Higher Calcium Levels & Stable pH

[chem geek]

This thread is in response to the question posed in this thread as it is a technical response (so belongs in The Deep End).

The short answer to why a lower pH with higher CH for the same saturation index results in a lower pH rise is "I don't know" since in theory it should not be any better and could be worse. The analysis below explains why a lower TA with higher CH might be beneficial.

The Calcite Saturation Index will determine the tendency to form scale (if positive) or to dissolve scale/plaster (if negative), but specifically for calcium carbonate via the following equilibrium reaction:

CaCO3(s) <--> Ca(2+) + CO3(2-)

The above reaction would say that water that is saturated with calcium carbonate would inhibit dissolving of plaster regardless of the level of calcium.

However, pool plaster itself is not only composed of a smaller amount of calcium carbonate, but is mostly composed of hydrated calcium silicate which is formed during curing as follows:

2 3CaO•SiO2 + 7 H2O ---> 3CaO•2SiO2•4H2O + 3 Ca(OH)2 + 173.6kJ

The Calcium Oxide is tightly bound to the silicate crystal structure, but can be dislodged via the following which is very strongly thermodynamically favored (were it not for the silicate structure):

CaO + CO2 --> Ca(2+) + CO3(2-) + heat

In the above reacion, saturation of calcium carbonate does not prevent the reaction from occurring though it reduces its likelihood. Reducing the carbon dioxide content further reduces the likelihood of the reaction and that means lowering the TA. If the TA is lowered, then either the pH or the Calcium Hardness (CH) must be increased to keep the calcium carbonate at saturation. Since one generally wants to keep the pH near 7.5 to be best for the eyes, this implies lower TA with higher CH could reduce the rate of dissolving of plaster. There is anecdotal evidence in extreme cases where calcium carbonate saturation in water very low in calcium results in pitting of plaster.

The result of having calcium carbonate dissolve or of having calcium oxide dissolve may be seen by giving an example starting with water at pH 7.5, TA 100, CYA 30, CH 300, TDS 525, Temp 80:

If one pound of CaCO3 as ions in 10,000 gallons is produced from dissolving Calcium Carbonate, this would raise the pH to 7.99 and the CH by 12ppm. If one pound of CaCO3 as ions in 10,000 gallons is produced from dissolving Calcium Oxide, this would raise the pH to 8.50 and the CH by 12 ppm. So the latter process raises the pH more for the same CH rise than the former.

So, in theory, having a lower TA and a higher CH could lead to a slower dissolving of pool plaster and a slower resulting rise in pH and CH over time.

Note that a lower pH instead of a lower TA with a higher CH to compensate (for saturation of calcium carbonate) does not produce the same effect. Instead the amount of carbon dioxide in this scenario is actually higher, not lower, so could have a faster dissolving of plaster. Therefore, if you see that a higher CH with lower pH has a lower pH rise, then it may be a different mechanism than described above or could just be a coincidence with something else happening in your situation.

As for the other questions in the original post, a lower pH of 7.4 would have slightly more carbon dioxide outgassing, but if the plaster dissolving were the main factor then that could overwhelm the outgassing effect. A lower TA instead of a lower pH would have the same effect on the dissolving of plaster, but would lower the outgassing as well so might result in an even lower pH rise and acid usage. As for eye comfort, I doubt there will be much of a noticeable difference between 7.4 and 7.5. The saturation index of -0.1 is only slightly negative. At the surface of a heat exchanger, the temperature may be 30F higher which would be around 0.2 higher in saturation index. As for the SWG cell, it has a very high pH at the hydrogen gas generation plate so the tradeoff is in having a negative index dissolve plaster faster. But remember that many of the measurements are approximate and have error so this isn't going to be perfect. Targeting -0.2 for the saturation index might be more reasonable, but it's not a big deal.

Richard

[aylad]

This thread is in response to the question posed in this thread as it is a technical response (so belongs in The Deep End).

Or in the China Shop!

Janet

[smallpooldad]

chem geek,

Thank you that was very clearand my response is here:

http://www.poolforum.com/pf2/showthr...ed=1#post56542

Aloha

[chem geek]

Regarding your reply, you shouldn't read into what I wrote that a slightly lower pH with a higher CH is a bad idea. There's a world of difference between theory and practice and the truth is we really don't know. Nevertheless, if you do lower your CH and raise your pH, it will be interesting to see if your pH rise returns -- if it doesn't, then something else happened to explain what you saw. If it does, then we've got a mystery on our hands.

