PH TA Relationship - Further Reading

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pH TA relationship – The Shallow End

Why is the TA and pH relationship important?

The primary purposes for looking at the TA and pH relationship are:

• Knowing how to manage TA and pH together, because they typically move together, and
• TA, pH, and their levels and relationship, are the number one cause of rising pH/Acid demand.

What is the relationship between TA and pH

Maintaining pH within a certain range is important because it can have an effect on plaster, liners, equipment and most importantly on people. TA levels have fewer effects on the pool than pH. Except in a limited number of situations, TA and pH generally move in tandem. If you increase TA, pH generally increases. If you lower TA, pH generally decreases. This article will explore the relationship between TA and pH, the relationship of TA and pH to pH rise/acid demand, how changing one affects the other, and the impact of the way that you adjust either one.

Adjusting TA will affect pH.

Raising TA

The TA of pool water is raised several ways, primarily through the addition of an Alkaline agent. Depending on what you use, it will have varying effects on pH. This table shows the how pH changes when you raise TA by 10 ppm.

Chemical	TA increase	Associated pH increase
Baking Soda	10 ppm	.01 ppm increase
Soda Ash/Washing Soda	10 ppm	.35 ppm increase
Borax	10 ppm	.6 ppm increase

Baking soda has a small impact on pH and is the preferred method to increasing TA. If you have a need to raise your TA, and decide to use Washing Soda or Borax, understand it will raise your pH somewhat significantly.

Particularly in the case of using Borax to raise TA, or add Borates, it has a significant impact on pH! If you decide to use Borax, you will need to add it in smaller increments, reducing pH each time with acid. Here is the process Borates in pool

Lowering TA

The TA of pool water is lowered using Acid. Adding Muriatic acid is the best way to reduce your TA, because alternative acids add other chemicals to your pool, or have other negative impacts on you or your pool. Dry acid should not be used in plaster pools or pools with a SWG. Adding acid lowers your TA, but it also lowers pH. This table shows the effect on pH by lowering TA by 10 ppm.

Chemical	TA decrease	Associated pH decrease
31.45% (20 Degree Be) Muriatic acid	10 ppm	.6 ppm decrease

This is a significant impact on pH! Assume your 5000-gallon pool has a TA level of 120. You look up the recommended levels for your pool, and determine that your pool should have a TA of 80. If you add .4 Gallons of 31.45% of Muriatic acid to reduce your TA by 40, you will CRASH your pH by an astounding 2.5 ppm. Lowering TA should be done in stages described here Lower Your TA.

Adjusting pH may impact TA

Raising pH

pH is raised the same way that you increase TA. However, there is a way to raise pH without affecting TA. That is through Aeration of the water. Aeration will raise your pH gradually while not affecting your TA. This table shows the effect on TA by raising PH by .5 ppm.

Chemical/Action	pH increase	Associated TA increase
Aeration	.5 ppm	No increase
Borax	.5 ppm	8.1 ppm increase
Soda Ash/Washing Soda	.5 ppm	16 ppm increase
Baking Soda	.5 ppm	243 ppm increase

The role each of these chemicals have in raising pH is the inverse of using them to raise TA. Baking soda is NOT a good choice to raise your pH. Soda Ash/Washing soda is a bit of a "pH Hammer" (TFP uses this term), use it carefully.

If TA is 50 or above, bring your pH up to 7.4 or so using 20 mule team Borax.....soda ash is not a good choice as it will raise the pH and raise the TA a lot.

If TA is below 50, then bring the TA up to 50-60 using baking soda and THEN raise your pH to around 7.4 using 20 Mule Team Borax.

Lowering pH

Again, pH is lowered in the same manner as TA. This table shows the effect of lowering pH by .5 ppm has on TA:

Chemical	pH decrease	Associated TA decrease
31.45% (20 Degree Be) Muriatic acid	.5 ppm	7.8 ppm decrease

It is easy to lower your pH without having a drastic effect on TA.

Can I raise pH without affecting TA?

Yes, Aeration

Can I lower pH without affecting TA?

You cannot directly lower pH without lowering TA, as Acid (used to lower pH) also lowers TA. You can, however, reduce pH with acid, then add baking soda to raise TA back to the same level TA level where you started. This chart shows how to lower pH by .5 ppm with acid and use Baking soda to restore TA. (amounts will vary by pool, use Pool Math to determine the correct amounts!)

Chemical/Action	pH decrease	Associated TA change
31.45% (20 Degree Be) Muriatic acid	.5 ppm	8 ppm Decrease
Baking Soda	0 ppm	8 ppm Increase

Can I raise TA without affecting pH?

Practically? Yes, use Baking soda. You can add up to about 20 ppm of TA using baking soda while changing pH <.1 ppm. Larger additions of baking soda will have at least a .1 increase in pH. As an example, 80 ppm TA addition of baking soda would raise pH .2 ppm. If you have a need to raise TA more than ~20ppm, add 20 ppm baking soda, allow it a period of time to circulate, correct pH if necessary, then add additional doses (<20ppm) and adjust pH, until you hit your target TA.

Can I lower TA without affecting pH?

No and Yes. You will end up with the same pH, but lower TA if you use this process: Lower your TA. However, you will change pH many times while you are lowering TA.

Causes of rising pH

The level of aqueous (dissolved) CO2 in pool water determines how fast pH will rise. For a deep dive, start here pH TA relationship - The Deep End. Many pool owners experience constant acid demand and pH rises quickly, requiring constant additions of acid. There are several causes of high levels of CO2, rising pH and acid demand.

