Borate Chemistry

[chem geek]

I just found out about an excellent article on borate chemistry in the September 2010 issue of Pool and Spa News here. The article talks about borate sequestering ability, but this is very weak so can pretty much be neglected as far as the saturation index is concerned where 50 ppm Borates would lower the index by about 0.02 units assuming you allow for the 5.2 ppm TA increase (i.e. keep the carbonate alkalinity the same).

 

[teapot]

Very interesting Richard, thank you.

 

[Melt In The Sun]

If I'm reading the article correctly, it says that carbonate alkalinity is continually changed by the outgassing of CO2. I thought that was not the case?

 

[chem geek]

If I'm reading the article correctly, it says that carbonate alkalinity is continually changed by the outgassing of CO2. I thought that was not the case?

The text in question here is the following:

Some technicians swear by borates and will use them as a complete replacement for alkalinity; after all, with the carbonate system it is continuously being lost from the water as CO2. However, borates also work well with carbonate and cyanurate to form three layers of buffering.

You are correct that they are wrong to imply that alkalinity is lost. There is a loss of carbonates (including carbon dioxide), but this causes the pH to rise with no change in TA itself -- essentially it is carbonic acid that is removed where the bicarbonate and hydrogen are both removed together (by outgassing carbon dioxide and producing water). However, in practice, when the pH rises, you add acid to compensate (or are using Trichlor which is acidic) with the net result that when the pH is restored (i.e. lowered with acid) the TA is in fact lowered. So in the long run, that is what they mean, though they could have worded that better.