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Topic: Anyone know pool chemistry?  (Read 2120 times)

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Offline Aggievet98

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Anyone know pool chemistry?
« on: July 16, 2013, 05:58:48 PM »
I am an engineer and studied enough chemistry to suspect what I am being told by a pool equipment manufacturer is wrong.  We have a saltwater pool that uses a titanium plate chlorine generator to produce free chlorine.  We bought a new unit from a new manufacturer 4 months ago.  When the unit is on, it does produce a cloud but our pool has no chlorine in it.  The manufacturer's technicians keep telling us that the problem is not their unit but our pool chemistry.  They maintain that even very low levels of phosphates and nitrates in our pool will 'consume' all the free chlorine or if our pH is off by even a little, the chlorine will somehow be all gone.  So we installed a valve about 1 foot downstream of the titanium cell.  Pulling a sample directly from that source still gives a chlorine level of 0 ppm.  They claim the nitrates and phosphates (even at less than 100 ppm for phos and less than 10 for nitrates) can instantly cause all the chlorine to be consumed or somehow inactivated to not show up on a test strip.  This doesn't seem plausible to me but I am not a chemist and do not want to assume anything.  Does anyone know enough about pool chemistry to answer this question?  I have tried pool forums but no one really understands the chemistry and is just using pool company talking points.

Offline magician4

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Re: Anyone know pool chemistry?
« Reply #1 on: July 16, 2013, 10:57:24 PM »
before we go into details: what would the pH of your "untreated" water be ?  (as this is the most important factor for "chlorine"-solubility)

if we're talking natural seawater, it should be around 8,0 , i.e. a littel on the alkaline side

this way, chlorine should have a chance to dissolve in water*) , whereas with a pH at 7 or even lower, indeed you'd run into some problems.

the talk about phosphates and nitrate being the problem sounds like complete rubbish to me, if they're really meaning it (i.e HPO42-/ H2PO4- ; NO3- ) : these ions wouldn't consume elementary chlorine (Cl2) under +/- neutral aqueous conditions in any meaningful way.

on the other hand, sometimes "phosphate, nitrate" are just shorthand for "sum total of organophosphates and organic nitrogen compounds" - meaning (pls. excuse me: its just for clarification purposes, no offence intended) that someone took a leak in the pool, for example, and now the usual suspects like urea would be in the water.

these in fact would be attacked by the chlorine (this is the whole point in adding it to the water: that it attacks organic material, bacteria and thatlike in a New York minute) and would consume it.

but then again, one foot downstream of the generator...
in my opinion, it would take the situation at a sewage treatment plant with your water to bring chlorine to zero in such brief a contact time, and I am convinced that the water in your pool isn't anything like that

for the time being, my bets hence are on the pH being low, and that's an easy problem to address, should it be so

regards

Ingo


*)
the solubility of chlorine - gas  in neutral (or even worse: acrid) water is very poor.
with (even slightly) alkaline water, chlorine will dissolve forming chloride ( Cl-) and hypochlorite (OCl-) as products, with hypochlorite being the active species for your purposes
Cl2 + 2 OH-  :rarrow: Cl- + OCl- + H2O
thereby improving the (indirect) solubility of the chlorine by several magnitudes
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