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Thread: SWG and acids

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    In the Industry
    Aqua Lab Rat's Avatar
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    May 2015
    Anoka, MN

    SWG and acids

    I did some searching on here and found lots of threads that mention potential damage to SWG plates or other potential problems from sulfates.

    The Wikipedia entry on Salt Water Chlorination says that HCL is a necessary part of the system to convert the Sodium Hydroxide back into Sodium Chloride and implies that built-up sulfates from sulfuric acid are a problem because they conduct electricity without giving you the benefit of producing a sanitizer.

    This all sounds reasonable to me (I'm not a chemist) - is it essentially correct or hogwash?

    I think my basic question is whether added CL from the acid is necessary to replace CL2. I would think you'd have to have an awful lot of sulfate for it to affect the efficiency of the SWG.
    Professional product developer using & maintaining a dozen or so spas from 150 to 550 gallons, many pools from 1200 to 40,000 gallons (above-ground and in-ground), and a half-dozen test tanks.
    If it's not one thing, it's usually several others!

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    bdavis466's Avatar
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    Aug 2014
    San Clemente, CA

    Re: SWG and acids

    Sulfates cause issues other than and more serious than false salt/chloride readings from a SWG. While sulfuric acid is much less hazardous to handle, the buildup of sulfates will cause problems down the road.

    The chloride content of acid is not needed to maintain the salinity levels in a pool. The conversion of sodium chloride into sodium hypochlorite (and eventually back to NaCl after the chlorine's consumption) does not consume salt in the process. Most salt levels decline from splash/carry out and leaks - not at all from SWG chlorine production.

    Pools that have high acid demands with little water loss (no leaks, slash out, etc) will see the salt level rise due to the high chloride content of the acid. I now know from experience that every gallon of acid added to my pool adds 18 ppm of salt. My salt level went up by 1600 in one year mainly from acid and a small part from liquid chlorine.

    Pool/Spa, Pentair Equipment

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    JoyfulNoise's Avatar
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    May 2015
    Tucson, AZ

    Re: SWG and acids

    Just to offer my experience - sulfuric acid is not safer than muriatic acid (but then again, no one uses sulfuric acid in pools, they use dry sodium bisulfate). You can actually get a drop of MA on your skin and have more than enough time to walk over to a spigot and wash your hand off. Worse that'll happen is you'll get a little white spot which will go away in a day or two. On the other hand, sulfuric acid on your skin will burn you very rapidly, denature the tissue and char the flesh. Oh, and it will hurt like h-e-double-hockey-sticks...

    Now, to the question at hand, this is one of those moments where I have to say that Wikipedia leaves a lot to be desired. Sulfates in your pool water will not get converted to sulfuric acid, that's not electrochemically possible. The real detriment of sulfates on SWGs comes from the sulfates enhancing the corrosion of the transition metal catalysts that cover the titanium plates. One component of that catalyst is ruthenium. Ruthenium can become quite soluble at low pH while in the presence of chlorides and sulfates. This is why acid washing/cleaning the plates should be done as infrequently as possible and why one should NEVER use concentrated MA to clean a cell, the ruthenium will get converted to ruthenium chloride which is very soluble in water.

    16k IG PebbleTec pool, 650gal spa, spillway and waterfall, 3HP IntelliFlo VS / 1.5HP WhisperFlo, Pentair QuadDE-100 filter, IC40 SWCG, MasterTemp 400k BTU/hr NG heater, KreepyKrauly suction-side cleaner Dolphin S300i robot, EasyTouch controls, city water, K-1001, K-2006 and K-1766 test kits, Mannitol test for borates

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