Phosphates II

Gill Man

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Feb 10, 2005
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I'm seeking to understand the difference between phosphate and orthophosphate and how they exist in our freshwater aquariums. I understand that orthophosphate is inorganic phosphate, Pi, and is a tetrahedron in structure. Fine. My new La Motte Phosphate test kit states that it tests for orthophosphate ONLY. So...what do we have in our tanks when we're adding KH2PO3 or K2HPO3, potassium monobasic phosphate or potassium dibasic phosphate...orthophosphate? Or is orthophosphate derived soley from H3PO4, phosphoric acid?

I read the following at, http://www.advancedaquarist.com/issues/sept2002/chem.htm
Orthophosphate exists in various forms in seawater, depending on the pH. At pH 8.1, seawater contains 0.5% H2PO4-, 79 % HPO4--, and 20% PO4---. At higher pH the equilibrium shifts toward more PO4--- and less HPO4--.....It may also surprise some people that so much of the phosphate is present as PO4--- while in fresh water only 0.1% is present in that form at the same pH. There are a number of reasons for this difference between salt water and fresh water that involve the effects of other ions in the seawater on the phosphate (such as calcium and magnesium ion-pairs)

So if the La Motte test kit only tests for orthophosphate, what the heck is it testing for exactly and why is 0.5 ml of H3PO4 added to the 10 ml sample of aquarium water? I'm not getting it and my sample turned a light teal color instead of any of the possible shades of navy blue, so it I couldn't match it to any color, just intensity of color. I will be retesting, but I wonder if I just spent $75 on a useless test kit.

I Googled orthophosphate but couldn't get a clear definition of what it is, what it isn't, what it is derived from and how its different from that derived from potassium phosphate.

I kind of think I'm getting it just by writing about it. Orthophosphates, derived from phosphoric acid, H3PO4, appears in various forms in freshwater depending on pH, which algae are capable of using but not plants. We add potassium phosphate, which dissociates in water, and gives us free PO3---, which can be used by plants, but won't show up on my La Motte test kit. Is this right?

Any comments would be appreciated.
 

Vladimir Zhurov

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Re: Phosphates II

Orthophosphates are any ion or salt of phosphoric acid (H3PO4).

If La Motte test kit only tests for orthophosphate then it determines concentration of PO4--- ion in a sample.

There can be many reasons why hues on the chart and your samples do not match. It can be something in the tank water that reacts with colour indicator, ambient lighting, yourself as perception of colours is an individual thing.

However, 1) do not test your test kit with your tank water or (even worse) make any dosing decisions immediately; 2) make samples of known concentration of phosphate using distilled or RO water and potassium phosphate you got; 3) see what colours you will get and that will be your new and tested reference. Then return to tank water testing.

Hope this helps.

Regards.

Vladimir.
 

Gill Man

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Feb 10, 2005
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Re: Phosphates II

Yes, it does. I'll do just that; make solutions of known concentrations, perhaps even the ones that came with my Seachem test kit, and test the new La Motte test kit.

The way you defined orthophosphates, it makes sense. Testing the same sample with the Seachem test kit gave me the usual, expected results after two days of nutrient assimilation/adsorption. Not sure why I NEEDED the La Motte test kit, I guess I thought it would be a bit easier to read. The La Motte nitrate test kit is awesome, though.
 

Tom Barr

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Jan 23, 2005
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Re: Phosphates II

Perhaps a better term: SRP, soluble reactive Phosphorus.
This is the bioavailable P.


Regards,
Tom Barr