Help validating kh2po4 diy solution strength

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Nickalls

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Hi all,

Hoping someone here can please offer some guidance. I'm running a 200l mixed reef tank which has been phosphate deficient for some time now. I've decided to start dosing kh2po4 (monopotassium phosphate), and have mixed up a solution. Now, I'd like to validate the solution strength using a Hanna Phosphate ULR Checker. But how???

Details;

Tank size = 200l
Container size = 800ml
Added 800ml of RO to container
Added 29.35g (Rounded to 30g) of kh2po4 to container
Dose size 1ml
to yield 0.128ppm phosphate

Testing device
HI-774 Marine Phosphate ULR Checker HC colorimeter
Range: 0.00 to 0.90 ppm
Resolution: 0.01 ppm
Accuracy: ±.02 ppm ±5% of reading

Required sample size = 10ml


How can I go about adding solution to the 10ml cuvette to confirm I've mixed up the right strength?

Cheers in advance!

Nick
 
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Hi all,

Hoping someone here can please offer some guidance. I'm running a 200l mixed reef tank which has been phosphate deficient for some time now. I've decided to start dosing kh2po4 (monopotassium phosphate), and have mixed up a solution. Now, I'd like to validate the solution strength using a Hanna Phosphate ULR Checker. But how???

Details;

Tank size = 200l
Container size = 800ml
Added 800ml of RO to container
Added 29.35g (Rounded to 30g) of kh2po4 to container
Dose size 1ml
to yield 0.128ppm phosphate

Testing device
HI-774 Marine Phosphate ULR Checker HC colorimeter
Range: 0.00 to 0.90 ppm
Resolution: 0.01 ppm
Accuracy: ±.02 ppm ±5% of reading

Required sample size = 10ml


How can I go about adding solution to the 10ml cuvette to confirm I've mixed up the right strength?

Cheers in advance!

Nick
I'm a little bit confused, so I'll need some clarification.

If I did my math right, 30 g MPP will yield about 20.94 g phosphate when dissolved. Since you used 800 mL water, the concentration of phosphate is approximately 20.94/820.94 or 2.55%. What did you mean by 0.128 ppm phosphate? Did you mean that is the phosphate rise when you add 1 mL of the solution to the 200L tank?
 

AllSignsPointToFish

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The issue you will run into is that the Hanna instrument is for ultra-low concentrations. If you took 1 mL of your solution and added it to 1 L of water, the phosphate would still be around 26 ppm.

Even 0.1 mL solution diluted to 1L total volume with DI water gives a phosphate concentration of around 2.6 ppm.

How much dilution are you willing to undertake to use the Hanna instrument to check your stock solution?
 

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Another option would be to add 0.1 mL of your solution to the tank. If you had before and after phosphate readings, you could attempt to calculate the predicted rise in the phosphate levels in the tank and compare that to the difference between before and after readings.
 
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Nickalls

Nickalls

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I'm a little bit confused, so I'll need some clarification.

If I did my math right, 30 g MPP will yield about 20.94 g phosphate when dissolved. Since you used 800 mL water, the concentration of phosphate is approximately 20.94/820.94 or 2.55%. What did you mean by 0.128 ppm phosphate? Did you mean that is the phosphate rise when you add 1 mL of the solution to the 200L tank?
Exactly right :) 1ml dose to raise the level to 0.128ppm (was using https://rotalabutterfly.com/nutrient-calculator.php)

Also...Little embarrassed to ask, but what's the math to work out the above, I've been scratching my head all night :p
 

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No problem :)

1 mole of MPP is 136.086 g, so 30 g is 0.22 mole MPP (30/136.086).

When dissolved, 1 mole of MPP yields 1 mole of phosphate in solution. 1 mole of phosphate is 94.97 g.

So, 0.22*94.97 = 20.94 g of phosphate in solution when 30 g MPP is dissolved.

800mL RO water is 800 g water, so concentration of solution is 20.94 g phosphate in 820.94 g solution, or (20.94/820.94) x 100% = 2.55%. 2.55% is 25500 ppm.

