pH keeps creeping up

[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?