COMMON MYTHS in Pool-Water Chemistry, Part 2
How many of these do you come across each day at the pool?
1. The ideal pH for any pool water is 7.4.
Impossible to know. Out of context, there is no “ideal” anything. The water’s hardness and other CSI variables, even the values read from the make-up (fill) water, help you determine the most appropriate value. In general, the best pH is “the lowest pH you can get away with” as chlorine works much better at lower pH values. That actual number is, of course, clearly limited by the bottom of the state health-code ranges, and otherwise chosen by pool operators considering the influences and trends of their sanitizer and their make-up water – wisely keeping that CSI thing in balance all along. Curiously, a pH of 7.4 is not even legal in South Dakota where pH 7.6 is dictated, and almost never used in Germany (the birthplace of water-chemistry science) where their national code’s working range is 6.5 to 7.3! Typically, they hold 7.0 or 6.9.
2. Green hair in blondes is caused by too much chlorine.
Not at all. Green hair is caused by copper in the water – but not right away during a swim… Copper must precipitate as a green salt during a high-pH shampoo, usually long after the pool water has dried in the hair. Two errors, then: The operator had allowed copper (pipes, heater, impeller) to be dissolved by his excessively aggressive water. No problem yet – crystal clear water still. Error two is the swimmer’s fault; she or he didn’t shower (rinse) or even towel dry the pool-wet hair. It dried with that half cup of copper-bearing water leaving its contents behind. Then the high pH of a shampoo (pH 9 or higher), hours or days later, caused the precipitation of copper oxide (and maybe some sulfide). Everything from Aspirin to vinegar has been used to reduce the coloration after the fact, but prevention is the much better approach. The so-called swimmer’s shampoo is simply a lower pH product that doesn’t clean as well but keeps the hair below pH 8.3 where all the dissolved solids otherwise begin to fall out of solution. Everybody, by the way, gets green hair under this sequence of events; it just shows up better in bleached blondes.
3. Significant chlorine can be saved by adding a supplemental sanitizer or sanitizing device to the pool’s treatment system, such as a UV lamp or an ionizer.
Not true, since the amount of chlorine consumed by virtue of its sanitizing activity is so small it cannot be measured. Supplementing chlorine in its bug-killing job may be nice, but it is not particularly necessary or economical. UV and copper/silver ions are sanitizers only; while sometimes serving useful functions, they don’t oxidize – chlorine’s toughest job. (See item 4, below.) One chlorine complement, ozone, on the other hand, sanitizes and oxidizes. Based on the type of ozone generator, its size, and installation arrangement, little, if any chlorine is saved.
4. Oxidizing and sanitizing are about equal burdens on the chlorine residual.
Not even close. Continuing with the explanation in 3, above, oxidizing (mostly burning up suspended organic particles) consumes much more chlorine than the related task of sanitizing. Losses to sunlight and other destruction or dissipation all exceed the tiny-but-effective amount used in making the water germ free. Help the chlorine in its oxidizing task, and minimize losses to sunlight, then there’s often money to be saved.
~ kw
