Better Option Than Baking Soda For Alk?

[Flagstone]

Hi all,

We have a 500 gal spa (using bromine as a sanitizer) and whenever we run the jets, the pH goes way up. I've been told that this is a result of the aeration driving out the carbonate in the water. I presume this carbonate comes from the baking soda we add to keep the alkalinity in range (we usually keep it at about 80 to 100). Is there a better choice, such as a phosphate, for raising alkalinity or is there another way to keep the pH from going up so much when we run the jets? Thanks!

[chem geek]

If the pH tends to go up, you do not need to maintain the TA at the 80-100 range. You could lower it to 50 ppm and still have plenty of buffer for pH. Phosphate is not a good option (though it is a pH buffer) because it is one of the nutrients algae needed for growth. A better option is to use Borates which may be found in 20 Mule Team Borax (Sodium Tetraborate Decahydrate) or in products such as Proteam Supreme (Sodium Tetraborate Pentahydrate) at a level of around 30-50 ppm. You do not want a dog drinking a pint or more of the water every day, but this is unlikely with a spa (though more of an issue for pools) -- otherwise, the dog will be at the first noticeable symptoms level with borates in terms of long-term exposure.

In 500 gallons, 24 ounces weight of 20 Mule Team Borax (just under 3 cups if settled), will add about 41 ppm Borates. Borax is a base so causes the pH to rise so you need to add 11.4 fluid ounces of Muriatic Acid (31.45% Hydrochloric Acid) or 15.4 ounces weight (about 1.3 cups) of dry acid (sodium bisulfate; pH Down) and can alternate adding the portions of each into the spa. The Borates are an additional pH buffer, but remember that this will not change the amount of acid you need to lower the pH when carbon dioxide outgassing occurs -- it only makes the rate of rise slower but then you need to add more acid when you do have to lower the pH. Only the lowering of the carbonate portion of TA will lower the rate of rise in pH. Also note that the Borates will effectively not show up in the TA test -- they will increase TA by about 5 ppm. The reason is that TA measures the capacity of the pH buffer against a lowering of pH, not a rise of pH, and the Borate buffer has much more capacity against a rise than a lowering of pH.

This chart shows the relative rate of carbon dioxide outgassing at various TA and pH levels, but note that this chart was for pools assuming a CYA of 30. Subtract 10 from the TA values in the chart to get the equivalent TA when there is no CYA as with your spa.

Finally, I assume you do not have any exposed grout (in tile) in your spa so do not need to maintain the pH, TA and CH to be saturated with calcium carbonate. Otherwise, a lower TA might require more calcium (higher CH).

Richard