Total acidity

[loop_pea]

There's a lot of talk on TFP about total alkalinity, and it's a routine test. The general literature on pools says TA is a buffer and a measure of the resistance of water to pH change. Elsewhere on here and if I recall correctly (I can't pinpoint the exact thread again), it says that TA is only a measure of the resistance to pH change downwards, while total acidity is the buffer against pH change upwards.

Why isn't total acidity routinely measured? Most people seem to have pools where pH keeps rising, so resistance to pH going up seems like a good idea.

What influence does CYA have? The mantra of the people who ran the pool before me is "you use tablets to manage your pH". They say that because the tablets are acidic, it helps to balance out the tendency for high pH due to having hard water. Late in the season this seemed to be true, but earlier on when overall CYA levels were lower, it didn't seem to work so well. pH would rise a lot during the day when the pool was in use, but settle back down overnight.

Where can I find out more? Thanks for being there to help.

 

[JasonLion]

While total alkalinity (resistance to PH change by acids) and what you are calling total acidity (resistance to PH change by bases) are not exactly the same thing, they are very closely related. Total acidity essentially tracks total alkalinity, so there is no need to measure it separately. If you know one, you essentially know the other.

The CYA level in the water doesn't have any significant effect on PH or TA. Adding CYA is mildly acidic, but that isn't what you are talking about. CYA does regulate the rate of chlorine breakdown, and so has small effects on the PH changes from chlorine breaking down, but nothing significant. Your differences in PH changes over the course of a season must be coming from something else, possibly changes in the rate at which tablets dissolve due to temperature changes or changes in the TA level over the course of the season.

I'm not sure exactly what kind of more information you are looking for. One good place to start is the Advanced Chemistry topic.

 

[chem geek]

(I wrote this while Jason posted his response.)

The Total Alkalinity (TA) measurement is one of capacity that is proportional to the amount of acid you could add before the pH "crashes" when the TA gets used up (the pH at that point is 4.5 as that is when the indicator dye changes color in the TA test). For that measurement, it is accurate and requires no compensating calculations. Roughly speaking, every 10 ppm TA will handle 3.2 cups of full-strength Muriatic Acid (31.45% Hydrochloric Acid) in 10,000 gallons. This amount of acid is also the amount needed (in 10,000 gallons) to lower the TA by 10 ppm.

The Total Acidity isn't what you are looking for since the pH doesn't "crash" on the way up and usually slows down in its rise as the pH gets higher, not from buffering but from slower carbon dioxide outgassing.

The number you are really looking for is known as "buffer capacity" though it's really a measurement of buffering strength (don't ask me why these terms are so confusingly named). If you wanted a rough idea of what that was, you could use the acid or base demand tests in the Taylor K-2006 test kit to see how much the pH moves when adding certain amounts of acid or base. If you are very bold, you can look at my spreadsheet where I calculate (at lines 417-422) buffer capacities for the two points of the carbonate buffer system and for the borates buffer system and where I just added the three points of the cyanurate buffer system. You can easily see how changing the pH changes the amount of buffering (in mmole/pH) for each system (be sure to put 50 ppm Borates into the Init and/or Goal). You can see that as the pH rises the carbonate buffer system gets weaker (until a pH of around 8.7), the cyanurate buffer system gets weaker, but the borates buffer system gets stronger.

If you were finding your pH rising in spite of using Trichlor tabs/pucks as your primary source of chlorine, then you must have had the TA be quite high and/or had a lot of aeration or perhaps curing of plaster. Most people using Trichlor find that their pH tends to drop. Even if the TA is high enough to have the pH be stable, the TA will still drop over time.

You should be able to get fairly decent pH stability in your pool. If the pH is rising, your TA level is probably too high. Though you could use 50 ppm Borates as an additional pH buffer, it won't change the amount of acid you have to add. It just makes it so you add it less frequently (but the total amount per time is the same). To lower the actual required acid amount, you need to reduce the SOURCE of rising pH and that is the TA level since a higher TA means more carbon dioxide in the water which means more outgassing that is the source of rising pH.

Richard

 

[loop_pea]

Thanks both JasonLion and ChemGeek, that helps a lot.

My theory at the time was that the pH changes were due to bather load and the chlorine not keeping up with demand. If I've understood things right then the chemistry backs this up. Production of monochloramine is slightly basic, and you have 120 kids in the water (not all at once) and chlorine levels on the low side, then pH would rise while they are in the pool and monochloramine accumulates, and then fall at the end of the day after they all get out and the chlorine reaction has time to complete. Does this make sense? Alternatively, 120 kids will aerate the pool pretty well, but would the pH then fall back?

This on its own doesn't really explain why the pH was fairly stable during the second half of the season. I wonder if TA could be the difference. We started the season with a fresh fill of water with TA of 220 (that's high?), but you say it would fall over time.

TA didn't get measured again because we had trouble getting hold of the tablets for the test, so unfortunately I don't know what it actually was later on. Some tablets were eventually purchased, but they turned out to be photometer tablets, and we do what I think is a titration type test for TA, only with tablets, where you keep adding them until the colour changes.

The reason I'm asking all these questions is that I'm trying to work out if we can run this pool better with the constraints we've got, including the use or trichlor tabs, and whether it is practical to change the dosing method. Making changes to startup procedures is one thing that could be handled by an individual, but changes to how the pool runs from day to day would involved re-educating a lot of people - people who don't spot the word "photometer" on the testing tablet box.

 

[chem geek]

The full oxidation of ammonia or organics is acidic and drives the pH down, but compensates for the rise in pH from adding chlorine from bleach or chlorinating liquid so is usually roughly pH neutral. If using Trichlor, the pH would normally go down. However, the TA of 220 in the fill water is high so would tend to make the pH rise. As for who wins, the Trichlor or the carbon dioxide outgassing, that depends on the amount of aeration and that will depend on the splashing, etc. As for the TA dropping over time, that would normally be the case from the Trichlor, but you may have add enough evaporation and refill of the pool water that the TA was stable or even increased, assuming you didn't add any pH Up or Alkalinity Up.

It sounds like you've got a high bather-load pool so almost anything can happen in that situation since it is so dynamic which is why many such pools have automated chlorine dosing and pH adjustment systems. If you were using Trichlor as your sole source of chlorine, I'm surprised your CYA level didn't climb rather quickly. For every 10 ppm FC added by Trichlor, it also increases CYA by 6 ppm.