that it would also raise the Potassium concentration by the same amount, does that sound right?

[SPACKlick]

Really sorry Munchy, I've added 2ppm KNO3 not 2ppm NO3. That's what I get for chemisting late at night.

KNO3 is 61.3% NO3 by mass 62.0049 / 101.1032

So we need to divide 16ml by 61.3% which is 26.1ml (I double checked myself with This Calculator which agrees with me.)

Because Potassium is only 38.67% of the mass it's only 2 * 38.67/61.33 = 1.26ppm

[Munchy2007]

Thanks SPACKlick, that makes sense. The calculator link is very handy, that will save me having to give my brain any more of a workout