FAQ: Questions about Swimming Pool Alkalinity Levels
What terms are used for "alkalinity"?
"Alkalinity", "Total alkalinity" (TA), "Carbonate alkalinity" (CA), etc.
There is a difference between CA and TA, but it's rarely important. Because of the way
I organized my testkit (PS23x series), it's usually abbreviated "ALK", here.
What should my TA be?
Somewhere between 50 and 200 ppm. If you've got a heater, it usually should be lower than 150 ppm.
If you've got a plaster or concrete based pool, it usually should be higher than 80 ppm.
Why does TA matter?
TA is actually just a measure of how resistant to change your pH levels are, over a particular range.
What level of resistance is OK varies.
Sometimes, if your TA is too low, your pH will tend to change a lot, and be hard to control.
high TA is often related to scale buildup inside your heater, or at your pool's waterline.
Why does CA matter?
CA is the portion of the TA that is due to "carbonates" from sodium bicarbonate (baking soda),
sodium carbonate (soda ash), and other sources. On pools with concrete or limestone surfaces, low
CA together with low pH will let your pool water DISSOLVE your pool surfaces, which is usually NOT a
good thing. If your CA is high, your pool will tend to cloud up, any time you add calcium hypochlorite
or raise the pH.
I still don't understand: what is alkalinity? Isn't the same thing as pH?
No, it's not the same as pH, but don't feel bad about being confused: most pool dealers, and
many pool chemical manufacturer's don't understand alkalinity, either. Again, alkalinity is a not
a 'thing', but a measurement. It's a measurement of how hard it is to change your pH. Exactly how
pH and alkalinity are related has been discussed on the PoolForum, but is too complex to get into
Why does my dealer (or the NSPI, APSP, or my builder , etc) say my TA / CA must be just exactly such and such?
Historically, the concern for getting alkalinity levels just so resulted from an attempt to use specialized
mathmetical formula*, developed for aiding boiler operators, on swimming pools. Later on, some of the
chemical companies realized that the more exactly pool owners tried to control things, the more chemicals
they'd sell. I suspect that this latter fact has a lot to do the pool chemical companies interest in 'just-so'
chemical levels today.
* The Langlier index. There are other saturation indices that you'll sometimes see mentioned, such as the Ryznar or Hamiltion indices.
Well, my TA is XX ppm; what should I do?
Usually, nothing! If you have a vinyl or fiberglass pool, no heater, no scaling, and no problem
with your pH level changin erratically . . . don't worry about it!
OK, but how to I raise TA / CA?
Add baking soda, at the rate of about 2 lbs (or 2 cups) per 10,000 gallons of pool water . . . then retest.
But, I need to raise my TA 40 ppm, what's the right dose for that?
Add baking soda, at the rate of about 2 lbs . . .
Seriously, don't make the mistake dealers encourage of trying to get there all at once.
Add some, and recheck. Then add a little more, if you need to do so.
There are all sorts of reasons why this is the best way.
My dealer told me that I need the Alkalinity Increaser he sells, instead
of baking soda. Why would he say that?
There are two reason. Either he's a liar or he's an ignoramus. Take your pick.
Actually, he's been taught to believe that, often by the salesman from his
chemical suppliers. Most pool dealers didn't major in chemistry in college.
But is baking soda really the same? The dealer stuff has a chemical name of "sodium hydrogen carbonate". Is that the same?
Yes, it's EXACTLY the same. Baking soda is the pharmaceutical grade (extra pure) of sodium bicarbonate or sodium hydrogen carbonate. The pool store stuff is the industrial grade which is slightly less pure and is not as fine. But, except for impurities, they are identical.
It's common in pool product labels to use the most obscure chemical name legally allowed, all for the purpose of allowing to sell you on their "sodium hydrogen carbonate" @ $1.50/lb instead of baking soda @ $0.40.
Everytime I add baking soda, my pH goes up. How can I raise alkalinity without affecting my pH?
Usually, you can't. That's one reason (among many) for the 'add a little, and then retest' approach.
My TA is way too high. How can I lower it?
You can reduce your CA (the portion of the TA cause by carbonates) by lowering the pH and then aerating. The procedure must be followed carefully, and is posted separately.
My dealer / builder / whoever said that the right way to lower TA is to add XX amount of acid. Should I do that?
No. The procedure for lowering TA (actually, CA) found in about 99.9% of the pool literature of the world does NOT work! What it does do is take you on an endless coaster ride up and down the pH and alkalinity scales. And, of course, you have to buy a ticket from your dealer each time you ride! As far as I know, it is STILL true that the ONLY place where a working process has been published is here.
My calcium hardness (CH) level is also too low. Can I raise my ALK or TA
level and raise my CH level at the same time?
Not unless you want to have a pool that looks like it's filled with milk!
When you try to add baking soda and calcium chloride (AKA calcium hardness
increaser), you almost always end up with a zone where both the ALK and
the CH are too HIGH, at least temporarily. And during that brief period the ALK
and CH combine to firm fine particles of calcium carbonate (limestone) that
cloud your pool.
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