Using Sodium Bicarb to lock pH where I want it, Help?

[gbergeron]

I'm going into my second season of managing two outdoor pools. I didn't open the pools last season so I didn't have to deal with the initial chemical balancing at all.

The thing that I want to do now is prevent one of the most common problems that I faced last year; pH swings. As an outdoor pool the pH is all over the place. I know that if the Total Alkalinity (TA) is high enough (around 200) the pH will likely lock and be hard to adjust. My objective is to lower the pH to a point that when I raise the TA from 70ppm to 200ppm my pH will be between 7.1-7.3. I've been scouring the internet for days trying to find ANY way to calculate the pH difference that amount of Sodium Bicarb will make.

Thing to know...
Pool size:16,500 gallons
Current TA: 70ppm
Target TA: 200ppm
Pool temp: 80 degrees F

Any help I can get will save me a mondo headache. Thanks!

 

[tim5055]

Any help I can get will save me a mondo headache. Thanks!

Sure, it's easy - your science is flawed and it won't work.

Higher TA amounts cause greater pH fluctuation, it has noting to do with "locking" a pH value.

Leave your TA whee it is or allow it to drop even lower (but not below 50) and you will generally see greater pH stability.

I know that if the Total Alkalinity (TA) is high enough (around 200) the pH will likely lock and be hard to adjust.

Where did this knowledge come from?

 

[gbergeron]

I'm drawing from the CPO handbook and the CPO course I took last spring.

Also, a little from experience. When I managed an indoor pool and spa my pH was all over the place at lower TA and much more stable at higher TA

 

[tim5055]

I'm drawing from the CPO handbook and the CPO course I took last spring.

Also, a little from experience. When I managed an indoor pool and spa my pH was all over the place at lower TA and much more stable at higher TA

Check out this article- Certified Pool Operator (CPO) training -- What is not taught

A quote form it:

TA and its Effect on Rising pH

The courses do not clearly explain how Total Alkalinity (TA), and specifically the carbonate portion of TA, is a SOURCE of rising pH so that as counter-intuitive as it may seem, a lower TA results in a lower rate of pH rise over time in most cases (when the pH rise is due to carbon dioxide outgassing). They do not teach that pools are intentionally OVER-CARBONATED to provide a pH buffer and to protect plaster and that this over-carbonation is the primary source of pH rise in pools using hypochlorite sources of chlorine. They do say to use a lower TA range of 80-100 ppm vs. 100-120 for acidic sources of chlorine such as Trichlor, but even lower TA levels may be needed in some cases (especially in spas using the Dichlor-then-bleach method). The ratio of how far out-of-equilibrium the carbon dioxide is in the water vs. the air at various pH and TA levels is shown in this chart though the rate of carbon dioxide outgassing appears to be related to the square of the TA rather than being linearly related (that is, the outgassing rate greatly outpaces the additional pH buffering at higher TA levels).

The handbook indicates that at higher levels of TA, the pH is usually higher than ideal and becomes very difficult to change, but does not indicate that TA itself is a source of rising pH due to carbon dioxide outgassing. In the Spa & Therapy Operations section, when discussing pH, there is a discussion of aeration and carbon dioxide outgassing causing the pH to rise (and it says the TA decreases which is incorrect; the TA only drops when acid is added as acid lowers both pH and TA). However, even in this discussion it is not made clear that this effect can be reduced by lowering the TA level and supplementing pH buffering with a different non-carbonate pH buffer such as borates.

You say both of the pools are chlorine sanitized, but what is the source of the chlorine?

What are you using to get the test values?