pH keeps creeping up

[frankcav]

Since opening the pool this year I need to add muriatic acid weekly (about 1 to 2 quarts) to keep the pH at 7.2-7.4 range or else it tends to gravitate to 7.8-7.9. I don't recall adding this much acid last year. My current numbers are:

FC 3.0
CC 0
pH 7.4 (after adding HCl today)
TA 70
CYA 30
CH 40

Inground vinyl 24,000 gallons

My question is would increasing the TA to around 100 or adding a second buffer like borates to about 50 help stabilize the shifting pH.


Thanks in advance,

Frank

[KurtV]

Increasing the TA would exacerbate your pH rise problem. Lowering your TA to 60 or even 50 will help some and the borates may help also. See the "Great Tetraborate Experiment" thread for more on that.

[frankcav]

Thanks for the input... will probably add borates.

Frank

[Jakebear]

I'm having the same problem (pH Creep). DH says it is normal since we add bleach automatically (pump) but I read one of Richards Posts about the chemistry where he showed that the chlorine reactions caused negligible change in pH (I think the thread was titled "Rising pH levels"). Anyway I've been adding about 15-20 oz per day to keep it in range.

Here's my numbers:

Water temp 85 F
FC 3.0
CC 0.0
TA 100
pH 7.6 (added 11 oz Muriatic yesterday)
CYA 22
CH 140 (but who cares with a vinyl liner)

Bleach Pump add about 20 oz 3% [EDIT: not 6% as originally stated] bleach per hour while circulator pump runs (8 hrs/day)

crystal clear, no algae, no problems, just have to dump acid every day:mad:

I'd like a better explanation if there is one.

[chem geek]

My question to Frank and Jakebear would be whether you have any water aeration features in your pools, such as waterfalls, spillovers (negative edge or pool/spa combo), fountains, showers, aerating jets, etc.? Also, Jakebear uses a bleach pump, but what source of chlorine does Frank use (SWG or bleach/chlorinating liquid)? Are you having a lot of rain (the splashing of raindrops provides a lot of aeration)? Are there any chemicals you are adding to your pools other than chlorine and acid?

As was pointed out, increasing the TA level only makes the problem worse and would require you to add more, not less, acid over time. With your vinyl liners, you can lower your TA level to 50 ppm without a problem and should find some improvement. Also, setting a target pH of 7.7 instead of trying to fight to maintain a lower pH will also help (for Frank, I would try that before lowering the TA further from 70; for Jakebear, you can certainly lower your TA to 70, see how it is, or lower it further to 50 as needed). Turning off water aeration features (if possible) will help. Using a pool cover should also help.

Adding Borates should reduce the frequency of acid addition (i.e. it will take longer for the pH to rise), but will not change the amount of acid you need to add over time (i.e. you will need to add more acid less frequently for the same net acid/time result to bring the pH back down). The main advantage of the Borates is that it gives you an additional pH buffer so that you can lower the TA level even further than you would normally do if the Borates weren't present. Note, however, that the borates do not have much capacity to handle a drop in pH so with low TA you need to be careful about any acid addition beyond that which you carefully monitor -- for example, CYA is acidic and Trichlor is very acidic.

This chart (http://richardfalk.home.comcast.net/pool/CO2.htm) shows the relative carbon dioxide outgassing rates at various TA and pH levels. This is proportional to the quantity of acid you need to add per time. It is not an absolute amount since that depends on the amount of aeration, whether the pool is covered, wind, etc. This chart (http://richardfalk.home.comcast.net/pool/CO2pH.htm) shows the relative rate of pH rise at various TA and pH levels. Though a lower TA significantly lowers the rate of outgassing, it is also less of a pH buffer so these somewhat balance out though the net result is a lessening of pH rise. You can see that going from 70 to 50 will help just a little, but maintaining a higher pH of 7.7 helps even more.

Richard

[Jakebear]

Hi Richard, We love your commentary an knowledge.

DH is fcfrey, you two have kicked around a lot of emails over the last year and we have learned a lot from you (and others too).

We have a fresh water pool with a pump like they use on a dialysis machine. He rigged it so the pump runs whenever the pool pump runs. It works well but he says he might have to add an acid pump to it??????

