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I am trying to better understand the chemistry of maintaining my pool so that I can use the least amount of chemicals to control algae growth and I'm confused by the need to measure both total alkalinity (recommended to be between 100 and 150 ppm) and pH. If pH is high then won't alkalinity always also be high, and vice versa? Or is there a specific need or advantage in checking for both pH and TA?

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  • $\begingroup$ How is defined the total alkalinity ? $\endgroup$
    – Maurice
    Oct 19 '21 at 20:10
  • $\begingroup$ @Maurice parts per million $\endgroup$
    – kidcoder
    Oct 19 '21 at 20:14
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    $\begingroup$ See Alkalinity. Alkalinity is the strength of a buffer solution composed of weak acids and their conjugate bases. ....... Alkalinity is expressed in units of concentration, such as meq/L (milliequivalents per liter), μeq/kg (microequivalents per kilogram), or mg/L CaCO3 (milligrams per liter calcium carbonate). The latter can be well approximated as ppm. $\endgroup$
    – Poutnik
    Oct 20 '21 at 7:18
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    $\begingroup$ The same pH can belong to various alkalinity. E.g. at the same pH=8, solution of NaOH would have much lower alkalinity than solution of Na2CO3 which would have much lower alkalinity than solution of NaHCO3. $\endgroup$
    – Poutnik
    Oct 20 '21 at 7:20
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Alkalinity is typically reported in terms of either bicarbonate (the dominant carbonate species from around $\pu{pH} \simeq$ 6.5 to 8.3) or carbonate. You can have a high $\pu{pH}$ and alkalinity (total carbonate) in the range you report. Basicity is frequently used as a synonym for alkalinity, which is not correct. You are introducing caustic alkalinity into the system by (likely) adding sodium hypochlorite to get chlorine into the pool, and there's typically lye ($\ce{NaOH}$) in that solution.

$\pu{pH}$ will give you a measure of acidity and basicity, while including alkalinity will add some information about the so-called buffer capacity (ability here to neutralize acid) of the solution. It's important for a pool to have buffer capacity to maintain the proper $\pu{pH}$ range, and the alkalinity part (total alkalinity will be carbonate + caustic) is needed to make that determination.

To do this you would likely need to combine a system of polyprotic acids and salts of those acids. I'm thinking about a combination of a phosphate and carbonate buffer, the mathematics of which I can't produce off the top of my head. But you would increase buffer capacity while maintaining that $\pu{pH}$. The relevant calculation/formulation of this sort of thing in general is represented in the Henderson–Hasselbalch equation.

Finally, I note that it appears to be commonplace to use soda ash and/or sodium bicarbonate to control $\pu{pH}$ in swimming pools, so my suggestion of the additional phosphate buffer might not relevant here: as it so happens, the carbonate/bicarbonate buffer system in our bodies (which tightly regulates blood $\pu{pH}$, among other things) would result from adding the two sodium salts in the appropriate quantities for the target $\pu{pH}$ range you specify.

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    $\begingroup$ Liquid bleach solutions (sodium hypochlorite) do not have NaOH in them. Also commonly used is calcium hypochlorite in powdered form, also without NaOH. And yes, usually just plain baking soda (sodium bicarbonate) is what's added to pools to increase the alkalinity. pH is adjusted with HCl (down) or NaCO3 (up). Stabilized chlorine (dichlor/trichlor) have cyanuric acid added, so people who use stabilized pucks tend to need to adjust pH up. Plain calcium hypochlorite tends to raise pH, so the pool usually needs downward adjustment of pH when chlorinating that way. $\endgroup$
    – J...
    Oct 20 '21 at 16:09
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    $\begingroup$ Really, the best tool a pool owner can get is a good drop-count test kit (Taylor, TF-100, etc). These have adjustment tables and saturation index calculators that make this whole process much easier for non-technical pool owners. It's not just TA and pH that need to be controlled in a pool - the total hardness also has a major impact on what the safe pH range for a pool is. $\endgroup$
    – J...
    Oct 20 '21 at 16:11
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    $\begingroup$ @J... Good points. Note that commercially available $\ce{NaOCl}$ does contain trace $\ce{NaOH}$, which is actually needed to stabilize the former. See this document from Oxy, one of the larger producers of industrial chemicals world-wide. $\endgroup$
    – Todd Minehardt
    Oct 20 '21 at 17:50
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    $\begingroup$ Fair point, but in the quantities present it's almost completely negligible as far as the pool chemistry is concerned. Didn't know that, though, so hey, +1. $\endgroup$
    – J...
    Oct 20 '21 at 18:01

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