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Reporting back as promised after running the pH experiment for more than a year. During this time my test salt-water-chlorinated pool always drifted up in pH, and never equilibrated below my intervention zone between pH 8.2-8.6. Over this period, I added more than 60 litres of 9 N HCl, and no...

Pinsky & Hu (cited above) explain why one would like measurements closer to pool conditions (and I doubt that O'Brien would have disagreed). Their motivation was right, though as Wojtowicz explained their new data analysis had problems. Nowell & Hoigne (1992) conclude from convincing analysis...

Thanks again chemgeek for the interesting perspectives. I agree about Brady’s good work, and I don’t think he concluded that it provided reliable constants for the very different conditions typical of swimming pools. In response to your request about spectra, see Sancier cited above...

Hi chem geek. I think we agree that there will be about a 50% increase in buffer capacity if pKa shifts about 0.2 units towards an operating pH about 1 pH unit from the pKa. This shows in the general curve (which can be explored graphically at http://academic.pgcc.edu/~ssinex/buffer_cap.xls) and...

Thanks chem geek. The caution to think also about saturation index is important. But focussing on pH for now: Many SWG pools are run way above 3000 ppm salinity. For example, Watermaid recommend 6000 ppm NaCl plus about 200 ppm MgCl2 (and there will be additional small molar contributions from...

Thanks chem geek, that table is another useful resource. It may be worth noting here a point that you have explained elsewhere: Buffering works in both directions. Over the same pH range the capacity of the same buffer is identical against pH rise and pH fall. Those who need regularly to reduce...

Thanks again chem geek and JasonLion. Your generosity in providing advice through this site, and your approach of basing that advice on careful analysis of the underlying chemistry plus a wealth of practical experience, are both outstanding. Swimmer safety does not seem to be an issue (ocean...

Thanks JasonLion. Maybe the page could say "the range should be 7.5 to 7.8 with 7.7-7.8 being ideal". Then someone would need to offer a suitable pH kit of course: cresol red covers the required range. Extensive experience is important, but there is probably very little experience (let alone...

One implication from chem geek’s analysis that seems to deserve faster diffusion to the shallow end is the changed effect of pH on HOCl:FC ratio, in the presence of CYA. Most sites continue to emphasise a pH target around 7.5. For example, in the Pool School Water Balance for SWGs page: “Monitor...

Pool School and the FC:HOCl chart on page 1 of this topic are great at pH 7.5; but for a concrete pool my SWG manual recommends to be content with pH 7.8 (which saves a lot of HCl use fighting what seems to be nearer the pH equilibrium from effects in the SWG). Hence the charts with a pH axis...

Here is the output from chem geek’s equations in the form I use directly to maintain outdoor pool effective chlorine (HOCl) levels. For a prevailing pH (measured using phenol red or at test strip) and CYA (which is more stable and also read from a test strip), the chart shows the required FC...