Why is Ph climbing?

[csevel]

I've added very small amounts of muriatic acid to my 500 gal hot tub to lower it's alkalinity. I've been successful with the blowers running in lowering the alk to 140 from 220 or so. but I don't understand why the ph has climbed to 8.2 (or over) How do I lower the ph in the spa now that I've got the alk under control?? I'm I missing something here?

[waterbear]

first, when you aerate two things happen, the alk goes down and the pH goes up.
Second, what is your sanitizer level and are you using chlorine or bromine? High santizer levels can cause interferance and give bogus high pH readings. I used to see this problem more with bromine when I used it in my spa....my pH would seem to be 8.2 whenever my bromine was above 6 ppm. With chlorine the cutoff seems to be around 10 ppm with a good test kit. some of the cheapie test kits will give inaccurate results on pH when the chlorine level is above 5 ppm!

If your high pH reading is not caused by interference from high sanitizer levels just use some acid to get it down to aobut 7.4-7.6. Your ALK will come down slightly but should still be within range.

[csevel]

I've been using stabilized chlorine granules for sanitizer. Our fill water is high on the alk side and since I have to drain it every month or two..I thought I would try to balance the water. After I reach a good level of PH, how often should I add chlorine? I used to add sanitizer only when we were about to enter the spa. I'm new to this BBB method and I thought I would use some of the principles on the hot tub. (We had the tub before the pool)

[waterbear]

If you are using dichlor you want to maintain a FC reading of about 4-6 ppm before use. after using the spa shock it with bleach (about 1/2 cup of ultra bleach is about right). this should raise the FC to about 10 ppm and get rid of any CC. the higher heat, smaller water volume, and higher bather load requires a higher FC than a pool. Some CYA is necessary in an outdoor spa exposed to the sun but if you check your FC before using it and add the dichlor if needed and shock with bleach or dichlor after use you should be fine. Since the spa is drained and refilled every 1-3 months buildup of high CYA levels in the water from the Dichlor is not usually a problem, The biggest problem is usually pH and TA control because of the constant aeration from the bubblers and jets.

[csevel]

Thank you...that clears it up nicely for me..

[chem geek]

first, when you aerate two things happen, the alk goes down and the pH goes up.

Evan,

I have a minor technical correction to this. Even though it is counterintuitive, when there is outgassing of carbon dioxide (from aeration, for example), the pH rises but the Total Alkalinity (TA) does not change at all. See Alkalinity for a good explanation of this. Another way to look at it is that any shift to reduce alkalinity combines a carbonate species (which is basic) with H+ (which is acidic) so there is no net change. This is why Ben's alkalinity lowering technique just increases pH during the aeration phase. It isn't until you add acid that the alkalinity (and pH) get lowered. Believe me, it took me a very long time before this sunk in and I still find it weird!

Richard

[waterbear]

Then explain to me why the TA increases when CO2 injection is used for pH control (and also why it causes an increase in TA in planted aquariums). Also if this is the case and the TA is lowered when the acid is added that would mean the 'slug method' works'. It would seem that by outgassing CO2 you have actually lowered the amount of the buffer system in the water since you have removed one of it's components. It would seem in the example that it was accounting for a slight increase or decrease in CO2 due to respiration and the effect is what would be expected with a buffer system. I do not believe in a stiuation where you have a forced continuous gassing off would be the same thing at all. YOur comments?

[chem geek]

The "slug method" does not work. If you add acid you lower both the pH and the alkalinity so if you are trying to just lower the alkalinity by itself with no change in pH, then you can't do so by just adding acid. Those that have claimed that the slug method works or "could work" claim that you outgas CO2 when you add the acid, and technically in the localized region where they are dumping the acid the pH is in fact so low that it does force the carbonate in that area to more rapidly outgas, but this area is so small that even if all of the carbonate was removed, it would have a negligible effect. By the time the acid is dispersed through more of the pool so that a larger area could be affected, the acid effect is weakened so that the increase in outgassing isn't very high and is so slow anyway that the total release of CO2 is small. Somebody somewhere probably carefully added acid in such a way that it spread near the surface and did in fact slightly lower alkalinity more than normal and hence this procedure was "born", but it is extremely ineffective (especially compared to Ben's sure-fired approach).

When you inject CO2 for "pH control" you are implying that there is some sort of pH drift (demand) already that you are trying to counteract. Since injecting CO2 makes the pH drop, the implication is that you have something else in the aquarium that is trying to make the pH rise (i.e. is basic or alkaline). So the combination of adding CO2, which just makes the pH drop, plus this basic source, causes the pH to be stable (or to drop less) and for the alkalinity to increase.

It is true that by outgassing CO2 you are removing carbonate from the system which is the primary component of the buffering system, but the very nature of the pH shift that is caused by this outgassing increases the buffer "start point" by having less hydrogen ion that needs to be "neutralized". Think of hyrdrogen ion (which is measured by pH) as being "anti-alkalinity" or "negative alkalinity". When you outgas CO2 you lose the alkalinity from carbonate but also lose the "negative alkalinity" from hydrogen ion so there is no net change in alkalinity (and the pH has risen). [EDIT] If you don't like the concept of "negative alkalinity", then consider that H+ and OH- are always in equilibrium (with water) so that when one goes up the other goes down. So when the pH rises, you get more OH- and clearly OH- is alkaline and increases alkalinity.

TA = 2*[CO3(2-)] + [HCO3-] + [OH-] - a(H+)

Even ignoring the a(H+) for the moment, the first two carbonate-like species decline with outgassing, but the pH rises so the OH- increases by exactly the same amount (and H+ decreases -- [OH-]-[H+] is a "net" excess of alkalinity from pH; the reason a(H+) activity is used is that the titration pH test measures the activity of H+, not its concentration, though they are pretty close to the same in pool water). It is as if each CO2 that leaves is the same as removing H2CO3 (CO2 + H2O) from the system and this takes away 2H+ and one CO3(2-), the latter counting double for alkalinity so there is no net change.
[END-EDIT]

Believe me, this is one of the most counter-intuitive things I have encountered in chemistry and it still doesn't "feel" right to me, but it seems to be true as measured by the titration alkalinity test.

Richard

[Rangeball]

N=1, but in my and my pool's experience when I was aerating ALK rose along with PH until I got the ALK below 200. Even then, ALK only drops after I add the acid and let PH drop THEN aerate, and ALK remains constant.

[chem geek]

N=1, but in my and my pool's experience when I was aerating ALK rose along with PH until I got the ALK below 200. Even then, ALK only drops after I add the acid and let PH drop THEN aerate, and ALK remains constant.

Did you phrase that correctly? When you are aerating, your alkalinity rose along with pH? Or did you see it drop?

Before you got started with the aeration, you may have had a tendency to drift in pH due some acidic or basic process in your pool. It's hard to isolate the different pieces since it is an open system.

Richard