Can't keep pH up!

[matpat]

Hello everyone,

I'm new to the forum and we inherited a 15,000 gal fiberglass pool with our house purchase last year. The pool has been great except for the fact that I cannot keep the pH anywhere near the 7.2-7.6 range I see recommended. I am worried about the heater failing due to the low pH readings I get.

The pool has a sand filter (6-7 years old and suspect for the low pH), heater, and I use tri-chlor tablets. I only "shock" the pool after pool parties which seems to be about monthly with Leslie's Fresh & Clear (potassium monopersulfate I believe) after pool parties. I used Calcium Hypochlorite last year but grew tired of the precipitant and the need to vac the pool after every "shock".

I keep planted aquariums and am very familiar with testing methods and know just enough chemistry to be dangerous :) All of the below tests were ran with Lamotte kits (all in date) with the exception of pH and TDS tests which were run with a freshly calibrated Hanna meter (accurate to 0.01 pH) Here are the results of the tests I ran tonight:

pH - 6.66 @ 72.7F (7.2 with LaMotte Pool Kit)
TDS - 1127ppm @ 72.7F
TA - 116ppm
FC - 1ppm
TC - 1.5ppm
Total Hardness - 228ppm
Ca - 144ppm
Mg - 84ppm

The only test I don't have a kit for is CYA. I suppose I could get it tested at the pool store tomorrow but I don't put much trust in them :)

Anyways, I'm open to suggestions as to how I can get my pH to keep from dropping. The sand filter will be changed out next spring unless I get recommendations to do it this fall.

I have went through nearly 80lbs of soda ash over the past two summers. I frequently add up to 4 lbs of soda ash to get anywhere from a 0.2 to 0.4 rise in pH but it is usually gone within a day or two. I know that volume of soda ash should raise the pH more, but it doesn't.

Just for an example, I had a pH of 7.0 this morning (LaMotte kit) so I added 2 lbs of Soda Ash. The pool filter ran all day (it runs 24/7 while the pool is open) and the pool sweep (Polaris 280) ran for about 2 hours after broadcasting the soda ash. I should have seen more than a 0.2 rise in pH after adding 2 lbs of Soda Ash!

Thanks and sorry for the lengthy first post but I wanted to give as much info as I could!

[doggie]

I'll bet those chlorine tabs are the culprit. For years, I had the exact same problem as you and I'd be dumping in tons of PH increaser constantly all season. The moment I stopped using the pucks and went to bleach, the PH stablizied. I've been on bleach almost 4 weeks and I haven't added PH increaser one time!

[duraleigh]

Doggie has the answer.

[chem geek]

Though the use of Tri-Chlor tablets would certainly seem to be the problem, and I used to have to add Soda Ash regularly when I (foolishly) used Tri-Chlor myself, something in your numbers just doesn't add up.

The Tri-Chlor tablets I used were 3" round and weighed 8 ounces. I know that some are 6 ounces so factor this into this discussion accordingly. Adding one 8 ounce Tri-Chlor tablet (and having it almost fully dissolve) into your pool size of 15,000 gallons adds 3.66 ppm of chlorine (FC) and 2.2 ppm of CYA. The amount that pH is lowered is a function of your starting pH and TA levels. If you were at a TA of 100 and a pH of 7.5, then your pH would drop by 0.07 while at a TA of 116 and a pH of 6.66 your pH would drop by only 0.04 instead. How many Tri-Chlor tablets do you use per unit time (per week, for example)?

Also, adding 4 pounds of soda ash to a TA of 116 and pH of 6.66 would increase the pH by 0.25 and the TA by 30.2. It would take 25 Tri-Chlor tablets plus the outgassing of carbon dioxide representing about 9.9% of your carbonate in your pool in order to get back down to the original pH and TA (given your low pH, it is likely you are outgassing a lot of CO2). You see, if you are truly using Soda Ash, then this is Sodium Carbonate (check the ingredients on your Soda Ash, or pH Up, to be sure) and the increase in TA will eventually outgas as carbon dioxide and this will cause the pH to rise, so the Tri-Chlor needs to lower the pH even more than you might think if BOTH the pH and the TA are to get restored to their original levels.

So though it is certainly true that Tri-Chlor is acidic, I would guess that something else acidic is also entering into your pool. Does it rain a lot where you are? Rain can sometimes be quite acidic. The shocking with Potassium Monopersulfate (MPS) is acidic. If you use 1.5 pounds of shock, then starting at a pH of 6.66 and with TA of 116 the MPS shock would lower your pH by 0.04 and your TA by 2.9. However, adding this once a month does not fully explain your situation.

If you switch to using liquid chlorine, please report back to see if you still have to add soda ash, even if it's not as much, since normally you shouldn't have to add any at all when using liquid chlorine or bleach.

[EDIT] If I use 4 pounds of Borax instead of Soda Ash in my calculations, then it only takes 5.5 Tri-Chlor 8 ounce pucks to compensate and no outgassing of carbon dioxide is needed. That sounds more reasonable, but you said you were adding Soda Ash and not Borax. [END-EDIT]

Richard

[matpat]

I'll bet those chlorine tabs are the culprit. For years, I had the exact same problem as you and I'd be dumping in tons of PH increaser constantly all season. The moment I stopped using the pucks and went to bleach, the PH stablizied. I've been on bleach almost 4 weeks and I haven't added PH increaser one time!

After reading everyone's replies yesterday, I turned off my tablet feeder and started to aerate the pool just for the fun of it :) I don't know if my low pH (6.6) is acidic enough to outg*** any CO2 but a few minutes ago I tested my pH and it has risen to 6.9 Not bad for really doing nothing at all! I think I will test my TA and see if it has changed any...


