spa phosphate buffers & removers

[Aqua Lab Rat]

I've been reading about phosphate removers, orthophosphates, HEDP metal sequestrants, and have found some good posts on TFP. One question I have, though, is whether the form of phosphates found in a spa pH buffer would be removed by a phosphate remover such as lanthanum. I figured there would be a previous post about it but haven't stumbled across it.

I'm just doing a little self-education exercise. It never hurts to learn more, unless you learn that you aren't just paranoid, they really ARE out to get you.

 

[JoyfulNoise]

What product or chemical are you using for a "phosphate buffer"? Are you talking about something like phosphate-buffered saline which uses a mixture of sodium hydrogen phosphate and potassium dihydrogen phosphate (KH2PO4/Na2HPO4)?

Note that phosphate buffering is incompatible with the calcium carbonate saturation found in pool & spa water because calcium phosphate is highly insoluble and will precipitate out almost immediately when phosphate buffers are added. This would drive the CSI to extremely large, negative values and it is the reason why phosphate buffers are never used in plaster pools or spas. I suppose one could use them in a vinyl or fiberglass hot tub but then you would not be able to hold any calcium hardness in the water (leading to possible foaming and or enhancing the likelihood of corrosion in the heat exchanger). I don't see using phosphate buffers as a practical or cheap alternative to what is normally found in all natural water supplies - carbonate buffers!

As for the reaction with lanthanum chloride, I see no reason why the lanthanum would not react with the phosphates found in solution (since they would be at very high concentrations) and lower them by precipitation of lanthanum phosphate? In fact, many divalent metals such as magnesium, zinc and iron would also cause the phosphate to precipitate out, lanthanum would just be a really expensive way to do it. One could also easily precipitate out some phosphate using an aluminum based floc such as hydrated aluminum sulfate or polyaluminum chloride.

Have you ever run across a spa using phosphate buffers?

 

[Aqua Lab Rat]

I haven't seen a spa using phosphate buffer, and I don't plan to use one except maybe as part of an experiment. I came across this http://www.spadepot.com/Control-the-pH-P664C634.aspx and I believe there are others out there. Just trying to learn.

 

[JoyfulNoise]

I haven't seen a spa using phosphate buffer, and I don't plan to use one except maybe as part of an experiment. I came across this http://www.spadepot.com/Control-the-pH-P664C634.aspx and I believe there are others out there. Just trying to learn.

Sounds good. Let us know how it goes. As long as you can fill a spa with softened water, the product should work well enough. The pKa for that particular phosphate system is roughly 7.4 (not accounting for ionic strength differences) so it should provide strong buffering to pH drift.