Why measure both alkalinity and pH in pools if pH alone tells us how acidic or basic something is?

Question

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?

Answer 1

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.