However, you add 1 mL of solution at 25500 ppm to your 200L tank. 200L is 200,000 mL of seawater at 1.026 SG, so 200,000 mL x 1.026 g/mL = 205,200 g seawater.

25,500 ppm /205,200 is approximately 0.127 ppm. Note that I rounded some of these numbers, and the rounding affects the precision of the last digit in the calculation.
 
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Nickalls

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No problem :)

1 mole of MPP is 136.086 g, so 30 g is 0.22 mole MPP (30/136.086).

When dissolved, 1 mole of MPP yields 1 mole of phosphate in solution. 1 mole of phosphate is 94.97 g.

So, 0.22*94.97 = 20.94 g of phosphate in solution when 30 g MPP is dissolved.

800mL RO water is 800 g water, so concentration of solution is 20.94 g phosphate in 820.94 g solution, or (20.94/820.94) x 100% = 2.55%. 2.55% is 25500 ppm.

However, you add 1 mL of solution at 25500 ppm to your 200L tank. 200L is 200,000 mL of seawater at 1.026 SG, so 200,000 mL x 1.026 g/mL = 205,200 g seawater.

25,500 ppm /205,200 is approximately 0.127 ppm. Note that I rounded some of these numbers, and the rounding affects the precision of the last digit in the calculation.

Mind blown! Thank you so much for taking the time to write this out for me :) I'll try the 3g MPP test procedure you suggested today and see what I get. Wonder how accurate my kitchen scales are hehe.

I've had a Green Dallas Acro in the top center of my tank for 3 months, which started out beautifully green, but is now pink, sitting there like an indicator something is wrong. It's growing but just no colour. I'll use the above detail to help lock in a new DIY dosing regime. Phosphate is currently 0.01ppm after 24 hours and a 2ml dose to bring it up to 0.256ppm.
 

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Could be. Also could be a mistake in my math. I haven't found an obvious error, but that doesn't mean there isn't one.

The Hanna ultra-low phosphate checker is just so sensitive that it doesn't take much error to saturate the reading.

I'm sorry that didn't work, man.
 
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Nickalls

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Could be. Also could be a mistake in my math. I haven't found an obvious error, but that doesn't mean there isn't one.

The Hanna ultra-low phosphate checker is just so sensitive that it doesn't take much error to saturate the reading.

I'm sorry that didn't work, man.
That's all good :) pretty sure it's my scales. Waiting on some milligram ones I've ordered which should be here this week, and will try again.

Good news though, dosing 1ml of the original solution raised po4 by 0.04ppm, which was then consumed within 3 hours. It's healing the Dallas Acro, which has gone from totally pink, to light green in some spots over the past 4 days. WIN!

Hoping you might have an idea on how to raise the PH of the kh2po4 solution which is at 4.43. I've stopped dosing it for now, as it's impacting the ph of the tank too much? If I could somehow increase the ph to 8.2 would be ideal... any ideas on a diy solution?
 
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It surprises me that such a small amount affects the tank pH so drastically. I think you should be able to adjust the pH with some sodium hydroxide. I know potassium hydroxide is soluble and I believe sodium phosphate is as well (i.e., TSP). I would think you add a drop of sodium hydroxide solution to a portion of your stock solution to ensure no precipitation occurs. The change in pH could affect the solubility of the phosphate, but I kind of doubt it.
 

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That's all good :) pretty sure it's my scales. Waiting on some milligram ones I've ordered which should be here this week, and will try again.

Good news though, dosing 1ml of the original solution raised po4 by 0.04ppm, which was then consumed within 3 hours. It's healing the Dallas Acro, which has gone from totally pink, to light green in some spots over the past 4 days. WIN!

Hoping you might have an idea on how to raise the PH of the kh2po4 solution which is at 4.43. I've stopped dosing it for now, as it's impacting the ph of the tank too much? If I could somehow increase the ph to 8.2 would be ideal... any ideas on a diy solution?
it can’t be dropping the pH much if you are adding an appropriate amount. What pH do you get after adding 1 mL?
 
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