r/ChemicalEngineering • u/eesemi77 • May 26 '25

Chemistry Question about the Chemistry of Swimming Pool "Total Alaklinity"

I don't understand the swimming pool maintenance concept of "Total Alaklinity"

From my High School Chemistry: If I mix Calicum Hydroxide and HydroChloric acid together in a swimming pool then I would expect any excess Hydroxide ions to combine with any available H+ ions to form water. The end result should be CaCl + H2O

I would expect the reaction to happen almost immediately, yet Pool maintenance talks about Total Alaklinity acting as a ph buffer to reduce swings in the water ph over time. To my thinking, the ph of the pool water will be determined by the residual ions either OH- or H+. there's no magical "ph Buffer" that stores this "Alaklinity" without itself changing the ph.

What don't I understand about this reaction?

Edit: Background a recent change in the Pool maintenance company has seen my chemical use more than double (before just HCL) now HCL plus "Alaklinity buffer". Result, I use almost 3 times as much acid as I used to.

Edit2: if anyone else is struggling this is the most useful site I found

https://blog.orendatech.com/total-alkalinity-role-water-chemistry

As others commented it's all about the Carbonic Acid > Bicarbonate + H+ reaction

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u/Altruistic_Web3924 May 26 '25

The chemical availability of the chlorine to kill the microbes in your pool is heavily dependent on pH. To help keep your pH in the 8-10 range a buffer is added to keep your pool alkaline, otherwise you will struggle to keep your pH consistent. Too low and you’ll have HCl acid in your pool, too high and you’ll get chemical burns.

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u/WorkinSlave May 26 '25

Im a bit confused by this response. pH is set to the 8-10 range for increased buffer. That makes sense if you see a pH / alk curve.

Chlorine loses efficacy as pH increases though. Not sure what you are trying to say.

Im genuinely curious.

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u/TeddyPSmith May 26 '25

“Chlorine” is more stable at higher pH. At a lower pH, the Cl prefers to be diatomic Cl2 which would prefer to be a gas. At higher pH, it prefers to be a hypochlorite ion. Still effective at oxidation but stable in liquid. Household bleach is high pH