Cheapest source for muriatic acid?

[cygnusecks]

If you find yourself adding lots of muriatic acid because you have a large pH rise, you might consider significantly lowering your TA level to at least 80 if not lower (but not lower than 50).

Richard

What? I thought if you lower your Alk, your pH will be able to move much more easily, and that higher TA means pH won't move so much.

[chem geek]

What? I thought if you lower your Alk, your pH will be able to move much more easily, and that higher TA means pH won't move so much.

This is one of those counterintuitive aspects of pool water chemistry. It is true that lower TA means that an amount of acid or base will make the pH move more compared with higher TA. However a high TA (especially in combination with low pH and/or high aeration) will result in a greater outg***ing of carbon dioxide. Essentially, a pool is intentionally "over-carbonated" (sort of like a soft drink) and contains more dissolved carbon dioxide than the air, so it will always be outg***ing. However, this is a relatively slow process but the rate of it is a function of TA, pH and aeration. The outg***ing causes the pH to rise with no change in TA. If you then add acid to lower the pH, this will lower the TA as well so the combination of carbon dioxide outg***ing plus adding acid results in a reduction in TA (this is how Ben's Lowering Your Alkalinity procedure works and he accelerates the process by lowering the pH and increasing aeration).

This chart shows the relative carbon dioxide outgas rates where I have roughly color coded in red the combinations of TA and pH where most people will find noticeable pH rise. The green combinations are where most people do not find a noticeable rise in pH. The orange area is mixed and in any event the amount of outg***ing is a function of aeration which is not easily calculable. I hypothesize that Salt Water Chlorine Generator (SWCG or SWG) pools have an increase in pH due to higher aeration from the hydrogen bubbles that are produced, but this has not been proven.

The lowering of TA will cut down the outg***ing of carbon dioxide and reduce the pH rise in pools that have a lot of such outg***ing, but it won't help and will actually make worse (as you pointed out) a pH rise that is due to some other external source. If you notice a reduction in TA over an extended period of time after you add acid to restore pH, then outg***ing of carbon dioxide is at least part of the source of pH rise, but this change in TA is very slow so has to be measured over weeks or a couple of months to be measurable.

The simplest way to reduce or virtually eliminate carbon dioxide outg***ing is to use a pool cover. For people with SWCG systems, another option is to add 50 ppm of Borates (Borax) to the pool (see this thread for more info) since this appears to lower the chlorine consumption and therefore generation with the result that there is less of a rise in pH (partly from the increased pH buffering from the borates) and acid demand (from the lower chlorine generation and therefore lower aeration).

Richard