Muriatic Acid is normally 31.45% Hydrochloric Acid and has a pH of -1 (yes, the pH scale can go negative since it's the negative logarithm of hydrogen ion concentration -- so that means a hydrogen ion concentration of 10 moles per liter since -log(10) = -1). A dilution of 10 to 1 with unbuffered water would raise the pH by 1 unit. If I assume a very slow pouring of one cup every 30 seconds (2 cups per minute) and a decent flow rate through the skimmer of 25 gallons per minute (GPM), then that is a dilution ratio of 200 to 1. If the water were unbuffered, it would dilute the acid to a pH of 1.3 (each factor of 10 adds 1 to the pH and the factor of 2 adds 0.3 to the pH). Remember that log(200) = 2.3 where "log" is to the base 10. So, -1 + 2.3 = 1.3 for the resulting pH. Or another way to look at it is that the concentration of hydrogen ion goes from 10 to 10/200 = 0.05 and the pH becomes -log(0.05) = 1.3
Pool water is buffered so will resist changes in pH, but the strong Muriatic Acid overwhelms this buffer essentially eliminating it in small volumes of water. When I run the calculations assuming a TA of 100 and a CYA of 30, it only helped by 0.2 in pH (at the most) so the resulting pH in buffered pool water is around 1.5 for the slow pouring assumptions above. That's VERY acidic (similar to strong gastric acid). So the pump, filter, gas heater, and any other inline equipment will be exposed to very acidic conditions while you are pouring the acid in the skimmer. There isn't much problem in the general body of pool water as the acid will get significantly diluted rather quickly after it comes out of the returns.
So I would never pour strong acid into the skimmer. It is true that the exposure would be relatively brief -- just a few minutes -- but the pH is very, very low. I just wouldn't risk it. It's far more harmful than anything else in the skimmer -- chlorine or even Trichlor tabs (the latter with the pump off). Pouring over a return flow in the deep end of the pool will offer the greatest dilution possible in the fastest amount of time. That minimizes exposure to low pH. Pouring it around the pool would probably also be OK except that near the edge of the pool it will be exposed to lower pH -- at least with a return flow it tends to push the acid away from the edge of the pool somewhat. Muriatic Acid, just like chlorinating liquid and bleach, is denser than water so you want to get it diluted quickly so that it doesn't settle at the bottom of the pool (I suspect that acid may mix more readily than chlorinating liquid, even though they both are about equally as dense, but I do not know that for certain).
Richard
