Total Alkalinity

[PaP dude]

I think I might have confused myself from reading just about every post on this forum.

What I want to know is what exactly does TA measure? I know what role TA plays in water chemistry, I just don't have a clear understanding on what all TA encompasses. Is the amount of sodium bicarbonate dissolved in the water, the amount of carbonates in general, or am I way off? I have read things about carbonate alkalinity, cyanurate alkalinity, and even borates. Can someone clear all of this up for me?

[chem geek]

Technically, the Total Alkalinity (TA) test measures the quantity of any chemical substance that can accept a hydrogen ion down to a pH of around 4.5 which is when the indicator in the test changes from green to red. Another way of looking at it is that TA is a measure of how much acid you can add to the water before it drops down to a pH of 4.5 so is a measure of the pH buffering capacity that resists a drop in pH. In fact, the test itself uses sulfuric acid as the titrant. In practice, this measures the amount of bicarbonate ion in the water since that is the largest component of TA (this portion is called carbonate alkalinity and technically includes a double-count of carbonate ion but that's negligible). The next largest component is cyanurate ion which at a pH of 7.5 is roughly 1/3rd of the total amount of CYA in the water (the 1/3rd mostly comes from the fact that the CYA ppm units are different than TA units). The borates are negligible for TA since even 50 ppm Borates only contributes 5 ppm to TA at a pH of 7.5 (most of the borates in the water are Boric Acid while a small portion is borate ion so the borates have a much larger capacity to resist a rise in pH than a drop in pH).

The related chemical substances, their relative quantities (at a pH of 7.5), and how much they count towards TA (per molecule) is shown below where I am ignoring the existence of ion pairs and the chlorinated isocyanurate species (chlorine attached to CYA):

CARBONATES

CO₂(aq) ... 5.7% ...... 0 ... Dissolved (aqueous) Carbon Dioxide

H₂CO₃ .... 0.009% ... 0 ... Carbonic Acid

HCO₃^- ...... 94.1% ... 1 ... Bicarbonate Ion

CO₃^2- ........ 0.2% ... 2 ... Carbonate Ion

CYANURATES

H₃CY .... 17.6% ... 0 ... Cyanuric Acid

H₂CY^- ... 82.4% ... 1 ... Cyanurate Ion

HCY^2- ... 0.01% ... 2

CY^3- ..... 0.00% ... 3

BORATES

B(OH)₃ ... 97.8% ... 0 ... Boric Acid

B(OH)₄^- .... 2.2% ... 1 ... Borate Ion

The units of TA are ppm CaCO₃ equivalent so you add up the molecular quantities (such as moles) using the weights I list above and then multiply by 100.0892 g/mole CaCO₃ and then divide by 2 since the CO₃ in calcium carbonate counts twice (and yes, this is all very confusing).

TA plays two roles in pool/spa water chemistry. First, it is a measure of pH buffering capacity -- technically against a drop in pH, but in practice pH buffers resist changes in pH in both directions. If you didn't have a pH buffer in the water, then a small amount of acid or base would cause a large swing in pH. The second role of TA has to do with the bicarbonate ion component (and carbonate ion, but that is very small) and that is known as carbonate alkalinity. Adjusted TA subtracts out the CYA component of TA leaving carbonate alkalinity (ignoring any borates in the water). The carbonate alkalinity is important for water balance in saturating the water with calcium carbonate which is determined by the combination of pH, TA and Calcium Hardness (CH). Saturation of the water with calcium carbonate prevents etching of plaster and grout surfaces or similar substances.

Another effect of the carbonate alkalinity is that it causes the pH to rise, all else equal. This is because the water is essentially over-carbonated so there is more carbon dioxide in the water than in the air so it outgasses (and does so faster with more aeration and also at lower pH when more of the carbonates are in the form of dissolved CO₂). When carbon dioxide is outgassed, the pH rises with no change in TA (for technical reasons I won't get into here).

Richard

[PaP dude]

Thank you for clearing this up for me.