Any way we have had about 2" rain as thunderstorms over the last 2 weeks which has kept up with evaporation and rug rat splash out. Since we have a DE we do not back flush very often (just bump). There are no fountains etc. We have not added anything other than chlorine and acid for 6 or 8 weeks except a bit of CYA (DH wants it up to around 30 to try to cut chlorine consumption a bit)

We do not have trouble with TA ---- Just the pH. It's not a big deal but since I don't have to lug jugs like last year -- maybe I'm getting lazy. Either way all last year on BBB (before the chlorine pump) we never had to add acid so I'm wondering ------ was-sup????

[Jakebear]

DH informed me I showed an incorrect amount of bleach being injected. It is actually only about 10 oz of 6% --- the pump pumps 20 oz/hour but the concentration is only 3% since he cuts the 6% 50/50 with pool water. Thought that might be important to correct. He says it amounts to about 1.8 ppm per day loss???

[chem geek]

Jakebear,

A daily 1.8 ppm FC loss in a 27,000 gallon pool is not at all unusual nor high. In fact, with the CYA as low as 22, the loss could readily be higher if your pool gets exposed to more direct sunlight. I used to think that running a pool at the lowest CYA you could while keeping the FC at the appropriate minimum level was the way to go, but recent analysis and experiments (starting around here (http://www.troublefreepools.com/viewtopic.php?p=4482#4482) in a thread in the TroubleFreePool forum) has me now believe that higher CYA protects chlorine more than the traditional industry chart would indicate. So maintaining a higher CYA with higher FC should actually result in a lower chlorine loss so I not only agree with dcfrey, but you could go even higher and probably see even more benefit (seems almost weird to give advice that is 180 degrees from what it used to be, but I can't argue with reality). The CYA to FC ratio should be around 8.6 as a target (or lower) which puts it in the middle of Ben's Min/Max columns and corresponds to the 0.05 ppm disinfecting chlorine level in this chart (http://richardfalk.home.comcast.net/pool/FC.htm).

Of course, I've avoided your rising pH issue and we now need to tackle that. There is no question that the rain would lead to a LOT of pH rise, but it sounds like you may have been experiencing this rise even before the rain hit. Perhaps the TA level is higher than last year, or there is more wind, or something else is going on. I doubt very much that the manual vs. automatic chlorine injection has anything to do with it unless dcfrey is secretly adding base to your pool water / chlorine mixture :D . Seriously, I think the best thing to do is to significantly lower your TA since we know that will help a lot with the total amount of acid, though may not help as much with the quantity of pH rise. So, first lower your TA a lot -- to at least 70 via Ben's Lowering Your Alkalinity (http://www.poolforum.com/pf2/showthread.php?t=191) process and see if you notice an improvement. If you do, you could lower it even more down to 50. You should also try and target a higher pH such as 7.7 and see how long it takes to move up to 7.8-7.9. If you find the amount of acid per time reduced, but the rate of pH rise is still unacceptable, then you could add the 50 ppm Borates which should slow that rise down though won't change the amount of acid you need to add.

One step at a time -- try the lower TA first since it's not hard to do and is easy to change back if for some reason you felt you needed to.

Richard

[mariner09]

I've also been experiencing pH creep. Over the last week, it's been hovering up around 7.8 - 8.0. To try something, I bought a bottle of pH down at Wal-Mart. I added about 24oz and got it down to 7.6. A few days later it was back up. Just finished the bottle this morning, 3 lbs of pH down in less than a week???

My TA is around 160.

I guess my bigger question is, is it OK to keep the pH up around 7.8?

We have had some big storms last week and more wind this week than usual, nice cold front came through in the middle of the week.

Come to think of it, one of my jets is pointed towards the surface and I have seen bubbles, maybe I need to lube the O-ring on the pump and re-prime..

[Amir]

Richard,
I can not access the link to the charts. Did you move them?
Thanks
Amir

[chem geek]

mariner09,

Yes, you can keep your pH at 7.8. The only downsides are a slight possibility of being more drying or stinging on the eyes since it's further away from the 7.5 that is around the pH of tears, but even that pH varies by person so I think you'll be fine. The other thing is that chlorine is a little less effective at that pH, but not by as much as the "traditional" industry graphs say because CYA helps to buffer disinfecting chlorine concentration. It's only around 10% less effective, so just bump your FC target up a bit to compensate.

