Sulfates - what's the problem?

Melt In The Sun

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Oct 29, 2009
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Tucson, AZ
Generally, the use of "dry acid" is not recommended on this site because it adds sulfates to your water. What's bad about sulfates :?: Since TDS is well-established as a non-issue, I can think of two possible things:

1. "montezuma's revenge" - politely called "gastric distress", is usually pathogen-related but can be caused by drinking water with several hundred ppm sulfate, if your body is not used to it. High sulfate will also make the water taste bad (not sure about the levels required for this).

2. "gypsum scaling" - I don't know if this is even possible. It would involve CaSO4 * 2H2O (gypsum) forming due to high CH and sulfates. Technically possible I guess, but gypsum is about 15x more soluble in water than calcite (normal calcium scale).

Is there something obvious I'm missing? Seems like the water taste would be the most serious of these.
 

JasonLion

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May 7, 2007
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High sulfate levels can damage the SWG cell plates. Unfortunately, I don't know how high the level needs to be before there is a problem. It also seems reasonable to assume that different brands may have different levels of sensitivity.

Sulfates have also been implicated in plaster damage, though again the exact levels where problems start is unknown. Sulfate damage to concrete at high sulfate levels is well documented, but relatively little information is available about the effects of lower level exposure on plaster.
 

chem geek

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Mar 28, 2007
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The concrete/stone damage comes from the fact that magnesium sulfate (and to some degree, sodium sulfate) has extremely high recrystallization pressure, mostly due to the much higher supersaturation achievable compared to regular sodium chloride salt. See this paper for more detailed info (unfortunately, you have to buy it).

As for calcium sulfate "scaling", it takes very high CH and sulfate levels for this to occur. At 500 ppm CH, it would take about 2000 ppm sulfate for scaling to even theoretically begin. At 1000 ppm CH, it would take around 900 ppm sulfate.

In an non-SWG pool, we're just being conservative. In an SWG pool, the SWG manufacturers describe problems with sulfate, but unfortunately they don't give a specific level at which problems occur or at what rate they occur.