Over in this topic:
This disagrees with our standard practice of not mentioning anything about PH beyond keep it between 7.2 (7.5) and 7.8. So the question is: In which, if any, situations is it important to think about PH before shocking?chem geek said:If one is going to shock at high levels using a hypochlorite source of chlorine (bleach, chlorinating liquid, Cal-Hypo, lithium hypochlorite), then one should lower the pH first before doing so. Adding chlorine raises the pH. Though it will come back down when the FC level drops, you don't want the pH to get too high because it can precipitate metals that can cause staining, can cause cloudiness from over-saturation of calcium carbonate, and at much higher pH the active chlorine level goes down significantly.
The following assumes a TA of 80 and CYA of 80 and adding the amount of chlorine indicated at different starting pH. I indicate in red the starting pH that would be reasonable to keep the pH reasonably low.
10 ppm FC: 7.5 to 8.00 (active chlorine cut by 13%); 7.3 to 7.60; 7.2 to 7.43; 7.1 to 7.27; 7.0 to 7.13
15 ppm FC: 7.5 to 8.37 (active chlorine cut by 19%); 7.3 to 7.86; 7.2 to 7.61; 7.1 to 7.40; 7.0 to 7.22
20 ppm FC: 7.5 to 8.63 (active chlorine cut by 25%); 7.3 to 8.21; 7.2 to 7.88; 7.1 to 7.58; 7.0 to 7.34
25 ppm FC; 7.5 to 8.80 (active chlorine cut by 32%); 7.3 to 8.50; 7.2 to 8.23; 7.1 to 7.85; 7.0 to 7.51
30 ppm FC; 7.5 to 8.91 (active chlorine cut by 38%); 7.3 to 8.68; 7.2 to 8.49; 7.1 to 8.18; 7.0 to 7.75
35 ppm FC; 7.5 to 9.00 (active chlorine cut by 45%); 7.3 to 8.81; 7.2 to 8.66; 7.1 to 8.44; 7.0 to 8.06; 6.9 to 7.59
If you have 50 ppm Borates in the water, then you don't have to lower the initial pH by as much:
10 ppm FC: 7.5 to 7.71 (active chlorine cut by 7%); 7.3 to 7.49; 7.2 to 7.37; 7.1 to 7.25; 7.0 to 7.12
15 ppm FC: 7.5 to 7.81 (active chlorine cut by 9%); 7.3 to 7.60; 7.2 to 7.47; 7.1 to 7.34; 7.0 to 7.20
20 ppm FC: 7.5 to 7.91 (active chlorine cut by 11%); 7.3 to 7.71; 7.2 to 7.58; 7.1 to 7.44; 7.0 to 7.28
25 ppm FC; 7.5 to 7.99 (active chlorine cut by 12%); 7.3 to 7.81; 7.2 to 7.69; 7.1 to 7.55; 7.0 to 7.38
30 ppm FC; 7.5 to 8.06 (active chlorine cut by 13%); 7.3 to 7.91; 7.2 to 7.80; 7.1 to 7.66; 7.0 to 7.50
35 ppm FC; 7.5 to 8.13 (active chlorine cut by 15%); 7.3 to 7.99; 7.2 to 7.89; 7.1 to 7.77; 7.0 to 7.61; 6.9 to 7.42
The bottom line is that when shocking by adding more than 10 ppm FC, one should lower the pH to at least 7.2 first. If shocking by adding more than 20 ppm FC, lower the pH to 7.0 first. You need to lower the pH before you add the chlorine so that you will be able to measure the pH accurately since the pH test will not be reliable when the FC is much above 10 ppm.