You can lower your TA to around 80 to see if that helps. Then use 7.7 or so as your pH target after you've lowered your TA and I think you'll find a huge drop in the amount of acid you need and some drop in the rate of pH rise.

Amir,

No, I didn't move any of the charts and the link works for me right now and I don't have it protected in any way (i.e. it's visible throughout the world). Do you get an error message like it's a broken link? Does anyone else see that problem? Here are some links to various charts and graphs:

Free Chlorine Chart (http://richardfalk.home.comcast.net/pool/FC.htm)
Disinfecting Chlorine Chart (http://richardfalk.home.comcast.net/pool/HOCl.htm)
Outgassing CO2 Chart (http://richardfalk.home.comcast.net/pool/CO2.htm)
Rate of pH Rise Chart (http://richardfalk.home.comcast.net/pool/CO2pH.htm)
Chlorine Half-Life Graph (will be changed in future) (http://richardfalk.home.comcast.net/pool/HalfLife.gif)
Disinfecting Chlorine Graph (http://richardfalk.home.comcast.net/pool/HOCl.gif)

Richard

[Amir]

Richard,
your links don't work! Would you please repost the links.
Regards

Amir

[fcfrey]

Richard / Amir

I just tried all of the links --- They work just fine. The first 4 are statistical charts not graphical but if you are skilled in Excel you could convert them if you wanted to. The last 2 links are logrithmic graphs.

All these should be Printed, Framed and kept where you can refer to them regularly. They are EXTRE E E E E MLY useful.

Quick related question for Richard, does aeration reduce pH along with TA?

Frank

[Amir]

Frank,
You are right! they work in internet Explorer but not in Firefox! I am not a big fan of microsfot so Firefox is what I used first and tried IE after your post.
Regards

Amir

[chem geek]

Amir,

I use Firefox on the Mac and the links work fine so this must be something specifically wrong with Firefox on a PC -- very strange.

Frank,

Aeration only causes the pH to rise by the outgassing of carbon dioxide. It does not change the TA. When you add acid, then that lowers both pH and TA so the combination of the two is a net result of lowering TA. Carl explains this here (http://www.poolforum.com/pf2/showthread.php?t=6746) and I have a chart about this here (http://www.poolforum.com/pf2/showthread.php?p=46352).

Richard

[fcfrey]

I don't know why I said "lower pH" I knew that it didn't do that. DAH!!:o
Thanks Richard,

I checked my pool test log spreadsheet from last year and the year before that and I see where we had a very minimal amount of pH rise last year unless there was rain or "rug rat" activity, but then it seemed to stabilize without acid addition. I will watch it for a while to see where it goes.

I will be discontinuing the use of the Bleach pump for a couple of weeks while I use some pucks to get the CYA up to 30. Right now it is below the capability of my test equipment but I believe it is around 20 ppm. If the pool is uncovered all day the FC depletion goes to around 3-4 ppm so I think a bit more CYA will be in order.

Thanks for your thoughts on the pH, after searching and finding a lot of pools getting several pints a day, I do not feel too bad.

By the way, NO!! I do not add anything surrepticiously that Jakebear is unaware of (See post #8).

I value my life :D

[mariner09]

I also use Firefox 2.0 on Linux and the links display just fine.

[Amir]

I run firfox on PC. I even copy and pasted the link
http://richardfalk.home.comcast.net/pool/FC.htm
and got "can not find server"
I can get to his site
http://home.comcast.net/~richardfalk/wsb/html/view.cgi-image.html--logo.html
but not to the links with firefox!!
amir

[chem geek]

Amir,

Just for the heck of it, I duplicated the FC.htm file and changed the name to FC.html so see if this link (http://richardfalk.home.comcast.net/pool/FC.html) is accessible. If it is, then Firefox on the PC requires the extension to be "html" instead if "htm". Let me know.

Richard

[nikonsal]

I am running Firefox v2.0.0.2 on a pc and I am able to pull up the pages that only have the .htm extensions.

