Does adding acid to a pool lower its hardness?

[NotYou]

Hi All,

If I add Hydrochloric Acid to a pool, will the calcium hardness be lowered? If so, by how much - is there a formula I can use to calculate the final value?

Thanks in advance.

[chenbeier]

No, because hardness is given by the amount of Calcium. Acid will not change it. You need a softener device. Its an ion exchanger which exchange Ca to Na. Or reversed osmosis. You could also add sodiumsulfate or phosphate and filter the water from the precipitate.

Edited April 25, 2021 by chenbeier

[exchemist]

9 hours ago, NotYou said:

Hi All,

If I add Hydrochloric Acid to a pool, will the calcium hardness be lowered? If so, by how much - is there a formula I can use to calculate the final value?

Thanks in advance.

I don't think it will. "Hardness", so far as I understand it, relates to the concentration of Ca²⁺ and Mg²⁺ ions in the water. This will not be affected by adding HCl. What you may do is reduce the amount of carbonate and bicarbonate in the water by lowering the pH and causing some of these to be converted to CO2, which is lost to the atmosphere. But you are introducing chloride Cl⁻ ions instead, so you are effectively replacing dissolved CaCO3 and Ca(HCO3)2 by CaCl2 - which I believe still counts as "hardness" according to most definitions.

However, reducing the amount of carbonates and bicarbonates will reduce the tendency of these to precipitate out as scale deposits, so it may look like a reduction in hardness in practice.

As least, I think that is how it works. Others more knowledgeable may correct me.