Soda: Baking versus Washing

[sks23cu]

I decided to raise my pH with Soda and found that Jason's Pool Calculator recommends using Washing Soda, which is Sodium Carbonate. But in the section for Total Alkalinity, the Pool Calculator recommends using Baking Soda, which is Sodium Bicarbonate.

I tried searching this forum for an explanation of how each affects pool chemistry, similarities and differences, but didn't find anything.

The Pool Calculator and its associated text page indicates, I think, that Baking Soda doesn't increase pH as much as Washing Soda. But both raise TA. I suspect that both also raise the salt level a little (from the Sodium).

Would someone please explain more fully why I should use one over the other and when?

Thanks.

 

[chem geek]

You summarized the situation correctly. Arm & Hammer Baking Soda, which is sodium bicarbonate, raises the Total Alkalinity (TA) but does not change the pH very much (it raises it slightly, depending on starting pH). Arm & Hammer Super Washing Soda, which is sodium carbonate, raises both the TA and the pH. Both have sodium so both will increase that part of the salt level, though technically the salt test measures chloride and not sodium. You use 20 Mule Team Borax to raise the pH with half the rise in TA and you aerate the water to raise the pH with no change in TA.

Chemically, Washing Soda produces the identical result as a combination of Baking Soda and Lye aka Caustic Soda (sodium hydroxide).

Baking Soda -- to raise the TA with little change in pH
Washing Soda -- to raise both pH and TA
Borax -- to raise mostly pH with about half the rise in TA
Lye aka Caustic Soda -- to raise mostly pH with half the rise in TA
Aeration -- to raise the pH with no change in TA

Richard

 

[sks23cu]

A clear and sparkling explanation!

Thank you Richard!

 

[no-mas]

Why would lye have any effect on TA?

 

[chem geek]

Any acid or base even if "pure" like lye (i.e. doesn't contain carbonates) will move BOTH the pH and the TA together. The reason is that lye raises the pH and this shifts more dissolved carbon dioxide (via carbonic acid) to become bicarbonate ion. TA is mostly a measure of bicarbonate ion.

So while it is correct to say that lye does not increase the total amount of carbonates in the water (in all forms, including dissolved carbon dioxide), it nevertheless increases both the pH and the TA. Borax is similar though it increases TA slightly more due to a fraction of the borax becoming borate ion that counts towards TA; most of the Borax becomes boric acid that does not count towards TA. However, at a pH of 7.5, even having 50 ppm Borates only increases the TA by 5 ppm, so for practical purposes Borax and lye are similar in their effect on TA.

 

[no-mas]

Makes sense - thanks for 'splainin'