[Amir]

Richard,
yes, the link was accessible. you are right it needs html. I guess 90% of the world uses IE so I am used to finding a way around or going to IE when nothing works.
Thanks

Amir

[chem geek]

Thanks Amir. Well, I think this should be logged as a bug against Firefox on Windows. I could change all the HTML files I've uploaded, but would need two versions so as not to break old links. I think getting Firefox fixed would be better, especially since it works on Mac OS X and Linux. Sorry for the inconvenience.

Richard

[cnix]

Richard,

I've been lurking(mostly) here for several years. You and many others have helped this new owner of a 25 yr. old IG plaster pool tremendously.

My pH has always tended to stay at the upper limit of the standard test kit. I usually let it run about 8.0, and don't do anything until it is clearly above 8.2. From memory (because my test results are at home, my TA is about 100-120. I have put about 40 lbs of borax in the pool (30,000 gal.)

This year, though, I have needed much more acid than before to keep it down. I think I'm starting my 7th gallon since opening in early April--more than I usually use in the whole season.

Rain isn't the problem here--East Tennessee is in the middle of the worst drought in 120 years. My refill water, from a well, tests at pH 6.8.

My chemistry degree was 25 years ago, and I no longer work in that field so I'm a bit rusty, but I look up the pKa of carbonate buffer, and find it is 6.35 & 10.33 with a useful buffering range of 6.0 - 8.0 and 9.5 - 11.0. For Borate, the pKa is 9.23 with a useful range of 8.5 - 10.2.

I assume the lower pKa for carbonate is the bicarbonate state, and the higher for the carbonate.

What I can't understand is how a borate addition would _decrease_ the number at which the buffer tends to hold the pH--since its active buffering range is higher than that of carbonate.

What am I missing?

As an aside, can anyone point to current, accurate resources for operating at higher pH levels? I have the info from the older Pool Solutions site, but wondered if there's anything more up to date.

Chuck

[chem geek]

Buffers do not move pH. The pKa numbers for the buffer simply indicate the pH at which half of each of the two species on either side of the pKa equilibrium equation are equal to each other and is the point of maximum buffer capacity in each direction. That's all. It is a myth or misconception that all buffers move the pH to their pKa value. After all, the pKa for the carbonate buffer is 6.3 and 10.3 yet the pH that pools will migrate towards depends on the TA level. If the TA is 100, then outgassing to equilibrium with the air will result in a pH of about 8.45 while at a TA of 19 (with CYA of 30 so carbonate alkalinity of 9) the equilibrium pH is 7.50.

In the case of Borates, they do nothing but buffer and have algaecidal properties. In the case of Carbonates, however, they also DO move pH, but it has nothing to do with their buffering capability. It has to do with the fact that they are over-saturated in the water with respect to air so outgassing will occur and that causes the pH to rise with no change in TA (because carbonic acid, which is dissolved carbon dioxide attached to water, is removed and that doesn't contribute to TA but has two hydrogen that are removed by turning them back into water).

So the Borates do not change the equilibrium state for the carbonates vs. carbon dioxide in air so do not change the tendency for the pH rise directly. They ARE an additional pH buffer so the rate of pH rise will be less, but you will still have to add as much acid as you would have without the Borates. IF the Borates kill algae that chlorine doesn't have to and IF you have an SWG, then turning down the SWG level (since you don't need as much chlorine) will lower hydrogen gas bubble production which lowers aeration which lowers carbon dioxide outgassing which lowers the rate of pH rise and amount of acid needed.

So if you are seeing a greater amount of acid you need to add, this has nothing to do with the Borates since they only affect the rate of pH rise for a given amount of outgassing, but do nothing in changing the amount of acid needed per unit time. For whatever reason, your pool is outgassing more carbon dioxide OR you have some other source of base. 40 pounds of Borax is 18 ppm in your 30,000 pool and of course adding that makes pH rise, but I assume you added acid to compensate for that and added the Borax to increase Borates, right? If you added Borax for any other reason, then it would have increased the pH and you would have needed acid to compensate.

Richard

[Amir]

Chuck,
I don't know if your are aware of this or someone else has pointed it out; if your are buying acid from home depot it is half the strenght of last year variety (15%) so you could be using twice as much acid by gallon than last year! I had the same issue until I read a post regarding lower strength and same price at HD!
Amir

[cnix]

I haven't added any Borax except in March at start up (to replace what was lost with partial drain/refill for the winter).

