# Please - help with maths and KH2PO4 doses!!



## Tom (3 Mar 2011)

Hi,
My maths is terrible and I tend to go blank when thinking about numbers. I'm trying to work out how much KH2PO4 to dose daily in my Mini M (approx 25l gross). Daily doses will be too small to use spoon measures, so I'd like to make a mix in a 250ml TPN bottle. Please could someone direct me as to how much to use per 250ml? Ideally I'd like to dose 1ml daily, to match the others I'm dosing. 

I'm using 1ml Spezial N so adding around 2ppm N daily and 0.4ppm K on top of what's in the Amazonia. I'm also dosing Brighty K at 1ml daily, whatever that adds up to. Just need to mix my P solution now. 

Sorry, and thanks in advance to any mathematicians!!

Tom


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## Tom (3 Mar 2011)

I'm going to have an educated guess at 16.4 grams per 250ml would let me dose at 1ml per 25l 3x weekly and get something around 1-3ppm. I'm doing daily 50% water changes, so if that's reasonable should I just dose that much daily? This gives me a headache and is probably why I went all ADA to start with!!

The weights were based on the weight of sugar I got from google which I thought would be similar!


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## ceg4048 (3 Mar 2011)

Hey Tom,
                If you're getting headaches, why not just use JamesC's Dosing Calculator 

All you need is to add 4 grams to your 250ml bottle to get a daily dosing of 0.45ppm for a 1 ml dose.

Cheers,


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## Tom (3 Mar 2011)

Clive, thank you! I've just discovered the calculator a minute ago, I didn't know it was there. And 0.45ppm will be enough? Thanks again, I'm such a numpty....


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## Drouthie (3 Mar 2011)

I worked out 6.25g for 1ppm and 18.75 for 3ppm.

My working went as follows:

Molecular weight of KH2PO4 = 136.08, MW of PO4 is 95
therefore the free phospahte would be 136.09/95 = 1.432
Therefore 1.432 in 1L would result in 1000ppm PO4 stock solution

However you need a different stock that would allow 1ml in 25000ml (25L) to result in 1ppm

If you made a 25000 ppm stock you could dilute it 1ml in 25000ml to give 1ppm
stock you want/ 1000ppm stock has a ration of 25  ( 25000/1000 =25)
so 1.432x25=35g in 1L
but you only want 250ml (1000ml/250ml=4) 35/4= 6.25

I've just seen Ceg's post and I *cautiously think* that James' calculator doesn't take into account free phosphate but it doesn't really matter for the amount you are out on this scale.

I can't believe I just sat and worked this out....


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## ceg4048 (3 Mar 2011)

Tom said:
			
		

> Clive, thank you! I've just discovered the calculator a minute ago, I didn't know it was there. And 0.45ppm will be enough? Thanks again, I'm such a numpty....



Hi mate,
              Well, numpties are people too..  
If you're doing this 3X per week then it would be on the low end of the PO4 requirements. We don't really know how much PO4 is in the tap, so it's difficult to say. If these are not fast growers then you can easily be OK. I typically dose about 10ppm PO4 weekly so there really isn't a "right" amount. More PO4 give you more growth. It's as simple as that.

Just start there and play with the dosing to see what you get. The tank will tell you if you need more. If you see GSA then this will tell you that you might need more.

Cheers,


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## Tom (3 Mar 2011)

Thanks again Clive and Drouthie  I'll watch it closely


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## Drouthie (3 Mar 2011)

I think for these things a general ball park is all you need. There are too many unknowns (ie water PO4 concentration) and inaccuracies (scales/ actual tank volume) to get too worried about it. It took me ages to relax about that as my work everything has to be so accurate! I nearly had a heart attack when Ceg mentioned using measuring spoons and not to worry about scales.


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## dw1305 (3 Mar 2011)

Hi all,


> Molecular weight of KH2PO4 = 136.08, MW of PO4 is 95 therefore the free phospahte would be 136.09/95 = 1.432


 Yes that is the right idea:
1.43g of KH2PO4 = 1g of PO4 (95/135 ~ 70%).
mg/l = ppm. 
1g PO4 is 1000mg, 1000mg in 1litre of H2O = 1000ppm PO4 stock sol.


> We don't really know how much PO4 is in the tap


 We don't but it is going to be a lot more than 0.45 ppm, as far as I know all tap water in the UK is now dosed with orthophosphate (PO4) ("for control of plumbosolvency") these combine with any lead (Pb) in the tap water and precipitates the lead out of solution as insoluble, pH stable, lead phosphate compounds. If you want the technical term it is - PIMS -"phosphate induced metal stabilisation". 

Wessex Water add phosphate to our water even though it is straight out of a limestone aquifer and almost infinitely buffered, softer water they buffer up to increase the pH. I've tested some water from the NW of England (on the AAS), and it had so much phosphate in it we had to re-calibrate with a new set of standards even after we had diluted it x1000, so I would imagine you don't need to add any.

cheers Darrel


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