Borates and TA

[rabbet]

Is it essential to bring the TA to the correct range when adding borates?
My TA never gets below 120.
Today's readings:
FC 4.5
CC .5
PH 7.7
TA 130
CYA 30

 

[RobbieH]

The thing is, with borates in the pool you are going to have a much harder time moving the TA. You can lower TA by bringing pH down to 7.2, then let it rise back to 7.7, etc.

Why are you adding borates? For pH control?

 

[CeeElGee]

rabbet, I tried and tried to get my TA down to recommended levels before adding borates and finally gave up. I went ahead and did the borates and just live with rising pH (which was happening before anyway). I'm happy with the results. I have since switched my fill water source and hope to get TA under control next season after a partial drain and refill in the spring.

 

[rabbet]

I hoped for reduced chlorine usage and pH would be more stable and it might help with algae.

 

[CeeElGee]

I hoped for reduced chlorine usage and pH would be more stable and it might help with algae.

My chlorine usage has been the same, if not slightly better, and algae control around my steps has been much better.

 

[RobbieH]

Borates won't affect chlorine usage, and isn't an algae control. But, it can help a bit with algae should you fall below target level for a very short period of time.

 

[SunnyOptimism]

If you want to do borate additions, use boric acid instead of borax. It is acidic going into the water and doesn't require MA like borax does to keep the pH down. The additional acidity of the boric acid could help lower the TA a bit but it probably won't be too measurable.

Because of the high pKa of borates in pool water (pKa ~ 9.0), about 90-95% of the boron will exist as boric acid at normal pool pH. Better to start that way from the beginning

Do note that boric acid is more expensive than borax/MA but way easier to use (and you can use the left over BA as ant poison!!)

Borates do not react with FC per se, but they do reduce chlorine consumption through the secondary effect of being a mild disinfectant. Both my acid-demand and FC were improved by borates. My SWG runs about 33% lower (from 45% power setting to 30% setting) and acid additions from once every other day to once every 5 days.

Borates also improved the look and feel of my water. My wife likes the pool water a lot better and she says showering off the pool water is a lot easier. Borates do act as a mild softening agent (binds with Mg and Ca) so washing off is easier.





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[chem geek]

If you want to do borate additions, use boric acid instead of borax. It is acidic going into the water and doesn't require MA like borax does to keep the pH down. The additional acidity of the boric acid could help lower the TA a bit but it probably won't be too measurable.
:
Borates also improved the look and feel of my water. My wife likes the pool water a lot better and she says showering off the pool water is a lot easier. Borates do act as a mild softening agent (binds with Mg and Ca) so washing off is easier.

Though boric acid is mildly acidic, it also contributes to TA because it is a weak acid. So the addition of 50 ppm borates (ppm Boron) from boric acid does not change the TA because the drop in pH lowers the TA by the same amount that it rises from the borate ion (there is a very minor drop of 0.1 ppm due to an increase in ionic strength).

........................... +TA ..... -TA
B(OH)₃ + H₂O ---> B(OH)₄^- + H⁺
Boric Acid + Water ---> Borate Ion + Hydrogen Ion

Starting with a pH of 7.5 and a normal initial TA level, the pH drops to 7.3 and the carbonate alkalinity drops by 2.5 ppm. So for practical purposes, one can ignore the TA effect from the borates, at least near a pH of 7.5.

Though the borates do form ion pairs with magnesium and calcium, this effect at pool borate concentrations and pH is negligible (only 0.1% of total calcium is bound to borate, but the change in ionic strength effectively keeps calcium ion concentration about the same). When 20 Mule Team Borax is used in the wash, the instructions are to use 1/2 cup or 1 tablespoon per gallon which would be 443 ppm, but it would also raise the pH to close to 9.1 where the borate ion concentration is much higher so at 300 ppm CH would sequester around 15% of total calcium.

Any effects on showering may be related to changes in surface tension so that the water feels like it's silkier since it may not bead up as much.

 

[SunnyOptimism]

@chemgeek,

The reference paper I read did not indicate that there was a measureable surface tension difference with or without borates. Have you come upon any newer information?

Reference - http://members.iinet.net.au/~jorobbirch/Boric_acid.pdf


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[chem geek]

I just wrote this post that refers to a recent experiment done to measure the surface tension difference. It's a pool owner's measurement and not a peer-reviewed published paper, but it's better than nothing.

 

[SunnyOptimism]

I saw that show up in my Tapatalk time line. Thank you!

And thank you for all of your expertise and clarifying posts. I greatly appreciate the rigorous scientific analyses you bring to the discussion threads.

As for surface tension, I believe using an optical microscope that directly measures the contact angles of a water droplet on a glass surface is the definitive measure of surface tension. We used dry oxidized silicon wafers and H-terminated silicon (HF stripped Si) to measure droplet sizes and contact angles. In our plating baths, we did crude surface tension testing using a loop of wire in which you would time how long a film of the solution would persist before breaking (like a magic bubble wand). The former being objective and definitive while the later was a great spot test. I'll have to think about the scale pull force test....



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