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Calculating the loss of pool water

September 5, 2016 | A version of this story appeared in Volume 94, Issue 35

In the otherwise interesting and informative article “What Lies Beneath” about the chemistry of swimming pools (C&EN, Aug. 1, page 28), it is stated that “about 1% of the water [in a pool] is lost daily. … Most of the water in a pool turns over after about 100 days.” While this phrasing is vague enough, it seems to reflect the erroneous thinking that the amount of water turned over is a linear function of time. This is incorrect—the turnover follows first-order kinetics, and thus, in 100 days, approximately 100/e, or about 37%, of the original water will remain.

The importance of not making this kind of error has historical relevance: During World War II, Lord Cherwell, an adviser to Winston Churchill, overestimated the amount of damage associated with a planned bombing campaign just by erroneously assuming that the damage would be proportional to the number of bombs dropped.

Mihaly Mezei
New York City

CORRECTION: On Sept. 8, 2016, this letter was updated to reflect that Lord Cherwell served as an adviser to Winston Churchill during World War II.


July 18, page 7: The news story about a mutant enzyme that produces novel triterpenes showed the wrong structure for the pentacyclic triterpene. Here is the correct structure.

Aug. 1, page 2: The chemical safety letter about peroxide formation should have referred to “2-propanol,” not “isopropanol,” which incorrectly combines two different alcohol naming conventions.

Aug. 15/22, page 49: The Talented 12 profile about University of California, Berkeley, chemist Ke Xu incorrectly stated that a technique he developed could distinguish between components in a cell that are less than 10 nm apart. It can distinguish between components that are 10 nm apart or more. The profile also incorrectly stated that, in the past, researchers had to use different cell samples in order to use superresolution fluorescence imaging and electron microscopy. Researchers have used both techniques on the same sample, but the sample had to go through a difficult, error-prone dehydration process.



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