pH adjustment versus TA

[Aqua Lab Rat]

I've been looking at this paper by Wojtowicz where he calculates the amount of chemical addition needed to adjust the pH, taking into account the total alkalinity and cyanuric acid (at least that's what I think he means by "CA").

Unfortunately his graphs for increasing the pH are targeted to reach 7.8 instead of, say, 7.2 or 7.4. As someone who likes to "get close enough to the answer" without putzing around doing acid demand tests or the continuous "add this much, retest, then do it again" cycle, I would think a full set of curves that are targeted to typical pool conditions would be very useful. I know that there is a lot going on in the water and that it is complex, in fact that's something I keep trying to drill into the minds of people I work with. And then there's the typical consumer who needs directions that just say "add X" even if they have to repeat it often.

Anyway, my point is that I'm surprised that I haven't seen such a set of curves anywhere else including here. Am I just not looking under the right rock, or do other people enjoy the added tedium of hyperbolically approaching the target?

 

[JVTrain]

I'm not aware of any curves for acid/base additions versus TA or CYA levels.

You could create your own using Pool Math outputs if you'd like...

I for one use Pool Math to tell me how much acid I should add and typically eyeball the volume of acid needed. I'll recheck the pH after a while but usually it's very close. Simply using Pool Math will "get close enough to the answer" for a given TA and CYA level.

 

[JoyfulNoise]

That paper by Wojtowicz is certainly an nice primer on pool chemistry and while the graphs are nice to look at, they are not that useful in my opinion. What he is essentially doing is an Acid-Base Buffer calculation which, for all but the simplest acid/base systems, can be a very complex calculation. There's no easy way to make a graph for that as you have multiple, independent axes (pH, TA, CYA, Borates, etc) and so you often have to make simplifying assumptions to reduce it down to something 2-dimensional. As you can see from the paper, just for swimming pool pH things can get very complicated because you have several polyprotic acid dissociations for carbonates, cyanurates and borates. There are mathematical software libraries written in various languages that implement these types of chemical calculations for doing pH and buffer capacity analysis.

This is why I think acid demand drops are actually quite important to a good test kits. The acid demand test is a test of your actual pool water sample and it tells you visually how much acid you will need to add based on the number of drops. One can easily adjust the standard acid-demand charts that come with a test kit to get better resolution around a specific pool volume (since most charts are graduated in steps of 5,000 gallons for pool volumes). One can certainly measure TA, CYA, borates and pH and plug that all into Pool Math to do the calculation but there's always the off chance you erroneously type in a value. The acid-demand test is pure chemistry right in front of your eyes and does not lie. It's also good to have for people who find themselves with a pool that has a really low or high pH (outside the normal testing range of the phenol red) and want to figure out how much acid it will take to get the pH right.

 

[JoyfulNoise]

So, just to post a "lazy man's" experiment, I knew I needed to add acid to my pool so I decided to go by what the acid demand drops say. I took a pH reading and the color on the comparator was between 7.8 and 8.0. I then added three drops of acid demand reagent (R-0005) and I got 7.4 on the pH comparator. That's a little lower than I wanted to go but it's perfectly fine. I know in my head that the 1 drop of acid demand reagent is a little less than 1 pint of 31.45% MA in my pool volume. So based on the acid demand drops, I would add 3 pints (52oz). I used the acid demand formula in that paper you cited and 3 drops turns out to be 44oz by the equation. 44oz is 2.75 pints.

So, the beauty of the A/D drops is that I was well within the ballpark of what I needed to add for MA and I did not have to break out my TA test or open up a laptop to get to Pool Math. The drops tell the whole story. I do have the Wojtowicz A/D equation programmed into my iPhone (pCalc for iOS ROCKS!!), so with my phone and the A/D reagents, I can completely acid dose my pool in confidence....

 

[Aqua Lab Rat]

Thanks, Matt & Joel. (BTW, I see that I need to put my name on my signature - it is Lyle).

Anyway, I know that there is no substitute for actually measuring the water. I just have a weird dislike for doing drop tests. I have Taylor K2006 kits and use them mostly just for TA, unless I really need to measure CYA or CH. Lazy and impatient. Anyway, I like to fiddle around with Excel, so maybe I'll do that to amuse myself.