I do remember (from Gen. Chem. labs, faintly) that buffers reduce the swing of pH for a certain amount of acid/base added, and that they only have much effect within their useful buffering range, usually about 1 pH unit above and below the pKa. I guess what I'm not getting is why borate is of any use as at alL as a buffer at more than 2 pH units away from the pKa. I understand why the quantity of acid needed would be the same with or without the buffer--a buffer that is resisting a rise in pH also resists lowering pH back down.

My understanding is that buffers work by providing a "reservoir" of H3O+ through. If borate doesn't affect pH, then why the common advice here to use it to raise pH?

I'm really not trying to be argumentative, or put you on the spot--I just don't get it.

Chuck

[cnix]

Amir:

Yes, it did come from Home Depot. I was sure the label said the normal 20 Baume (31.45%, I think), just as it always has.

I'm due for a water test tonight, though. I'll check that out this evening, and thanks for the tip.

Chuck

[chem geek]

Chuck,

It is true that buffers work more efficiently within 1 pH unit from the pKa, but it is not true that the don't work outside that range. As I said, having the pH near the pKa means that the buffer has capability in BOTH directions of pH swing. The Borate buffer at a pH of 7.5 is mostly all boric acid and not borate ion. That means it is a good buffer at resisting a rise in pH, but it is not as good and has very little capacity at resisting a lowering in pH.

For example, at a pH of 7.5, the Borate buffer has almost 40 times as much Boric Acid as Borate ion. If I have a TA of 80 and 0 ppm Borates, then if I add 1 cup of Muriatic Acid (31.45% HCl) in 10,000 gallons, the pH gets lowered from 7.5 to 7.29 or a drop of 0.21 in value. If I add 3 ounces of lye, then the pH goes from 7.5 to 7.75 or a rise of 0.25 in value.

If I repeat the above but also have 50 ppm Borates, then adding 1 cup of acid goes from 7.5 to 7.37 or a drop of 0.13 in value. If I add 3 ounces of lye, then the pH goes from 7.5 to 7.61 or a rise of 0.11 in value.

So you can see that the Borates do buffer in both directions, but are a better buffer at preventing a rise in pH. If I repeat this experiment starting with a pH of 9.1 which is the Boric Acid pKa adjusted for ionic strength, then with no Borates, 1 cup of acid has the pH drop from 9.1 to 8.96 or a drop of 0.14 while adding 3 ounces of lye has the pH go from 9.1 to 9.20 or a rise of 0.10 in value. With 50 ppm Borates, 1 cup of acid has the pH go from 9.1 to 9.0773 or a drop of 0.0227 while 3 ounces of lye has the pH go from 9.1 to 9.1198 or a rise of 0.0198 in value.

So yes, the buffers would be more effective if near their pKa, but they still work even away from their pKa. So at a pH of 7.5, the 50 ppm Borates with 80 ppm TA improves buffering of acid by 38% and of base by 56%. At the optimum pH of 9.1, the 50 ppm Borates with 80 ppm TA improves buffering of acid by 84% and of base by 80%.

As for the advice of using Borates to raise pH, this is because Borax IS a base (boric acid, on the other hand, is a weak acid). Borax is sodium tetraborate decahydrate. When dissolved in water, the following occurs:

Na2B4O7•10H2O --> 2Na+ + 4B(OH)3 + 2OH- + 3H2O
Borax --> Sodium Ion + Boric Acid + Hydroxyl Ion + Water

then an equilibrium is established between Boric Acid and Borate Ion:

B(OH)3 + H2O <--> B(OH)4(-) + H+
Boric Acid + Water <--> Borate Ion + Hydrogen Ion
(since the pKa is around 9.1, most of what is in the water is boric acid, not borate ion, so this equilibrium is mostly shifted towards the left)

So Borax in water produces hydroxyl ion which is why this is a base that raises pH. However, once in the water and having raised the pH, it no longer has an effect or movement on making the pH change. That was my point. This is unlike the carbonates that DO have an effect or movement on pH just by being in the water in an over-saturated state relative to air. The pool is over-carbonated. So not only do the carbonates and borates both provide a buffer against changes in pH, but the carbonates themselves FORCE the pH to rise because they are out of equilibrium with the air -- there are too many carbonates in the water relative to the carbon dioxide in the air. The fact that the carbonates are a pH buffer is irrelevant. If carbonic acid were a substance that didn't dissociate (which it's not), then the same pH rise effect would still take place if there was too much of it in the water relative to carbon dioxide concentration in the air.