I had one other question. (sorry, I just want to know all of these things because I am try to pass on this knowledge to co-workers of mine.) I have read many of your post (chem geek) about your explanation of CO2 being out gassed from pools causing a rise in pH with no change in TA. You also mention that pools with SWGs aerate the water because of the hydrogen bubbles it produces at the electrolytic cell. (I understand all of electrochemistry behind that) I guess I am confused on how hydrogen gas bubbles causes carbon dioxide to out gas from the pool. Is this a shift in equilibrium? (from products to reactants) If so, what compounds are actually in equilibrium that causes this?

[chem geek]

It's much simpler than that. It's just a phase transfer from dissolved (aqueous) carbon dioxide in the water into a gaseous phase bubble that is passing through. The fact that the bubble is hydrogen is not relevant. It could be mostly air from an ozonator and the effect would be essentially the same. The concentration of carbon dioxide in the water is higher than its equilibrium concentration in ANY gaseous bubble that does not contain carbon dioxide, including a bubble of hydrogen gas. The concentration in water is also higher than that in an air bubble from an ozonator or regular aeration jet. The equilibrium involved is a phase equilibrium:

CO₂(aq) <--> CO₂(g)

where it does not matter what gasses are in the gas phase since the equilibrium only depends on the partial pressure of carbon dioxide in that gas phase (for ideal gasses that do not have unusual interaction). Essentially, the hydrogen gas bubbles are just like air bubbles in terms of the equilibrium except that air does contain some carbon dioxide so the rate of transfer will be slightly slower (since there will be a small reverse rate of carbon dioxide in the gas going back into aqueous carbon dioxide in the liquid phase).

Ignoring special circumstances such as facilitated transport, the rate of transfer is proportional to the surface area of the phase boundary and the contact time. So having smaller bubbles is more efficient since they have a higher surface area to volume ratio and smaller bubbles tend to linger longer. This is why using an air compressor connected to a nozzle with small holes to produce many small bubbles is very effective at lowering the TA (at low pH and with acid addition) and is also why spas with ozonators have a much stronger rise in pH unless one uses a more (net) acidic source of chlorine.

Richard

[PaP dude]

Ok, one last question regarding this topic. Where does the aqueous carbon dioxide come from in your pool? Does it come from carbonic acid, or a bicarbonate ion breaking down?

[chem geek]

Initially, the carbonates (as a whole) come from the water you used to fill the pool. If that Total Alkalinity (TA) level was too low, then you added more using sodium bicarbonate (baking soda or Alkalinity Up) or could possibly add it over time using sodium carbonate (washing soda, pH Up). The equilibrium between the carbonates is relatively fast and nothing gets destroyed -- the amount of each species is determined solely by the pH of the water. The only really slow reaction is that of the phase transfer between the dissolved (aqueous) carbon dioxide in the water with the carbon dioxide gas in the air. The following shows the equilibrium equations:

CO₂(g) <--> CO₂(aq)

Carbon Dioxide in Air <--> Carbon Dioxide in Water

CO₂(aq) + H₂O <--> H₂CO₃

Aqueous Carbon Dioxide + Water <--> Carbonic Acid

H₂CO₃ <--> HCO₃^- + H⁺

Carbonic Acid <--> Bicarbonate Ion + Hydrogen Ion

HCO₃^- <--> CO₃^2- + H⁺

Bicarbonate Ion <--> Carbonate Ion + Hydrogen Ion

If your fill water has carbonates in it, then evaporation of the water and refill can lead to an increase in TA over time. This is true with everything else in the fill water as well such as Calcium Hardness (CH).

At a pH of 7.5, the equilibrium amount of carbonate alkalinity (which is the same as TA if there is no CYA or borates) is a little less than 10 ppm. So you can see that pools and spas are quite a bit over-carbonated. Though this over-carbonation isn't as much as a carbonated beverage, the principles are the same. If you stir up a beverage vigorously, you release carbon dioxide and make it go flat. Likewise, if you blow bubbles into it through a straw, then you aerate it to remove carbon dioxide and make it go flat. If you were to add acid to the drink, you could have it bubble even more which is similar to adding vinegar (acetic acid) to baking soda (sodium bicarbonate) which is always a fun home experiment!

Richard