Though the use of Tri-Chlor tablets would certainly seem to be the problem, and I used to have to add Soda Ash regularly when I (foolishly) used Tri-Chlor myself, something in your numbers just doesn't add up.

The Tri-Chlor tablets I used were 3" round and weighed 8 ounces. I know that some are 6 ounces so factor this into this discussion accordingly. Adding one 8 ounce Tri-Chlor tablet (and having it almost fully dissolve) into your pool size of 15,000 gallons adds 3.66 ppm of chlorine (FC) and 2.2 ppm of CYA. The amount that pH is lowered is a function of your starting pH and TA levels. If you were at a TA of 100 and a pH of 7.5, then your pH would drop by 0.07 while at a TA of 116 and a pH of 6.66 your pH would drop by only 0.04 instead. How many Tri-Chlor tablets do you use per unit time (per week, for example)?

I use probably 6-8 of the 3" tablets per week to keep my chlorine levels in the 1-3ppm range. Some weeks I probably add a few more and some weeks I probably add less. I've never really paid that close attention to it but went on the recommendation of the guy who opens/closes my pool every year (not that he was an expert but at the time I figured he knew more than me). I'm not sure of the weight of my tablets...***uming these are 8oz tablets, I have been adding ~15-20ppm of Cl and ~9-12 ppm of CYA per week. According to these numbers, my CYA should be through the roof by now! I will see about getting it tested in the next day or two and report back.


Also, adding 4 pounds of soda ash to a TA of 116 and pH of 6.66 would increase the pH by 0.25 and the TA by 30.2. It would take 25 Tri-Chlor tablets plus the outg***ing of carbon dioxide representing about 9.9% of your carbonate in your pool in order to get back down to the original pH and TA (given your low pH, it is likely you are outg***ing a lot of CO2). You see, if you are truly using Soda Ash, then this is Sodium Carbonate (check the ingredients on your Soda Ash, or pH Up, to be sure) and the increase in TA will eventually outgas as carbon dioxide and this will cause the pH to rise, so the Tri-Chlor needs to lower the pH even more than you might think if BOTH the pH and the TA are to get restored to their original levels.

I am adding Soda Ash (sodium carbonate) to raise the pH. It seems to raise the pH pretty close to the numbers you mentioned, giving some leeway to test kit inaccuracy and color judgement. I will contineu to use the Hanna meter since there has to be some innacuracy in the color comparitor with the Taylor/Lamotte kits. The meter is accurate to 0.01pH :)


So though it is certainly true that Tri-Chlor is acidic, I would guess that something else acidic is also entering into your pool. Does it rain a lot where you are? Rain can sometimes be quite acidic. The shocking with Pot***ium Monopersulfate (MPS) is acidic. If you use 1.5 pounds of shock, then starting at a pH of 6.66 and with TA of 116 the MPS shock would lower your pH by 0.04 and your TA by 2.9. However, adding this once a month does not fully explain your situation.

We have had a bit of rain this year but I had the same problem last year! Last year I was adding water to the pool on a weekly basis to keep the levels up. I have only had to add water twice this year due to rain. As far as the MPS, I use 2lbs per shock when I use it. I figured it was best to use the extra half pound since I have 2 young children running around and someone is bound to get inot it. Ignorance is bliss isn't it. I am out of the shock now and don't plan on purchasing anything else from the pool store, now that I have found this forum :)


If you switch to using liquid chlorine, please report back to see if you still have to add soda ash, even if it's not as much, since normally you shouldn't have to add any at all when using liquid chlorine or bleach.

I purchased 6% bleach today along with some 20 Mule Team Borax and Baking Soda. It should be much cheaper than using Pool Chemicals and everythig has a household use as well. I will report back on my findings in a week or two. given the time of year, I probably won'tbe able to see much of a result before we close the pool in the next couple of weeks.

You did say above that something doesn't add up in my numbers so let me throw this out there for you...

According to what info I could find, the City of Dayton "softens" the source water with "lime" and I ***ume they are using "slaked lime" or Calcium Hydroxide. It seems to work well and according to my Comprehensive Water Quality Report, the Calcium Hardness is reduced from ~220ppm to ~70ppm, the Magnesium Hardness is reduced from ~190ppm to ~60ppm and TA is reduced from ~270ppm to ~70ppm. This process also removes about 25ppm of CO2 from the source water. I ***ume the use of Calcium Hydroxide precipitates the Ca and Mg from the water supply and it is then filtered out?

On top of that they also add 6ppm of Phosphate Alkalinity, I ssume to raise the pH of the water in the distribution system. What effects, if any does all of this have on my pool water? I know, probably not an easy question to answer but it has given me fits trying to determine CO2 levels in my Aquariums and I figure it just may have some relevance here also.

Thanks to everyone for their help thus far and I will report back my findings ouver the next week or two.

[matpat]

I think I will test my TA and see if it has changed any...[QUOTE]

My TA tested at 104ppm this morning. It has dropped 12ppm with the aeration and acidic (6.6 pH) water.


[QUOTE=matpat;37349]I purchased 6% bleach today along with some 20 Mule Team Borax and Baking Soda.

I added half the box of Borax and 5L jug of bleach to the pool yesterday and the other half of the box today along with another 5L jug of bleach to the pool about noon and here are my results as of a few minutes ago:

TA - 116 ppm
pH - 7.13
FC - 1 ppm
TC - 1 ppm
CC - 0 ppm

It appears that the Borax raised my TA by the 12ppm it hd prevfiously been lowered and it also raised my pH by about 2. Hopefully these numbers seem about right as I have not done the calculations :)