Does Alkalinity Cause pH To Increase Faster?

[jereece]

Just an update. My TA has now dropped to 50 and I can't be happier. My pH upward drift has slowed to a crawl and the amount of acid to bring it back down is at least half what it used to be. I have checked my pH after each nightly addition of bleach (after an hour of recirc time) and see no increase in pH. I have been running a pH of 7.4 for a week now. I am much happier.

My plan for the rest of the season to let TA continue to trend down (if it does) and see what happens. I will particularly watch for pH swing problems and if so note at what TA that happens.

This is very interesting and I will keep you informed.

Thanks,
Jim

[chem geek]

Jim,

For the most extremes in pH, measure the pH just before you add chlorine, presumably at night. Then measure the pH about an hour after you've added the chlorine (with the pump running during that hour). This would be one of the extremes of pH. Another time to measure would be soon after a heavy bather load since that can sometimes lower pH. Another time would be after a rain storm (if it rains in the summer where you live).

Richard

[medvampire]

Jim
Thanks for the up date. It would be great if you do let us follow your restults and plot at the end of the year.
Steve

[jereece]

My Alkalinity is holding fairly steady at 50ppm. I have been taking notes about how adding bleach initially affects the pH. My pool is about 19,000 gallons. Adding one 3/4 gallon jug of 6% bleach adds about 2.4 ppm chlorine. I ran each test adding bleach after sunset and allowing to mix with the pool pump on for about 2 hours. I ran the tests several times with a starting pH range from 7.4 - 7.8.

In a nut shell, adding one jug of bleach to my pool at 50ppm Alk causes an initial pH increase of .2 or slightly less. I ran a few tests adding 2 jugs which would add 4.8 ppm chlorine to my pool. Adding 2 jugs resulted in an initial pH increase of .4 or slightly less. I say slightly less because some times the color did not look like it was fully developed to the mark on my Taylor comparitor. After 24 hours, my pH was back down to where it started before I added the bleach.

My in pool results are comparable to what I got in my jug tests. So I am convinced that, at least for my pool water, that running low alkalinity is the way to go. I am having to adjust my pH half as often as when I was running alkalinity around 100 and it takes about half as much acid to bring the pH down. So I am using less chemicals and from what I can tell I am getting better results.

If anyone can think of a reason to be concerned, please let me know. Otherwise I plan to continue to run low alkalinity.

Thanks,
Jim

[chem geek]

Jim,

I don't think there is any problem with you running at the lower alkalinity of 50. The main reason to have higher TA would be to prevent pH swings, but you are seeing less of that (due to less CO2 outgassing) so no problem there. You are technically in a more corrosive pool water balance state which normally means either increasing CH or pH to adjust for that, but I think you are fine so long as you don't plan on running at a low pH (much less than 7.4). This only applies to a plaster/gunite pool situation. If you are concerned, you can post a full set of numbers and I can be more specific.

Richard

[jereece]

Thanks Richard. I guess corrosion could be a concern. I have an AG pool. I don't use a heater, but I suppose the pool walls and pump shaft are possible corrosion concerns? Below are my current numbers,

pH: 7.4
FC: 5
TC: 5
CYA: 30
Alk: 50
CH: 10

As noted, I have very soft water, so yes CH is only about 10 ppm. Next season I plan to replace my pool liner, so I will be draining & refilling. At that time, I may use Calcium Hypochlorite for a while to get the CH up some.

Also, how would adding salt affect this? Next year I may add salt after reading some dicussions on the board. Would adding salt just put me in a more corrosive environment? If I continue to run at low Alkalinity, is there anything I should do to minimize corrosion?

Thanks again for your insight and suggestions.

Jim

[chem geek]

You are fine so long as you don't have any exposed grout in tile or anything like that -- it sounds like there isn't any and of course there is no plaster/gunite. So "corrosion" in the sense of dissolving calcium carbonate from plaster is not possible for you (aren't your pool walls vinyl?). As for "corrosion" of metal, that is very unlikely at your pH so long as you keep it well above 7.0 (7.4 is fine). You'd have to have a heck of a lot of dissolved oxygen in your pool to corrode metal at this higher pH so I wouldn't worry about that. For a little extra safety, you can do as you suggest and add some more calcium by using Cal-Hypo for a while, but I don't think that's necessary.

Technically, the fact that you've been running with virtually no calcium means that the TA level doesn't really matter -- you are already corrosive for calcium carbonate, but that isn't relevant for vinyl pools with no grout/plaster/gunite exposed to water.

As for salt, it is technically more corrosive for metal since it increases conductivity, but again this isn't a problem in your situation because you don't have a lot of dissolved oxygen in your water and you are keeping the pH alkaline (> 7.0). If you had some sort of iron parts exposed to your pool, then slow rust could happen, but generally any metal designed to be exposed to pool water is either stainless or galvanized steel which resists corrosion or is copper (in heaters) which doesn't corrode as readily as iron (raw steel).

Richard