Can Polyquat raise pH???

[chem geek]

It's not the Polyquat, but rather the chlorine you added. When one adds a hypochlorite source of chlorine to the water, the pH rises. When the chlorine is used/consumed, it falls back down, but since you are adding it to shock the pool AND you are maintaining the high level, the pH stays high. Specifically, if I use your starting numbers of pH 7.6 and TA 70 and I assume you have CYA in the water (I assumed 50 ppm) and I assume you went from an FC of 5 ppm to 13 ppm, then the pH would rise to 8.2 from the addition of chlorine.

Also note that the pH test isn't accurate at high chlorine levels, but I would assume that your reading is in fact correct -- the pH is probably around 8.2 as predicted.

Ironically, having CYA in the water exacerbates this effect. CYA is an active chlorine (hypochlorous acid) buffer so as the pH changes it resists changes to active chlorine concentration. But this makes a large change in chlorine concentration result in a larger swing in pH. Basically, most of the hypochlorite you add combines with CYA and in the process this raises the pH (mostly OCl- + H2CY- ---> HClCY- + OH-). If there were no CYA in the water at all, then the pH would have risen only to 7.8 (less than half OCl- + H2O --> HOCl + OH-). In your case, the lower TA doesn't help either and I assume you have no borates in the pool.

This is why I usually recommend for people to lower the pH first before shocking. The main reason for this is to prevent metal staining that can occur from higher pH (scaling can occur as well, but only if the water is already fairly saturated with calcium carbonate -- that is, the saturation index is already >= 0). Otherwise, as Ben wrote, there isn't usually a problem from higher pH.

If you had initially lowered the pH to 7.4 before you started shocking, then the pH would have risen only to 7.8-7.9. If you had a TA of 100 instead of 70, the pH would have risen to 8.0 - 8.1 instead. If you had 50 ppm Borates in the pool, then the pH would have risen to only 7.8 (yet another great reason for having borates). For the masochists among you, you can calculate these things using my Pool Equations spreadsheet.