Do I Need to Add Phosphate?

[GPHowell]

Quick Question:

I'm in the process of making up some fert solutions (KNO3 and annhydrous MgSO4) and was wondering, since my tapwater seems to routinely have 3-4 ppm PO4 already, do I need to add any more?

I have an HMA unit and do 30-40% water changes weekly...

Thanks

 

[Alastair]

I wouldn't have thought adding more would be a bad thing. I've never tested what's in my tap water and dose everything fully in the way of EI.

 

[dw1305]

Hi all,
I think all tap water in the UK is now treated with ortho-phosphate (PO4), this is designed to precipitate out any free heavy metal ions (to control "plumbosolvency"). "PIMS" is the technical term, even our tap water is treated, although it is about 18dKH. <http://www.edie.net/library/view_article.asp?id=1814>

The question of whether there is any free phosphate will probably depend on the calcium content of the water. If you were really worried about deficiency you could use a chelated source of P.

cheers Darrel

 

[GPHowell]

Ah, that's interesting. I knew that phosphate was added to UK tap water but I didn't know it was to precipitate out lead-salts.

I'm not sure what you mean about "free-phosphate depending on calcium content"?

In the typical pH range of tap water (6.5-7.5 say), any PO4 source (sodium phosphate, phosphoric acid etc) will be present as H2PO4- or HPO4-- and (as long as it is not part of an insoluble metal-complex) should be available.... or is that not true?

THanks

G

 

[dw1305]

Hi all,

......as long as it is not part of an insoluble metal-complex) should be available.... or is that not true?

I think it is likely to re-form insoluble calcium phosphate complexes like Tricalcium phosphate (Ca3(PO4)2) and precipitate out of solution at higher pH, but don't quote me on that as I don't know much about calcium & phosphorus interactions, and other forms of the calcium phosphate complex are soluble.

cheers Darrel

 

[GPHowell]

Thanks again; more interesting stuff.

I might do some digging and see if I can find any solubility data for Ca3(PO4)2...

g