I seriously doubt that near calcium carbonate saturation that any pool sees any sort of serious pH rise because of it. Most well maintained pools saturated in calcium carbonate that have pool covers show virtually no rise in pH and that includes my own pool. It's possible that higher calcium levels may have something to do with carbon dioxide outgassing, but that would be the first time I've heard or read about that. We'll see...

Richard

[smallpooldad]

Chem Geek,

Just drained the pool down to get Calcium at 375 and a pH of 7.5, so we will see. Anyhow should I drop dead tomorrow at least the pool person that my wife will hire will not be able to give her some "Cock and Bull" story about the Calcium being too high.

Unless of course he is really good looking and young. That again could seriously influence the story/numbers.

I will let you know how it goes.

Aloha

[CarlD]

Unless of course he is really good looking and young. That again could seriously influence the story/numbers.

I will let you know how it goes.

Aloha

"Go home to your wife. Better still, let me go home to your wife and apart from the improvement, she'll never know the difference. Pull over to the side of the road and let me see your marriage license!"
-- Prof. Quincy Adams Wagstaff, better known as Groucho Marx, (From "Horse Feathers")

[smallpooldad]

Well the pH is holding fine with the following numbers. Small amounts of acid are being inputed into the pool by the PoolPilot Digital input face but the tank seems to have used relatively little acid once the Alk got to 80. The tank is mixed 1 part muriatic to 4 parts water.

Here are the numbers:

Measurement
Measured pH 7.5
Total Alkalinity (ppm CaCO3) 80
Free Chlorine (ppm Cl2) 6.0
Cyanuric Acid (ppm CYA) 35
Calcium Hardness (ppm CaCO3) 400
Total Dissolved Solids (ppm) 3,450
Total Sulfate (ppm SO42-) 0
Total Borate (ppm Boron) 50.0
Total Ammonia (ppm Nitrogen) 0.0
U.S. Gallons 10,000
Temperature (oF) 83

Total Chloride (ppm NaCl) 3019
Carbonate Alkalinity (ppm CaCO3)61.9
Langelier Saturation Index (LSI)-0.09
% HOCl (vs. Total Free Chlorine)1.3%
OCl- (as ppm Cl2) 0.099
HOCl (as ppm Cl2) 0.080
Calcite Saturation Level (CSL) 0.63
Calcite Saturation Index (CSI) -0.20

A pH of 7.5 seems less irritating to the eyes. These seem fairly ideal numbers. One reason the pH holds so well is the pool is covered except from 5pm to 9pm for swimming. If the cover is left off over night the pH rises to 7.55-7.6 according to the digital control, mostly to 7.55

[chem geek]

Thanks for the update. At least this now makes more sense -- keeping the pool covered and having more stable pH is directly related to carbon dioxide outgassing and wouldn't have anything to do with the CH level. So though I don't know why the pH seemed to be more stable at higher CH, at least keeping on a cover makes it stable as we would expect.

[smallpooldad]

Further follow-up.

Increased Calcium to 425. The strange thing is that although the pool starts of at a pH of 7.5 if it is very, very sunny the pH actually drops if the cover is off to about 7.45. Have no idea why this happening. Later in the day (5 hours later at about 5 pm) it returns to 7.5, sometimes it might get up to 7.52. This only occurs on very sunny days. SWG runs from 11 am to 3 pm.

This is confirmed by my friends pool that also displays the same tendency. The only difference in his pool is that he has no borates and the calcium is 375. His SWG runs from 8:30 am to 4:30 pm.

I do not expect a reply but I thought it might be useful to you to follow-up, maybe not.

Aloha and thank you for all your kind and polite replies.

[CarlD]

That variation in pH is totally statistically insignficant. You could play around with various hypotheses, but my guess is the water is moving around and reacting to the air, out-gassing a little in the local area you're testing (as in aeration), re-mixing a little unevenly, etc.

Sometimes we over-measure and over-organize. One company that made sort engines as a gag produced a calendar that was completely alphabetical for days of the week and months of the year...
Friday, Monday, Saturday, Sunday, Thursday, Tuesday, Wednesday...
April,August, December, February, January, July, June, March, May, November, October, September
Kinda useless but very funny!