Ph Won't Budge From 7.9-8.0

[KenM]

I guess I am also a newbie since I haven't figured out the chemistry. 400 gal Hot Springs Vanguard came with house purchased in 2006. For 3 years we enjoyed it, just used test strips to keep FC, pH and TA in recommended range. I've just had to replace the cover and lifter, and since I now have several hundred $ invested, I decided I would buy a Taylor K-2006 kit and do things right. Kit arrived about a week after refill. Measured levels at that point: CH=40, TA=50, pH=7.9. I first used Leisure Time Calcium Booster, aimed for CH of 150 but ended up with 170. I then focused on getting TA up, so added 4 oz sodium bicarbonate (SpaGuard Total ALkalinity Increaser). At this point I measured CH=170, TA=100, pH=8.0. I did the Taylor acid demand test, which said I needed 0.7 oz dry acid to get pH down to 7.5. I added the dry acid (using SpaGuard pH decreaser and measuring with a digital kitchen scale with 0.1 oz resolution), ran all jets for an hour then let sit for an hour. Remeasured pH with Taylor kit and got 8.0 again. I repeated this 3 times, with same result, no change at all in pH, although TA measurement was dropping. Taylor acid demand called for 0.7 oz dry acid to get down to pH=7.5, but adding this much gave NO CHANGE in pH reading. After 4th addition of 0.7 oz dry acid I finally got to 7.9. Three more cycles of adding dry acid I finally got to pH=7.7, this time I added 0.4 oz dry acid. Next day pH reads 7.9 again, with TA back down to starting point of 50 ppm.

What is going on here? I started with pH=8, I have added 5.3 oz dry acid, incrementally over 3 days, pH is at 7.9. While the saturation index is pretty low with these numbers, that pH value seems pretty high.

For the record, I am using Nitro's dichlor/bleach sanitation procedure. My CYA measures 30 using the Taylor K-2006.

Should I be doing something different, or should I live with pH values that high?

[chem geek]

Your TA level was too high and that causes the pH to rise or stay higher. As you have noted, the TA gets lowered from acid addition (and the pH gets lowered as well, but you didn't measure it right away), but your aeration from running the jets causes the pH to rise. The net result is a drop in TA with no change in pH -- basically, this is the TA lowering procedure.

I suggest you keep your TA low at around 50 ppm. When adding chemicals, use the circulation pump but not the aerating jets. You can also consider adding 50 ppm Borates which will also help to keep the pH more stable.

[KenM]

Your TA level was too high and that causes the pH to rise or stay higher. As you have noted, the TA gets lowered from acid addition (and the pH gets lowered as well, but you didn't measure it right away), but your aeration from running the jets causes the pH to rise. The net result is a drop in TA with no change in pH -- basically, this is the TA lowering procedure.

I suggest you keep your TA low at around 50 ppm. When adding chemicals, use the circulation pump but not the aerating jets. You can also consider adding 50 ppm Borates which will also help to keep the pH more stable.

Thanks Chem Geek. If I am using the Pool Calculator correctly, it tells me with TA=50ppm and CH=170, I need pH 7.8 - 7.9 for the lowest CSI. Is this what you are recommending? If I put 50 ppm Borates in the calculation, then the pH needed to minimize CSI goes even higher. Which is more important, keeping the right pH or the minimum CSI?

Also I'm curious why Hotsprings recommends TA in the 125-150 ppm range, and quite a few suppliers recommend the 80-120 ppm range.

[chem geek]

The pH is most important. The saturation index is important in pools and spas that have plaster/gunite/grout exposed to the water and its somewhat important for fiberglass when a calcium carbonate based gelcoat is used. Most spas don't usually have such surfaces. Some believe that a saturation index closer to zero helps prevent metal corrosion (though that isn't necessarily true as described here) while others have noted that some pumps may contain ceramics that may do better with water more saturated with calcium carbonate. If you wanted the saturation index to be higher, you could target a somewhat higher pH such as 7.7 or 7.8, but I wouldn't go much higher than that. You can increase the Calcium Hardness (CH) if you want the saturation index higher. To minimize the potential for scaling in the gas heat exchanger, target a CSI of around -0.2. So if you do keep your TA at around 50 ppm and use 50 ppm borates as well, then you could have a CH of 250 ppm to get a CSI near -0.2. I would wait to increase the CH until after you are sure you can maintain a stable pH that doesn't get too high as you really do want to avoid scaling in the heater.

