- 1 What is Total Alkalinity in Pool Water?
- 2 What does Total Alkalinity do in Pool Water?
- 3 What is the Relationship Between TA and ph?
- 4 What is the Best Value for TA in Pool Water?
- 5 Why Should TA be Lowered with a SWG?
- 6 Is TA a Factor in the Pool Waters Calcium Saturation Index?
- 7 Is TA is affected by CYA levels?
- 8 How is TA Raised in Pool Water?
- 9 How is TA Lowered in Pool Water?
What is Total Alkalinity in Pool Water?
Total alkalinity (TA) is the measure of your pool waters ability to resist pH changes. Technically, TA is defined as the sum of all of the concentrations chemical ion species in your pool water that can react with (or consume) a hydrogen ion (H+).
The vast majority of your water’s alkalinity comes from the dissolved inorganic carbon (DIC) in it. DIC is primarily in the form of carbonates - dissolved or aqueous CO2, bicarbonate ion (HCO3-) and carbonate ion (CO32-). However, cyanurates from your stabilizer (cyanuric acid) as well as borates contribute to your water’s alkalinity. For both carbonate and cyanurate alkalinities the buffering intensity increases with a decrease in pH while borate is the opposite where buffer intensity increases with an increase in pH.
What does Total Alkalinity do in Pool Water?
The easiest analogy for TA is this - the water’s TA is a lot like a shock-absorber on a car. It keeps the car from bouncing around wildly when you drive. If your water has very low TA, then the pH of the water can bounce around wildly like a person being attached to a bungee cord.
If the water has too much alkalinity, then the pH will change very slowly and, in the case of excess carbonate alkalinity (which all pool water has), there will be upward pressure on the pH to constantly rise (the chemical reactions that govern this are explained in more detail in the forum threads).
What is the Relationship Between TA and ph?
TA and pH go hand-in-hand. When TA gets too low (near zero), your pool water’s pH can crash down to values lower than 4.5. When TA is too high, the outgassing of dissolved carbon dioxide and subsequent chemical reactions involving the bicarbonate alkalinity will cause hydrogen ions to be consumed and your pH will rise.
What is the Best Value for TA in Pool Water?
Just like pH, there is not “optimal” TA value - your optimal TA value is the value at which your pH is most stable (longest times between acid additions). Typically, people find that a TA somewhere between 80-100ppm works well enough.
However, if a pool is experiencing constant acid demand and pH rises too quickly, TA can be safely lowered as far as 50ppm to compensate for this. Despite what the “pool experts” will say, keeping your TA on the low end will not “rot out your equipment” or “destroy your plaster surfaces”.
The TA range in the Recommended Levels is designed to protect your pool surface as long as the pH, TA, and CH can be kept in those ranges.
Why Does the TF-100 state the TA should be between 100-120?
The TA range given in the TF-100 is from Taylor. TFP does not follow that. It is based on using trichlor for chlorination purposes.
Why Should TA be Lowered with a SWG?
With high TA and aeration from the bubbles the SWG creates, you'll get faster pH rise inside the cell and so there's more potential for scaling. With current reversal, the cell tends to stay clean but you'll get more snowflakes out of the returns. Calcium hardness is only one part of the equation.
If the SWG cell pH rises much above 10 (and it can easily do that) and if there is sufficient levels of carbonate ions available (CO3--), then you will get calcium carbonate precipitation. The higher the TA and pH, the more carbonate anions are available.
Borates are particularly helpful preventing scale in a SWG because the pKa for boric acid / borate anion buffering is approximately 9. That essentially means you get the maximum buffering capacity at a pH of 9 and that tends to hold down the pH rise inside the cell. Keeping the pH below 10 int he SWG cell reduces the risk of calcium and magnesium scaling (insoluble magnesium hydroxide precipitates at a pH of 10.2 or so).
Is TA a Factor in the Pool Waters Calcium Saturation Index?
Like pH, TA is a quantity used to calculate the Calcium Saturation Index (a variant of the Langlier Saturation Index) and, as long as your CSI is within the “balanced” limits, then it does not typically matter what the individual pH or TA values are.
Is TA is affected by CYA levels?
Cyanurate does contribute to the TA.
Some test procedures say you should deduct 1/3 of your CYA from your TA to really get a true reading but this in effect is producing the carbonate alkalinity value. PoolMath accounts for the effect and no adjustment is needed for TFP Methods. Taylor, for example, describes the practice of deducting 1/3 of the CYA value which in effect is removing the cyanurate alkalinity portion to produce the carbonate alkalinity value. Taylor then use this carbonate alkalinity value to calculate their Saturation index (SI). In practice the concentration of the cyanurate ion is pH dependant where at a pH of 7.0 it is 0.22% of the CYA. The percentage increases with pH where at a pH of 8.0 it is 0.36% of the CYA value. 
When cyanuric acid is added to the water, the hydrogen ions that are released lower the TA as much as the resulting cyanurate raises the TA. So, adding cyanuric acid has no net effect on TA.
How is TA Raised in Pool Water?
The simplest way to raise TA is with baking soda (sodium bicarbonate or sodium hydrogen carbonate). In fact, if you look at the ingredient in the chemical bottle that pool stores sell as “alkalinity increaser”, it is precisely that, baking soda!! And, it is often 2X to 3X the price of the stuff in your grocery store.
Other chemicals are sometimes suggested for raising TA. Their effects are:
- Baking Soda = big TA change, small pH change
- Borax = Big pH change, small TA change
- Soda Ash/Washing Soda = big pH change, big TA change.
It is often best to make large TA adjustments in a couple of steps, testing the water after each one, as adding large quantities of baking soda can raise the PH a little and you don't want the PH going out of range.
How is TA Lowered in Pool Water?
TA is lowered every time you add acid to your water.
If you need to lower your TA level aggressively, see How To Lower Total Alkalinity.
- Taylor. (2015). Pool & Spa Water Chemistry. A Testing & Treatment Guide. Sparks: Taylor Technologies, Inc. p.14