pH keeps creeping up

[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