1. Water features and aeration release CO2
2. Lower pH water has higher levels of CO2
3. Higher levels of TA allow higher levels of CO2
4. When pool water has low pH and high TA, it has the most amount of stored CO2

pH levels have a larger influence on CO2 levels (and pH rise) than TA levels. For explanation, see pH TA relationship - The Deep End

Reducing pH rise and acid demand

For a deep dive on the causes of rising pH, start here pH TA relationship - The Deep End.

Water features and aeration cause the release of CO2, which raises pH. The ways to combat this are to turn off the aeration or limit the time that one runs the water feature.

Practically, as pH rises, the rate of rise will slow. If the pH rise is faster than you care to maintain, reduce your TA until the rate of rise is reasonable to manage. There will become a point where the TA is sufficiently low, and your pH will slow to a reasonable level. In many pools, particularly SWCG pools, the TA level may be as low as 50 and the resulting pH may be at 7.8-8.0. All pools are different, you need to find the right level for your pool.

A reasonable starting point for most pools is a TA of 80-100. Most find however, particularly with SWCG pools, that a TA of 60-80 stabilizes their pH, hence the TFP recommended levels. TA can be safely lowered as far as 50 ppm to compensate for this. 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 recommended ranges.

The scope of this article is to help you understand the TA, pH relationship and to better manage the two. It will not get you to a permanent, zero acid demand pool. It will significantly extend the period between acid additions.

If TA in the water contributes to pH rise, can I just get rid of all the TA?

TA levels below about 50 ppm lead to insufficient levels of TA to buffer pH to prevent sudden pH crashes.

Why should one ignore TA if their pH is steady?

You now understand the relationship between TA and pH, and that one of the primary objective of TFP is to have a pool that has reasonable rising pH levels and acid demand. Once you have achieved steady pH, you have met the objective. Your pool is in reasonable equilibrium. Sit back and enjoy!

pH TA relationship - The Deep End

Summary

1) Higher Aqueous (dissolved) CO2 levels lead to higher pH rise.

2) Lower pH water has higher levels of CO2

3) Higher levels of TA allow higher levels of CO2

4) Combined, the lower the pH and higher the TA, the more a pool will tend to outgas CO2, resulting faster pH rise.

5) Reducing your TA until you have a fairly stable pH, reduces acid demand and the need to constantly add Acid

Henry’s Law & Pools

Henry's law is one of the gas laws formulated by William Henry in 1803 and states: "At a constant temperature, the amount of a given gas that dissolves in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid."

In terms most of us can understand, Henry's law basically states that the amount of a gas dissolved in water wants to be directly proportional to the amount of that same gas in the atmosphere. Our atmosphere has a small percentage of CO2, so CO2 leaves the pool (or outgasses) trying to equalize with the atmosphere. This process is not immediate, it happens over a time.

CO2 and Water

CO2 dissolves in water, and some of it reacts with water molecules to produce a slightly acid solution called carbonic acid. The (aq) indicates water solution.

CO2(aq) + H2O ←→ H2CO3(aq)​

Carbonic acid is weak, so some of it dissociates to produce H+ ions, hence it’s a slightly acidic solution, forming the hydrocarbonate ion, and Bicarbonate, and with release of more H+ ions, Carbonate.

H2CO3(aq) ←→ H+(aq) + HCO3-(aq) And HCO3-(aq) ←→ H+(aq) + CO3–2(aq)

All These are all reversable.

pH and CO2

pH is a measure of how basic/acidic water is. pH is measured by the concentration of Hydrogen (H+) and hydroxyl ions in water. So, the more CO2 in water, the more H+ ions in the water, the more acidic and the lower the pH.

Remember Henry’s law? Our atmosphere tends to have less CO2 than our pools, so the CO2 in our pools desire to be in equilibrium with the atmosphere, as a result, they outgas, reducing the H+ ions, increasing pH.

pH and TA

Total alkalinity (TA) is the measure of water's ability to neutralize acids. The role of alkalinity (TA to make is simple) is to buffer (slow down) a reduction in pH. When acid is added, carbonate ions can absorb Hydrogen to create bicarbonate ions. And with enough acid, bicarbonate ions absorb another Hydrogen ion and convert into carbonic acid, which is dissolved CO2. The reverse of our equations above.

Net, net, having alkalis (like Carbonate/Bicarbonate) in the pool allow CO2 to be stored in the pool, counterbalancing the pool’s natural desire to outgas, increasing pH.

So What?

Your pool wants to be in some form of equilibrium, and that is affected by the pH and the TA of the water. If we were to plot HOW FAR OUT OF EQUILIBRIUM the water will be at various levels of TA and pH, it would look like the chart below.

The larger the number in the chart, the further the water is out of equilibrium.

• Perfect equilibrium would be 0 (Zero) on this chart.
• If the number is GREEN (<10), the pool is close to equilibrium.
• If the number is Orange (<20) it is somewhat out of equilibrium.
• If the number is Red (>20), there is a large pressure on the water to outgas CO2 and you would see a faster rise in pH.

Conclusion: To reduce your pool's rate pH rise rate to a level you can reasonably manage, reduce your TA to the point your pH rises at a rate that you can reasonable manage. Worry about pH (keep it in range) and pH rise (what you can manage), worry less about the absolute TA number (except to keep it above 50).