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  Kim Skinner January 11, 2010 at 1:31pm

  David,
  When I started working for Pool Chlor Inc. (a chlorine gas pool service company in southern California) in the late 60's, they were adding borax to pools. Starting with a pH of about 8.0 to 8.2 and adding an acidic form of chlorine would help buffer the pH and prevent it from going to low. That was the main reason.
  
  Today, my service company in Livermore uses bleach as our primary sanitizer and we add some borax to buffer the pH from going too high, along with some other advantages as Richard Falk describes in his post. I am in agreement with most everything he cites.
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  Richard A. Falk January 9, 2010 at 7:19pm

  When borates are added to water at typical pool/spa pH near 7.5, the primary resultant form is boric acid which is a very weak acid. So mostly pH neutral products such as ProTeam Gentle Spa for spas and ProTeam Supreme Plus for pools are mostly boric acid. ProTeam Supreme (without the plus) is essentially the same as 20 Mule Team Borax except for water content (hydrated in the powder). If you add either of these latter two products, which are sodium tetraborate (pentahydrate for the first, decahydrate for the second), you need to add Muriatic Acid to compensate for pH since tetraborate is a strong base. You can use The Pool Calculator to calculate dosages.
  
  Boric Acid is a pH buffer, just like the carbonates and even Cyanuric Acid. There are two main differences. One is that, unlike the carbonates, it is not a SOURCE of rising pH itself as it does not outgas carbon dioxide. The second is that it has more buffer capacity against a rise in pH than a fall. It will buffer in both directions, but the buffering gets stronger as the pH rises so it's particularly well suited to situations when the pH would otherwise tend to rise as when using saltwater chlorine generators (SWGs). In particular, it helps reduce scaling at the hydrogen gas generation plate by helping to prevent the pH from rising as much. A rather detailed comparison of the buffering capability of the carbonates, CYA, and borates is in this post that I wrote. Roughly speaking, having 50 ppm Borates cuts the rate of pH rise in half. However, note that you need to add more acid to make the pH go back down so it does not cut down the amount of acid needed -- only removing the source of pH change will do that, such as lowering the TA if the pH tends to rise over time.
  
  Another benefit of boric acid is its ability to inhibit algae growth -- that is, it is a mild algaecide. This is described in more detail in this WHO summary. Note that drinking water with 50 ppm Borates is at the edge of first symptoms of toxicity for dogs drinking pool water daily though 50 ppm is over a factor of 100 away from such levels for humans (including children). This is described in more detail in this post I wrote. The algae inhibition sometimes results in lower chlorine usage, though this depends on whether you were using too little chlorine (too low an FC/CYA ratio) in the first place. In some SWG pools, this results in being able to lower the SWG on-time and that helps to reduce the rate of pH rise and amount of acid addition needed by reducing normally increased aeration from the SWG and possibly reducing outgassing some undissolved chlorine gas (especially for short pipe runs) which causes a pH rise over time. In general, if one lowers the TA to reduce the rate of pH rise from carbon dioxide outgassing, then one can use borates to compensate for the lower pH buffering from the lower TA.
  
  Some people report that their water sparkles more after adding the Borates. This may be due to the apparent lower surface tension of the water -- something I've noticed in my own pool when I added 50 ppm Borates at the start of this year's swim season.
  
  Boric acid in bulk may be purchased at The Chemistry Store or at AAA Chemicals. It is not cheap, but is a one-time addition with the only maintenance amounts required being that to make up for water dilution. It's less expensive (for consumers) to buy 20 Mule Team Borax and Muriatic Acid. So it would get expensive in commercial/public pools with higher bather loads where there is generally a lot of water replacement. For residential pools, this usually isn't as much of an issue, especially for larger pools or for those with oversized cartridge filters that don't need to be cleaned very often.
  
  Another use for borates, specifically from 20 Mule Team Borax (or Proteam Supreme, but that's a lot more expensive), is as a base to raise the pH (obviously in quantities far lower than for 50 ppm). Using Borax to raise pH results in half the rise in TA compared to using pH Up (sodium carbonate; Arm & Hammer Super Washing Soda). So if you want to raise the pH but don't want to raise the TA as much, you can use Borax. If you want to raise the pH with no change in TA, then aerate the water, but if you are using acidic sources of chlorine (especially Trichlor) then normally you need both pH and TA to be raised.
  
  Richard