Most spa manufacturers recommend a higher TA level because 1) it's the only pH buffer in the system (i.e. there are no Borates) and 2) they recommend using Dichlor which is net acidic when accounting for chlorine usage/consumption. So if you were to use only Dichlor, then you are effectively adding acid regularly to your spa so having a higher TA has the pH be more stable with the net result of a slow drop in TA over time. However, when using the Dichlor-then-bleach method, when you are using bleach then the pH will tend to rise from carbon dioxide outgassing and this happens faster when the TA is higher (or the pH is lower).

Richard

[quantumchromodynamics]

Some believe that a saturation index closer to zero helps prevent metal corrosion (though that isn't necessarily true as described here)

I think that it's important to note that metal corrosion is an oxidation process where the metal atoms are initially in a zero oxidation state and they lose electrons to become positively charged ions.

The dissolution of Calcium carbonate does not involve oxidation. Therefore, they are two fundamentally different processes. Calcium carbonate dissolves; metal is oxidized.

The Calcite Saturation Index is useful to determine the tendency of water to dissolve calcium carbonate or to precipitate scale. Scale can be various metal compounds such as calcium, magnesium, copper, iron etc. The CSI is relevant to metal ions.

Each metal has to be evaluated specifically to determine its risk of being oxidized. The most important factors in metal oxidation are pH and oxidizers, such as chlorine and dissolved oxygen.

[chem geek]

The Calcite Saturation Index is specifically for calcium carbonate (and even more specifically for the calcite form which precipitates or scales first) and not for magnesium, copper or other metals. The index is calculated from the equilibrium constant of calcite with calcium (not other metals) and carbonate ions. I think that some believe that a thin layer of scale from slight over-saturation can form on metal surfaces protecting them from corrosion, but as the link points out this is far from widely accepted. Nevertheless, you often hear those in the pool/spa industry talk about a low saturation index being "corrosive" to metal. As you point out, it's low pH and high oxidizer levels (including dissolved oxygen and active chlorine) that are most relevant for metal corrosion.

[KenM]

Back to pH: what's going on with Taylor Acid Demand test?

Thanks for the very useful information, this is helpful to me in my struggle to settle on a balanced tub. I made a couple more dry acid additions and managed to get pH down to 7.5. It is still tending to creep up, but now between 7.5 and 7.7 which makes me a little more comfortable.

Now I am curious why the Acid Demand test does not give me a useful suggestion for lowering pH. It repeatedly suggested 0.7 oz of dry acid would get the pH down to 7.5 in my tub, and I never found that to work. In the end it took 8 to 9 times that amount to get pH to 7.5. BTW, after the last 3 additions of dry acid I measure no change in TA (to the resolution of the Taylor kit), it remains at 50, but pH is now responding.

[Nitro]

Back to pH: what's going on with Taylor Acid Demand test?

Thanks for the very useful information, this is helpful to me in my struggle to settle on a balanced tub. I made a couple more dry acid additions and managed to get pH down to 7.5. It is still tending to creep up, but now between 7.5 and 7.7 which makes me a little more comfortable.

Now I am curious why the Acid Demand test does not give me a useful suggestion for lowering pH. It repeatedly suggested 0.7 oz of dry acid would get the pH down to 7.5 in my tub, and I never found that to work. In the end it took 8 to 9 times that amount to get pH to 7.5. BTW, after the last 3 additions of dry acid I measure no change in TA (to the resolution of the Taylor kit), it remains at 50, but pH is now responding.

Forget trying to set pH directly. Instead you need to adjust TA to the level at which pH stays stable. Read my links below for more info. If your pH is stable in range, your TA should be good for now. Just keep an eye on pH and adjust TA as needed.

[KenM]

Still curious about the Taylor test

Thanks Nitro, the articles are great! I had found them, read them and have been following your advice the past month or so, in fact did your decontamination process, tested chlorine demand, and am using your dichlor/bleach method. My pH is much more stable with TA=50 ppm.

I remain curious about the acid demand (or base demand) test. Does anyone find them useful? Do they only work in a certain range of TA? Taylor's testing guide book states "correcting pH is effortless through the use of either the acid or base demand test." Is this just hype?

[PaulR]

I've used the acid demand test on occasion but mostly I rely on the Pool Calculator.

Your procedure mentioned above was to add acid, run all jets for an hour, let sit for an hour. If "all jets" includes air or turbulence, that will tend to raise pH. What happens if you add the acid, run circulation only for 10-15 minutes, and then recheck pH?

--paulr