Ph And Alkalinity Adjustments/carb/bi-carb?
Posted 01 October 2007 - 02:14 PM
Which would be the ideal way to balance the water if you have a high pH and a low alkalinity reading?
Just a few "chemical" problems I have been fuzzy on, thanks for any help yet again guys (and gals)!!!
Posted 01 October 2007 - 04:11 PM
Sodium carbonate, aka soda ash, the chemical in "pH Up" and "Arm & Hammer Washing Soda" raises the pH, but ALSO raises the alkalinity. In fact, one pound of Sodium carbonate is chemically identical in its result to a combination of 6.2 ounces weight of Sodium hydroxide (aka lye or caustic soda) and 12.7 ounces weight of Sodium bicarbonate. The Sodium carbonate is lighter by 2.9 ounces weight since it reacts with water to form bicarbonate using the hydrogen from the water and produces hydroxyl ion from the water as well. The chemical formulas might make this more (or less ) clear.
Sodium bicarbonate: NaHCO3 which in water is sodium ion, Na+ and bicarbonate ion, HCO3(-)
Sodium carbonate: Na2CO3 which in water is two sodium ions, 2Na+ and carbonate ion, CO3(2-)
Sodium hydroxide: NaOH which in water is sodium ion, Na+ and hydroxyl ion, OH-
Adding carbonate to water mostly produces bicarbonate through the following reaction with water:
CO3(2-) + H2O --> HCO3(-) + OH-
Carbonate Ion + Water --> Bicarbonate Ion + Hydroxyl Ion
So you can see that essentially Carbonate is the same as Bicarbonate and Hydroxide. The hydroxyl ion is why the pH rises.
If you want to raise the pH with half the amount of increase in TA, then you can use Sodium Hydroxide (lye or caustic soda) as I referred to above. Or you can use 20 Mule Team Borax (which is Sodium tetraborate decahydrate) at about double the weight amount as Sodium carbonate. If you want to raise the pH with no change in TA at all, you can aerate the water (more on that below). You might wonder why the TA changes at all because there is no "carbonate" in these products and the reason is that TA also comes from pH itself (technically the difference in concentration between the amount of hydroxyl ion, OH-, and hydrogen ion, H+ and these also are what determine the pH which is technically the negative logarithm of the hydrogen ion concentration). This is why adding a pure acid, such as Muriatic Acid (which is 31.45% Hydrochloric acid, HCl) lowers both TA and pH. If you added equivalent concentrations of Muriatic Acid and Lye to pool water (one after the other), you would end up just producing saltier water (sodium chloride, NaCl) with no change in pH or TA.
As for the acid column or slug method, it doesn't work well and technically doesn't work at all (see this link) but if you add enough acid, then you effectively lower the pH of the entire body of water and that, combined with aeration (which there always is some naturally if the pool is exposed to air), DOES work. The real way to lower alkalinity is through a combination of low pH and aeration of the water. You have to drive the carbon dioxide out of the water -- it is already out-of-equilibrium with the air because pools are intentionally over-carbonated, but you need to help it along. It's just like taking a carbonated beverage and blowing bubbles in it (or stirring vigorously) to make it go flat, which essentially removes the carbonation (i.e. removes some of the carbonate, bicarbonate, carbonic acid and dissolved carbon dioxide from the water).
So the procedure for lowering TA most effectively is to first lower the pH to around 7.0 -- the lower the pH, the faster the outgassing of carbon dioxide, but you don't want to damage vinyl liners or pool equipment (metal) so 7.0 is a practical lower limit. You then aerate the water as much as you can -- using an air compressor with a nozzle that produces lots of tiny bubbles and putting that in the deep end works really well. Turning on waterfalls, fountains, showers, etc. also works. Even turning up the eyeballs in the returns helps. Aeration makes the pH go up with no change in Total Alkalinity (TA) (you remove carbon dioxide via H2CO3 --> H2O + CO2 so are removing carbonic acid so you are removing BOTH carbonate which increases TA AND acid, hydrogen ions, which decrease TA and these exactly cancel each other out). When the pH rises to 7.2 (or more), you add acid to lower the pH back down to 7.0 and repeat this process until the TA is where you want it. Then, just aerate without adding acid to have the pH rise to where you want it -- usually around 7.5
By the way, one great source of aeration is a saltwater chlorine generator (SWG) as it produced tiny hydrogen gas bubbles that you can see streaming out of the returns at night if you turn on a pool light (and turn on the pump and SWG, obviously). This is why SWG pools tend to have a strong tendency to rise in pH. They are frequently aerating the water. The solution is to lower the TA level, which is counterintuitive, but it works (you can raise the Calcium Hardness level somewhat if you lower the TA a lot so that the water remains saturated with Calcium carbonate if you have a plaster pool or exposed grout from tile). You can also add 30-50 ppm Borates (Borax) to the water which acts as an additional pH buffer and is also an algaecide which helps reduce chlorine usage and lets one lower the SWG output, making less aeration, so less rise in pH. Using 60-80 ppm CYA also reduces the chlorine loss from sunlight, again allowing for the SWG to be turned down in output, etc.
For high pH and low TA, you would add acid to lower the pH and then add Sodium bicarbonate to raise the TA. Since adding acid will lower the TA, if you added the Sodium bicarbonate first you are more likely to not add enough and would have to add some more. However, I doubt that your TA is really too low unless you are using an acidic source of chlorine, such as Trichlor pucks/tabs, and if that is what you are using then I bet your Cyanuric Acid (CYA) level is high and your pool is at risk of developing algae unless you are using some sort of algaecide or phosphate remover. If instead you just watched your CYA level and used unstabilized chlorine always keeping an absolute minimum Free Chlorine (FC) level of 7.5% of the CYA level (a decent target is a little over 10% of the CYA level since that gives some room for error), then your pool can be kept free of algae without the need for algaecides or phosphate removers.
Technically, to lower the pH with no change in TA, one can inject carbon dioxide directly into the water, but that is generally only done in some commercial pools and is not practical nor really necessary for residential pools.
Posted 02 October 2007 - 01:17 PM
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