very high alk-very low ph

Problems relating to pH and total alkalinity.
Increase ph, increase TA. Reduce pH, reduce TA.
pH chemistry advice and techniques for the pool.
barhip

very high alk-very low ph

Postby barhip » Sun 02 Sep, 2007 11:53

our pool - 25,000 gal in ground vinyl lined, has struggled all sumerr with high total alk. and very low ph. i was told to first adjust the total alk and then try to get the ph up. every time i have maintained the right alk. level the ph still wants to be low. when i add the ph + it just raises the tot alk again and i'm back to where i started. the water gets real milky cloudy. i have added muriatic acid each time to get the total alk down. what else can i do to raise the ph and not the total alk?
barb


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Postby mr_clean » Sun 02 Sep, 2007 13:51

read the topic in this same section by chem geek who explains how to do this by aeration. it's under really HIGH alkalinity for a long time....
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Postby chem geek » Mon 03 Sep, 2007 02:12

Actually, the more usual problem with high TA is that the pH tends to rise. So in your case since your pH is usually low, I suspect you are using an acidic source of chlorine such as Trichlor (or possibly Dichlor). With Trichlor, you actually do want the TA to be higher, usually at least 120 ppm.

I also suspect that you have lower than usual aeration in your pool. Do you use a pool cover? If so, then a combination of Trichlor plus a pool cover would tend to lower pH rather quickly and you'd need to add pH Up (sodium carbonate or Arm & Hammer Super Washing Soda) regularly, and yes, the TA will go up with this. You can also raise the pH without raising the TA as much by using 20 Mule Team Borax (another option is to use Lye aka Caustic Soda). The only way to raise the pH without raising the TA is to aerate (keeping the cover off, running waterfalls, spillovers, fountains, pointing the returns up, etc.).

Do you have a full set of water chemistry numbers? How high is the high TA and how low the low pH?

Let us know more about your pool and we can figure something out that works for you. If you've been using Trichlor, you should check your CYA levels to make sure they don't get too high or consider using an algaecide if you don't want to switch to unstabilized chlorine.

Richard
barhip

Postby barhip » Tue 04 Sep, 2007 12:37

thank you for your suggestions. yes - i do use the trichlor tablets. the pool is usually covered. the ta was above 200 ppm - i quit dropping at 20 drops because i knew i had to get it down. we purchased some 20 mule team and will use it instead of the baking soda. i do not have a way to measure the cya - our test kit does not show that. the ph was below 6.8 - that is as low as the number goes on our test kit. we have used algaecide when needed. i have tried to maintain the higher ta count because of the type of tablets but as i stated before - i would get the ta around 140 and try to increase ph by adding ph+ and up goes the ta to 180-190. then the milky cloudy water came back - happened once before when ta was real high and ph was real low. now that it's clearing up it's going to be time to close it. thanks again for your help
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Postby chem geek » Tue 04 Sep, 2007 14:18

If you are using only Trichlor as your source of chlorine, you should have your Cyanuric Acid (CYA) level checked since as that gets higher you will need a higher Free Chlorine (FC) level to keep away algae or you'll have to use an algaecide (weekly PolyQuat 60; or 30-50 ppm Borates; or Copper ionization, etc.). For every 1 ppm FC added by Trichlor, it also adds 0.6 ppm to CYA.

Keeping the pool covered is preventing the normal outgassing of carbon dioxide which would have the pH rise to compensate for the pH drop from the Trichlor. That's how this works in most pools using Trichlor. Of course, having a cover has other benefits (heating the pool; keeping junk out of the pool).

You might consider using another source of chlorine other than Trichlor. Bleach or chlorinating liquid will be far more stable in pH -- especially with a covered pool. Cal-Hypo would be similarly stable, but will increase Calcium Hardness (CH) over time. For every 1 ppm FC added by Cal-Hypo, it also adds 0.7 ppm to CH. In spite of what the pool industry thinks, these hypochlorite sources of chlorine are actually pH neutral -- they increase pH upon addition, but then the consumption of chlorine decreases pH and you end up where you started. The observed increase in pH in some pools using hypochlorite is from the carbon dioxide outgassing and with your pool covered most of the time, you won't see that.

I only use chlorinating liquid in my own pool which is covered when not in use with an opaque safety cover. The pH is rock solid stable. I can also get away with adding chlorine every 3 days instead of every day because the cover keeps out the sun's UV rays.

Richard
barhip

Postby barhip » Tue 04 Sep, 2007 21:42

our chlorinator is obviously attached to the pump and we put our tablets in it - how would we go about trying the liquid chlorine? can you use something like that with a chlorinator?
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Postby chem geek » Wed 05 Sep, 2007 01:10

You pour it slowly over the return flow usually at the deep end of the pool. If you want something more automated, take a look at The Liquidator . Several people have used this and love its convenience.
barhip

Postby barhip » Wed 05 Sep, 2007 21:59

can you help me again with aerating the pool? you said this will help increase the ph w/o affecting the ta. i finally have the ta around 150 ppm -today is wed. the chlorine level is good and the cya count is good. the ph level is still showing 6.8 or below. i ran a pump/waterfall into the pool for a couple of hours tonight. i will again check the ph tom. am. i don't understand how this aeration will increase the ph but not the ta - they seem to work hand in hand.
Guest

high chl in pool

Postby Guest » Wed 12 Sep, 2007 13:28

wat can i do for high chlorine in a pool over 10ppm.
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Postby chem geek » Wed 12 Sep, 2007 16:14

barhip wrote:can you help me again with aerating the pool? you said this will help increase the ph w/o affecting the ta. i finally have the ta around 150 ppm -today is wed. the chlorine level is good and the cya count is good. the ph level is still showing 6.8 or below. i ran a pump/waterfall into the pool for a couple of hours tonight. i will again check the ph tom. am. i don't understand how this aeration will increase the ph but not the ta - they seem to work hand in hand.

Aeration drives carbon dioxide out of the pool. Pools are intentionally over-carbonated in order to provide a pH buffer and in plaster pools to help saturate the water with calcium carbonate. That's what adding Alkalinity Up (sodium bicarbonate) and pH Up (sodium carbonate) do -- both add carbonates to the pool which is a form of dissolved carbon dioxide in water.

When carbon dioxide leaves the pool (because there is less of it in air then there is in the water), it essentially converts carbonic acid, H2CO3, into water H2O, and carbon dioxide, CO2. Removing carbonic acid increases the pH because two hydrogen are removed from the system and this shifts the other components of the carbonate equilibrium (the negative logarithm of the concentration of hydrogen ion is the very definition of pH). The reason that the TA doesn't change is more subtle, but essentially by removing carbonic acid, one removes two hydrogen, H+, and one carbonate, CO3(2-) and these exactly cancel each other out in terms of TA so there is no net change in TA.

You are right that when adding a pure acid or base to a pool that pH and TA will move together. The outgassing of carbon dioxide does two things at the same time -- it removes an acid from the pool (carbonic acid) which by itself would raise the pH and the TA, but outgassing carbon dioxide also removes carbonates from the pool which reduces TA exactly compensating for the normal rise in TA that comes from rising pH.

Richard

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