Richard

[cnix]

Thanks, Richard, for the great explanation. I have since found some additional technical discussion of this in Advanced Aquarist Magazine http://www.advancedaquarist.com/issues/may2002/chem.htm and some other articles linked to it. I'll try to digest them this weekend.

BTW, I checked my acid (from Home Depot) and it is the regular 31.45% strength.

Here are my numbers as of this morning:
FC 4.5
CC 0
pH 8.0
TA 130
Cal 240
CYA 35
Temp 85

For the three seasons since I've been operating this pool, the Cal has stayed 220-250 consistently..

I try to keep the TA in the 130-150 range. Over about 4 months, it will slowly drop to about 90, then I bump it back up. I usually don't worry about the pH until it climbs to 8.2+, then I add about 2-3 pts of acid, which drops it into the 7.4-7.6 range

The pool has had 50 lbs of borax total added, but I've partial drained and refilled about 5000 gal for each of two seasons now, so I calculate it has just under 35# in it now.

We have a very low swimmer load (me and my wife only), and we never use sunscreen.

After all the horror stories I've heard about pools, this one has been trivial--under 45 min per week total during the season. Since I've never found any CC in the pool, I only check every two weeks or so. I add a large jug of bleach 5-1/2 qts? every 3 days or so, and check/adjust the pH once a week.

I've had very mild mustard algae twice--just a little on the tile. I bump the FC up to 10+, brush, and hold it up for 24 hrs., and it's all gone. Both times, I had neglected the Chlorine addition for a couple of days too long--just after the water temp moved up into the 80s.

However, I've never heard any precise recommendations for how much borate to have for proper buffering.

I guess I have four questions:

1) Any particular reason to fight my pH lower than the 8.0 that's been working well?

2) Recommended borate level?

3) Recommended TA and Cal level for best protection of my plaster?

4) Should I go higher with the CYA? (and hold the FC correspondingly higher?)

Thanks for all your help.

Charles Nix

[chem geek]

Charles,



1) Any particular reason to fight my pH lower than the 8.0 that's been working well?

The main issue with running at the higher pH is that it is further away from the pH of human tears which runs closer to 7.5. Chlorine is less effective at the higher pH, though not by as much as the traditional graphs show because of the disinfecting chlorine buffering effect of CYA.


2) Recommended borate level?

50 ppm Borates will be a decent buffer (the numbers in my previous post used this amount) and will act as an algaecide. The "ppm" is technically measuring Boron. The test strips (AquaCheck) will accurately measure this level.


3) Recommended TA and Cal level for best protection of my plaster?

The water parameters of your pool -- especially pH, TA (and CYA), CH indicate that your water is over-saturated with calcium carbonate. That is, it will tend to scale or become cloudy. The index is at +0.6 and usually people start to see problems around +0.7 to +1.0 so you are on the edge for that.

The easiest thing to do is to lower your TA to 70 ppm and use a pH target of 7.7. That will give you about the same rate of carbon dioxide outgassing as your current TA 130 with pH 8.0. However, this should improve your acid usage since you add acid to have the pH go down to 7.4-7.6 where it outgasses more to rise to 8.0. At the new targets, it should take a lot less acid to keep the pH near 7.7 and you could swing between 7.6 and 7.8, for example.


4) Should I go higher with the CYA? (and hold the FC correspondingly higher?)

That is up to you, but if you increase your CYA, then have your TA level higher accordingly. The CYA contributes one-third of its ppm level to the TA value. So if you go to 70 ppm CYA, then make your TA target 80 instead of 70 (when you add more CYA, the TA will go up so you shouldn't need to change it -- just note the higher target TA).

