I clean and service pools in East Tennessee as well as work for a Builder, I have been working around pools for 40 years in construction as well as service and maintenance.
Now besides actual fighting Phosphates in 2 to 3 pools a year that are running FC 3 to 4, TA of 100-125, CYA levels at 50 and PH at 7.0 -7.2 and one of those is a sub division pool that sees regular inspections by the health department and you come in on your weekly visit and the black algae is growing, phosphate levels are 200-250 PPB. Treat the problem and get rid of the phosphates and the algae stops growing and goes away
I left the name of the product out since I am not advertising any thing. Orthophosphates are what Phosphates turn into and is used by the Algae as food.
http://www.poolsupplyworld.com/blog/what-are-phosphates-and-how-do-they-affect-your-swimming-pool/
This explains what phosphates do to chlorine
http://blog.orendatech.com/phosphates-affect-chlorine-efficiency
Now I realize I am the new guy, I have learned stuff from your site and have it saved on Taptalk, hence the reason this came as one of the suggestions.
You are mistaken and so is that blog post (ie, not a scientific peer-reviewed paper).
Basically that Orenda Blog post is horribly misinformed on how buffer chemistry works or their chemists tried to explain it to the marketing guys but then the marketing guys changed it so they could more easily justify selling a product. The one chemical equation they actually write out is the basic chlorine dissociation equation and then the equilibrium equation between hypochlorous acid (HOCl) and hypochlorite (OCl-). They then state that phosphates interfere with the equilibrium of HOCl/OCl- by causing the reaction to shift toward hypochlorite (OCl-). Great little equation but they leave out some HUGE details. For one, there isn't enough phosphate (PO4) around to have any appreciable buffering effect as your are talking about part per billion levels is most pools (somewhere around 50-100 micro-molar concentration). At those concentrations the carbonate buffering species and cyanurate buffering species are 100X times more concentrated and much more effective at holding pool water pH to a specific value. Phosphates are simply irrelevant as a hydrogen ion consuming species and will have no practical effect on the HOCl/OCl- equilibrium. Also, that blog post gets the entire notion of buffering wrong - the pH of the solution is determined by the concentration of all alkaline species in the water, both permanent and temporary forms. The pH is what it is at any given moment and so the equilibrium concentration of HOCl/OCl- is set
no matter what chemical species are actually contributing to the buffering of the water volume. Because phosphates do not directly react with chlorine, then it's pointless to consider their role in hydrogen ion concentration changes because they simply do not matter. If phosphates reacted directly with chlorine, then there would be some appreciable change in pH in a manner that is similar to how the outgassing of CO2 causes the pH to shift. But, that is simply not the case - chlorine and phosphates do not react with one another. Finally, there are some very technical details about the ionic strength of pool water (caused mostly from the presence of salt) that shifts the pK2 value of phosphates in water to values much lower than normal pH levels found in pool water (the pK2 of phosphates can easily drop below 7.0 when salt is present in water). This drop in pK2 means that the peak buffering levels of the phosphate ions will be far away from the pH of water thus rendering the buffering capacity of phosphates to very low levels.
As for why you see a difference when phosphate removers are added, that's easy. Phosphates are algae nutrients (along with nitrates and dissolved carbon species). If you remove nutrients from the water volume, you make it harder for algae to regrow once it has been sanitized by chlorine. Black algae is more prevalent in your part of the country and will typically grow in pools where there is limited sunlight (shaded walls) and low chlorine levels. Maintenance companies and uninvolved pool owners typically do not keep appropriate FC levels in pools and so there is much more of a chance of the active chlorine levels falling well below the concentrations needed to keep algae at bay. Black algae is a very persistent nuisance to pools because it grows in spots and grows
into the pool surface with root-like structures. Black algae also develops a fairly thick hydrophobic coating on its surface making it highly resistant to chlorine exposure. Normally one needs to treat black algae with SLAM levels of chlorine and lots of vigorous brushing to ensure that chlorine makes it to the surface of the algae. Stainless steel brushes are often needed on plaster pools to help break up the algae's protective coating. Finally, after the algae is visually cleared away, the pool owner needs to increase the FC to an elevated shock level for 24 hours to ensure that all of the algae "root" is killed. Very often it is the case that pool owners with persistent black algae problems simply need to maintain higher baseline levels of FC to avoid regrowth as the chlorine slowly attacks the algae deep in the pool wall surface. Supplemental algaecides like polyquat-60 can and should be used to help hold the algae growth at bay. One can certainly use phosphate removal treatments to reduce algae nutrients and make it harder for the algae to regrow.
My suggestion to you is that if you are truly interested in learning the science and chemistry of what we teach, you do a more thorough job of reading through what is taught here. Richard Falk (aka,
chem geek) has written most of the definitive threads on pool water chemistry on this forum and other pool & spa forums as well. He has worked with and contributed to much of the current recreational pool water standards set by industry trade groups and the EPA over the last 10 years. While a lot of what he has written falls on deaf ears because it is not simple or profitable, truth is not dependent on what people want to hear, it is simply truth. I'll leave you with Richard's thread on advanced pool water chemistry and you can bookmark that and read it at your leisure -
Pool Water Chemistry