Potassium mix

[hawkinsrgk]

Since I don't have a chemistry background I wanted to see if this is correct or if I am going about this the wrong way. This is the very first time I have tried to calculate something like this. I generally just ask the question, get the answer and go from there.

I have been using Potassium Chloride to dose my tank whenever it gets below 400ppm
After Jim's response to the thread about the DIY Real 2 part on using Sodium Sulfate, wanted to see if I could incorporate this idea with Potassium.

Potassium Chloride is 52.445% Potassium and 47.555% Chloride

Typical seawater has 18.98 mg/L Chloride and 2.649mg/L Sulfate which comes to a ratio of 7.16 to 1

If I added 275g of KCl to 1L of water that would give me (275)(.52445) = 144.2g of K and
(275)(.47555) = 130.776g of Chloride

So with 130.7g of Chloride being added we would need 130.7/7.16 = 18.26g of Sulfate.

Sodium Sulfate is 67.629% Sulfate so I would need 18.26/.67629 = 27g of Sodium Sulfate

Therefore to make a balanced mix of 144g/L solution of Potassium I would need to add 275g of Potassium Chloride and 27g of Sodium Sulfate to 1L of water.

Thanks
Randy

 

[Randy Holmes-Farley Verified Community Expert]

Always good to try it yourself, and the calculation looks good!

I would also add that it won't matter much if you dose only potassium chloride or the mix you suggest because there is so little being added relative to the huge amount of chloride and sulfate already present.

But there's no reason to not be perfect if you can.

 

[hawkinsrgk]

Always good to try it yourself, and the calculation looks good!

I would also add that it won't matter much if you dose only potassium chloride or the mix you suggest because there is so little being added relative to the huge amount of chloride and sulfate already present.

But there's no reason to not be perfect if you can.

Thank you and really appreciate the confirmation. I didn't think it would matter too much since I have only dosed 200g of KCl over the last two years. Just wanted to be sure I was looking at the calculations the correct way.

 

[JimWelsh]

Another approach (that I've used in the past) is to simply use a balanced ratio of potassium chloride to potassium sulfate. Calculating this using your approach, potassium sulfate is 55.13% sulfate, so you would use 18.26/0.5513 = 33.12 g of potassium sulfate, which would add 40.7 g of potassium, increasing the total amount of potassium in the solution to 184.9 g/L.

 

[hawkinsrgk]

Thank you Jim. Would it be a better solution to use potassium sulfate over sodium sulfate?

 

[JimWelsh]

Typical seawater has 18.98 mg/L Chloride and 2.649mg/L Sulfate which comes to a ratio of 7.16 to 1

I do want to take issue with your values. Typical NSW at S=35 has 19.35 mg/kg Cl-- and 2.712 mg/kg SO4--. When multiplied by the density of seawater at 20C (1.024763), these become 19.83 mg/L and 2.779 mg/L respectively. It looks like your numbers may have been arrived at by dividing by the density rather than multiplying by the density. The resulting ratio remains the same, of course.

 

[JimWelsh]

Thank you Jim. Would it be a better solution to use potassium sulfate over sodium sulfate?

Not "better", just different. One might call it "better" because it has more of the element of interest.

 

[hawkinsrgk]

Got it. Thanks again.

 

[hawkinsrgk]

I do want to take issue with your values. Typical NSW at S=35 has 19.35 mg/kg Cl-- and 2.712 mg/kg SO4--. When multiplied by the density of seawater at 20C (1.024763), these become 19.83 mg/L and 2.779 mg/L respectively. It looks like your numbers may have been arrived at by dividing by the density rather than multiplying by the density. The resulting ratio remains the same, of course.

I'm glad you brought that up. I got the information by going to http://www.lenntech.com/composition-seawater.htm
where they posted this.

 

[hawkinsrgk]

I redid the calculations based on Jim's corrections. The ratio is 7.13 instead of 7.16. The amount of sulfate needed would be 18.33 instead of 18.26. So we would need 27.1g instead of 27g of sodium sulfate. Hopefully I did that correctly

 

[JimWelsh]

In terms of what I can link to online, I feel that a more authoritative source than the Lenntech site is Pilson's Table 4-1 on the first page of this PDF: http://www.ocean.washington.edu/courses/oc421/Lecture_Notes/CHPT4.pdf. Please note the bit about mmol/kg vs. mmol/L, and the footnote about multiplying by density (1.024763 @ 20C, and 1.023343 @ 25C) to arrive at mmol/L. FWIW, the full Pilson text can be found here.

 

[hawkinsrgk]

In terms of what I can link to online, I feel that a more authoritative source than the Lenntech site is Pilson's Table 4-1 on the first page of this PDF: http://www.ocean.washington.edu/courses/oc421/Lecture_Notes/CHPT4.pdf. Please note the bit about mmol/kg vs. mmol/L, and the footnote about multiplying by density (1.024763 @ 20C, and 1.023343 @ 25C) to arrive at mmol/L. FWIW, the full Pilson text can be found here.

Thank you Jim for supplying that. Looks like good reading.