Richard

[cnix]

OK--Thanks--

I'll get the borate up higher. If you said 40# puts me at 18 ppm. Then I need to add 85# to get to 50ppm. I can get it at my local Farmer's Coop in 50# bags (It's used to supply trace Boron mostly for Alfalfa, I think). BTW, Baking soda is usually available at Coops too in 50# bags.

One bit of clarification--I knew I was close to scaling conditions--I had understood that this would help preserve plaster from normal erosion. I know Calcium is essential for this, but does the Carbonate play any role in plaster preservation?

Charles

[chem geek]

It will take 70-71 pounds to get you to 50 ppm Borates, not 85 pounds.
( (50-18)/18 ) * 40 = 71.

It will take 67 cups (4.2 gallons) of Muriatic Acid to compensate for the pH rise from the additional Borates. You can add them one after the other in batches (say, 10 pounds Borax, then 9.5 cups acid, etc.).

Preventing dissolving of plaster/gunite/grout surfaces requires saturation of the water with calcium carbonate. Over-saturation is not necessary and can lead to scaling or cloudiness. Being a little under-saturated is fine -- it's only when you are way out of balance that problems are seen (i.e. well out of the +/- 0.5 range for saturation index -- problems are usually not seen until at least 0.7 to 1.0 or higher in either direction). It is the combination of calcium and carbonate that is important -- not each individually. The reason is that it is calcium carbonate that is in plaster/gunite/grout and that dissolves -- this originally comes from limestone (which is calcium carbonate) and is used as part of the cement making process.

Richard

[cnix]

I did a lot of reading over the weekend, that has brought up additional questions.

Richard, your explanation of saturation with calcium carbonate only being necessary to protect the plaster makes perfect sense. so:

1) How does one determine when the water is fully saturated?

2) If CYA contributes 1/3 of its ppm to TA, how much does Borate contribute to TA? (Assuming the Methyl Red/Bromcresol Green indicator--so that we know what the pH endpoint is)

3) How is Saturation index determined?

[chem geek]

1) How does one determine when the water is fully saturated?

One uses the Langelier Saturation Index (or the equivalent Calcite or Calcium Carbonate Saturation Index). See below for more info.


2) If CYA contributes 1/3 of its ppm to TA, how much does Borate contribute to TA? (Assuming the Methyl Red/Bromcresol Green indicator--so that we know what the pH endpoint is)

Borates contribute a negligible amount to TA. At a pH of 7.5, 50 ppm Borates contributes 5.7 ppm TA so figure an 11% contribution to TA.


3) How is Saturation index determined?
There are many online calculators, some that are good and some that are not, but I believe that Michael Smith's BleachCalc calculates the saturation index. It is a function of pH, TA, CH, TDS and temperature. When the index is near 0, your pool is at saturation for calcium carbonate. Negative numbers indicate risk of dissolving plaster/gunite/grout but only when it's quite negative -- don't even think of worrying until it gets to -0.5. Positive numbers indicate risk of scaling or precipitating calcium carbonate (including cloudiness). Again, don't even think of worrying until it gets to at least +0.5 and realize that most pools don't see any effect until at least +0.7 and some until +1.0 or more.

For a non-salt pool (i.e. a TDS around 500-1000), an ideal saturation (0.0) occurs at a pH of 7.5, TA of 100, CYA of 40, CH of 300, and temp of 80F. These are the mid-point values for the recommended ranges from NSPI. The pH directly affects the index so a 0.1 change in pH changes the index by 0.1 in the same direction. The TA and CH affect the index logarithmically so a change in adjusted TA (that is, TA reduced by one-third the CYA at a pH of 7.5) or CH by a factor of 10^0.1 = 1.26 changes the index by 0.1 in the same direction. The variation with temperature is small -- index increases by about 0.1 with a 10F temperature increase. The Langelier index, or at least the approximation used by the pool industry, has the index change by -0.08 when going from 525 to 3200 TDS (i.e. from non-salt to salt pool) while my calculations show that calcium carbonate saturation really changes by -0.22 instead. The temperature and especially the TDS calculations in the traditional industry formula are the least accurate portions of that formula. My calculations track the Taylor Watergram so if you have a Taylor K-2006 test kit then you can just use the watergram (using adjusted TA) to get an accurate index.

Richard